Margalef revisited: A new phytoplankton mandala incorporating twelve dimensions, including nutritional physiology

Typhoon-induced stormwater drives nutrient dynamics and triggers phytoplankton blooms in Laizhou Bay, China

In this study, we investigated the hydrological and ecological impacts of heavy rainfall caused by the storm Rumbia and Typhoon Lekima on Laizhou Bay (LZB) through land‒sea synchronous field surveys, online remote sensors, and simulated enclosure experiments. Within two weeks of Rumbia, approximately 9% and 16% of the annual riverine total nitrogen (TN) and total phosphorus (TP) fluxes, respectively, were transported to the LZB and the proportions were 17% and 35%, respectively, for Lekima. The land use on the watersheds increased the rates of land-derived nutrient loading and altered their biogeochemical forms. Consequently, the average concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the LZB increased by 2.6 and 1.0 times post-Rumbia and by 3.5 and 1.3 times post-Lekima, respectively. Relatively lower salinity and temperature, sudden increases in DIN, and strengthened coastal currents stimulated the growth of highly adaptable and small diatoms, resulting in the first diatom blooms. Subsequently, a bloom of Noctiluca scintillans formed.

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

Oligotrophic waters of the Northwest Atlantic support taxonomically diverse diatom communities that are distinct from coastal waters

Diatoms are important components of the marine food web and one of the most species-rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the genera Bacteriastrum, Haslea, Hemiaulus, Pseudo-nitzschia, and Nitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems.

Temporal changes in zooplankton indicators highlight a bottom-up process in the Bay of Marseille (NW Mediterranean Sea)

Sixteen years (2005-2020) of zooplankton monitoring in the Bay of Marseille (N-W Mediterranean Sea) are analyzed in relation to physical, meteorological, climatic and biotic data. Samples were collected every two weeks by a vertical haul (0-55 m) of a 200 μm plankton net. Different indices characterizing the mesozooplankton are compared: biomass dry weight of four size fractions between 200 and 2000 μm; abundances of the whole of the mesozooplankton and of 13 main taxonomic groups defined from plankton imagery; seasonal onset timing of each zooplankton group; and two other types of indices: the first characterized diversity based on abundance data, and the second was derived from zooplankton size spectra shape. The clearest pattern in the environmental compartment was an overall decreasing trend in nutrients, shifts in phytoplankton metrics (i.e. size structure and particulate organic matter), and changes in winter conditions (i.e. increasing temperatures, precipitation and NAO). Interannual patterns in the mesozooplankton community were: (i) a decrease of total abundance (ii) a decrease in biomass for the four size fractions, with an earlier decrease for the 1000-2000 μm size fraction (in 2008); (iii) a reduced dominance of copepods (calanoids and oithonoids) and a concomitant increase in abundance of other taxonomic groups (crustaceans, pteropods, chaetognaths, salps) which induced higher diversity; (iv) a first shift in size spectra towards smaller sizes in 2009, when the 1000-2000 μm size fraction biomass decreased, and a second shift towards larger sizes in 2013 along with increased diversity; and (iv) a later onset in the phenology for some zooplankton variables and earlier onset for salps. Concomitant changes in the phytoplankton compartment, winter environmental conditions, zooplankton community structure (in size and diversity) and zooplankton phenology marked by a shift in 2013 suggest bottom-up control of the pelagic ecosystem.

Phytoplankton Communities Miniaturization Driven by Extreme Weather in Subtropical Estuary under Climate Changes

Estuarine ecosystems are subject to extreme weather and climatic events, particularly global climate change. To characterize the effects of extreme weather, such as heavy precipitation and cold waves, on phytoplankton communities, four seasonal cruises were conducted in the subtropical Pearl River Estuary. Among the main phytoplankton taxa, small (pico- and nano-sized) phytoplankton accounted for approximately 50% and 70% in the upstream estuary. In spring, summer, and autumn, small-sized phytoplankton communities were dominated by Chlorophyta (20-45%), Ochrophyta (Bacillariophyceae; 25-40%), and Dinoflagellata (20-25%), associated with high phytoplankton network stability and river discharge. In winter, small phytoplankton communities were dominated by Cryptophyta (50%), Dinoflagellata (25%), and Ochrophyta (Bacillariophyceae; 10%), which were associated with low network stability and temperature. Low light and high nutrient conditions induced by large river discharge favored the growth of the smallest known brackish Chlorophyta, such as Ostreococcus tauri. Low temperatures provided a competitive advantage for the survival of the small-sized cryptophyte genus Teleaulax, especially in the upstream estuary during the 2020/2021 extreme cold wave period. Our findings highlight the impact of extreme weather induced by climatic events on the miniaturization of phytoplankton communities in subtropical estuaries by altering temperature, light availability, and nutrient dynamics.

Using climate reanalysis and remote sensing-derived data to create the basis for predicting the occurrence of algal blooms, harmful algal blooms and toxic events in Santa Catarina, Brazil

This study aimed to form a basis for future predictive modeling efforts in support of the harmful algal blooms (HAB) surveillance program currently in force in the Brazilian State of Santa Catarina (SC). Data from monitoring toxin-producing algae were merged with both meteorological and oceanographic data and analyzed. Data from four sources were used in this study: climate reanalysis (air temperature, pressure, cloud cover, precipitation, radiation, U and V winds); remote sensing (chlorophyll concentration and sea surface temperature); Oceanic Niño Index; and HAB monitoring data (phytoplankton counts and toxin levels in shellfish samples obtained from 39 points located in shellfish farms distributed along the SC coastline). This study analyzed the period from 2007-01-01 to 2019-12-31 (7035 records in the HAB database) and used descriptive, bivariate and multivariate analyses to draw correlations among environmental parameters and the occurrence of algal blooms (AB), HAB and toxic events. Dinophysis spp. AB were the most registered type of event and tended to occur during the late autumn and winter months. These events were associated with high atmospheric pressure, predominance of westerly and southerly winds, low solar radiation and low sea and air temperature. An inverted pattern was observed for Pseudo-nitzschia spp. AB, which were mostly registered during the summer and early autumn months. These results give evidence that the patterns of occurrence of highly prevalent toxin-producing microalgae reported worldwide, such as the Dinophysis AB during the summer, differ along the coast of SC. Our findings also show that meteorological data, such as wind direction and speed, atmospheric pressure, solar radiation and air temperature, might all be key predictive modeling input parameters, whereas remote sensing estimates of chlorophyll, which are currently used as a proxy for the occurrence of AB, seem to be a poor predictor of HAB in this geographic area.

A coherent status of summer monsoon phytoplankton communities (2017-2018) along the Western Indian continental shelf: Implications for fisheries

The western Indian continental shelf is characterized by contrasting biogeochemical features from south to north mostly governed by monsoon wind forcing. Here we present the first detailed study on the phytoplankton community (analyzed by marker pigments and microscopy) during the summer monsoon addressing the interannual variability (Aug 2017 and 2018) along the Western Indian Continental Shelf waters (8° N-21° N, at 200 m isobaths). A distinct interannual variability in monsoon impacted the hydrography and nutrient stoichiometry that was further cascaded to the phytoplankton community structure. The timing of the monsoon onset, wind speeds, and the strength of the alongshore wind component were the major factors that governed the interannual variability in the development and progression of the upwelling as well as phytoplankton bloom. The upwelling-dominated southwestern shelf was characterized by cold, nutrient-rich waters dominated by the marker pigment of diatoms, fucoxanthin, and microscopy also revealed the presence of large, chain-forming diatoms (Chaetoceros sp.; Dactyliosolen fragilissimus). In contrast, low nutrient warm waters in the northern shelf housed nanophytoplankton and picophytoplankton along with small diatoms (Thalassiosira sp.) and dinoflagellates (Gymnodinium sp.). Interestingly, in a few stations in the southern shelf, the surface waters were low saline that was intensified in 2018 compared to 2017 inhibiting upwelled water from reaching the surface. Consequently, due to low nutrient levels caused by reduced upwelling and low salinity, picoplanktonic cyanobacteria dominated replacing the larger diatoms that were found plenty in 2017. Likely, such a niche shift adversely impacted the planktivorous fish catch (Indian Oil Sardine) that was seen to be less in 2018 relative to 2017. Any further changes in monsoon variability and freshwater flow to this region may have direct consequences on the phytoplankton community as well as fisheries.

The underappreciated role of biotic factors in controlling the bloom ecology of potentially harmful microalgae in the Hudson-Raritan Bay

Despite widespread distribution of harmful algal blooms (HABs) and new and improved methods for detecting and quantifying them, no unifying ecological explanation has been found. Improved understanding depends upon local, ecological studies that include analysis of phytoplankton species data in relation to both abiotic and biotic parameters. Ecological network analysis was used to detect co-occurrence patterns among abiotic and biotic parameters in a long-term monitoring dataset (i.e., 2010-2021) from the eutrophic Hudson-Raritan Estuary (HRE) between the states of New York and New Jersey. The regular co-occurrence of potentially harmful bloom-forming species with companion species observed through microscopy was supported by the results of ecological network analysis, which showed that there were far more associations between HAB species and biotic parameters (∼95%) than abiotic parameters (∼5%). Temperature was the environmental variable that was most associated with HAB species throughout the estuary. The numerous network associations of HAB species with one another and with diatoms, dinoflagellates, and zooplankton highlight the complexity of planktonic food webs and interactions. Results also suggest that some taxa may play a central role in structuring the HRE plankton communities. These findings demonstrate that biotic associations play an underappreciated role in plankton structure and the value of examining the ecology of HAB species within the breadth of their biological communities. While network analysis does not fully explain and confirm complex associations among species, it does provide fresh insights and testable hypotheses to strengthen understanding and improve prediction.

Early warning of red tides using bacterial and eukaryotic communities in nearshore waters

Anthropogenic discharge activities have increased nutrient pollution in coastal areas, leading to algal blooms and microbial community changes. Particularly, microbial communities could easily be affected with variation in nutrient pollution, and thus offered a promising strategy to predict early red tides warning via microbial community-levels variation and their keystone taxa hysteretic responses to nutrient pollution. Herein high-throughput sequencing technology from 52 samples were used to explore the variation of microbial communities and find the significant tipping points with aggravating nutrient conditions in Xiaoping Island coastal area. Results indicated that bacterial and microeukaryote communities were generally spatial and seasonal heterogeneity and were influenced by the different nutrient conditions. Procrustes test results showed that the comprehensive index of organics polluting (OPI), total nitrogen (TN), inorganic nitrogen (DIN), and total phosphorus (TP) were significantly correlated with the composition of bacteria and microeukaryotes. A SEGMENTED analysis revealed that the threshold of TN, DIN, and NH₄-N for bacterial community were 0.23 ± 0.091 mg/L, 0.21 ± 0.084 mg/L, 0.09 ± 0.057 mg/L, respectively. Tipping points for TN, DIN, and NH₄-N agreed with the concentration during Ceratium tripos and Skeletonema costatum blooms. Co-occurrence network results found that Planktomarina, Acinetobacter, and Verrucomicrobiaceae were keystone and OPI-discriminatory taxa. The abundant changes of Planktomarina at station A1 were significantly correlated with the development of C. tripos blooms (r = 0.55, p < 0.05), and also significantly correlated with TN, DIN, and NO₃-N (r≥|0.55|, p < 0.05). The abundant changes of Acinetobacter and Verrucomicrobiaceae at station C1 were significantly correlated with the development of C. tripos blooms (r ≥ 0.77, p < 0.05), and also significantly correlated with PO₄-P (r ≥ 0.64, p < 0.05). The dynamic abundance of keystone taxa showed that the trend of rapid changes could be monitored 1.5 months before the occurrence of red tide. Therefore, this study provides an assessment method for early warning of red tide occurrence and factors that trigger red tide.

Variability patterns and phenology of harmful phytoplankton blooms off southern Portugal: Looking for region-specific environmental drivers and predictors

Harmful algal blooms (HABs) negatively impact coastal ecosystems, fisheries, and human health, and their prediction has become imperative for effective coastal management. This study aimed to evaluate spatial-temporal variability patterns and phenology for key toxigenic phytoplankton species off southern Portugal, during a 6-year period, and identify region-specific environmental drivers and predictors. Total abundance of species responsible for amnesic shellfish poisoning (Pseudo-nitzschia spp.), diarrhetic shellfish poisoning (Dinophysis spp.), and paralytic shellfish poisoning (G. catenatum) were retrieved, from the National Bivalve Mollusk Monitoring System public database. Contemporaneous environmental variables were acquired from satellite remote sensing, model-derived data, and in situ observations, and generalized additive models (GAMs) were used to explore the functional relationships between HABs and environmental variables and identify region-specific predictors. Pseudo-nitzschia spp. showed a bimodal annual cycle for most coastal production areas, with spring and summer maxima, reflecting the increase in light intensity during the mixed layer shoaling stage, and the later stimulatory effects of upwelling events, with a higher bloom frequency over coastal areas subjected to stronger upwelling intensity. Dinophysis spp. exhibited a unimodal annual cycle, with spring/summer maxima associated with stratified conditions, that typically promote dinoflagellates. Dinophysis spp. blooms were delayed with respect to Pseudo-nitzschia spp. spring blooms, and followed by Pseudo-nitzschia spp. summer blooms, probably reflecting upwelling-relaxation cycles. G. catenatum occurred occasionally, namely in areas more influenced by river discharges, under weaker upwelling. Statistical-empirical models (GAMs) explained 7-8%, and 21-54% of the variability in Pseudo-nitzschia spp. and Dinophysis spp., respectively. Overall, a set of four easily accessible environmental variables, surface photosynthetically available radiation, mixed layer depth, sea surface temperature, and chlorophyll-a concentration, emerged as the most influential predictors. Additionally, over the coastal production areas along the south coast, river discharges exerted minor negative effects on both HAB groups. Despite evidence supporting the role of upwelling intensity as an environmental driver of Pseudo-nitzschia spp., it was not identified as a relevant model predictor. Future model developments, such as the inclusion of additional environmental variables, and the implementation of species- and period-specific, and hybrid modelling approaches, may further support HAB operational forecasting and managing over complex coastal domains.

Environmentally induced functional shifts in phytoplankton and their potential consequences for ecosystem functioning

Phytoplanktonic organisms are particularly sensitive to environmental change, and, as they represent a direct link between abiotic and biotic compartments within the marine food web, changes in the functional structure of phytoplankton communities can result in profound impacts on ecosystem functioning. Using a trait-based approach, we examined changes in the functional structure of the southern North Sea phytoplankton over the past five decades in relation to environmental conditions. We identified a shift in functional structure between 1998 and 2004 which coincides with a pronounced increase in diatom and decrease in dinoflagellate abundances, and we provide a mechanistic explanation for this taxonomic change. Early in the 2000s, the phytoplankton functional structure shifted from slow growing, autumn blooming, mixotrophic organisms, towards earlier blooming and faster-growing microalgae. Warming and decreasing dissolved phosphorus concentrations were linked to this rapid reorganization of the functional structure. We identified a potential link between this shift and dissolved nutrient concentrations, and we hypothesise that organisms blooming early and displaying high growth rates efficiently take up nutrients which then are no longer available to late bloomers. Moreover, we identified that the above-mentioned functional change may have bottom-up consequences, through a food quality-driven negative influence on copepod abundances. Overall, our study highlights that, by altering the phytoplankton functional composition, global and regional changes may have profound long-term impacts on coastal ecosystems, impacting both food-web structure and biogeochemical cycles.

Spatial-temporal variability of microphytoplankton assemblages including harmful microalgae in a tropical semi-enclosed strait (Johor Strait, Malaysia)

Harmful algal blooms (HABs) were not new to the tropical semi-enclosed Johor Strait, with incident records that could trace back to the 1980s. HAB monitoring in the area, often, is reactive, focusing only on HAB taxa previously causing problems but neglecting potential emerging HABs. To develop datasets on HABs that can better inform and improve management practices, monitoring should expand to sample whole microphytoplankton communities. In this study, microphytoplankton community structure across the Strait was investigated. Abundances of microphytoplankton and a suite of in situ water parameters of temperatures, salinity, pH, dissolved oxygen levels, macronutrients, and chlorophyll-a contents were collected at ten sites across the Strait at monthly intervals from January 2017 to December 2018. A total of 48 genera (51 taxa) microphytoplankton were identified microscopically. Diatom was the most diverse group (32 genera), followed by dinophyte (15 genera). Bloom-forming species included diatoms Chaetoceros, Coscinodiscus, Eucampia, Pseudo-nitzschia, Rhizosolenia, Skeletonema, Thalassiosira, and dinophytes Blixaea quinquecornis and Scrippsiella. Diatom taxa that exhibit high in situ growth rates were predominant in the low-nutrient marine-influenced environment. Bloom-forming taxa including HAB taxa were found dominant in the environment with high nutrient levels and mesohaline, salinity-stratified conditions. This study provides valuable baseline data that could assist in monitoring and prediction of HABs in the Johor Strait and could be of reference to other similar tropical coastal systems.

The Eco-Evo Mandala: Simplifying Bacterioplankton Complexity into Ecohealth Signatures

The microbiome emits informative signals of biological organization and environmental pressure that aid ecosystem monitoring and prediction. Are the many signals reducible to a habitat-specific portfolio that characterizes ecosystem health? Does an optimally structured microbiome imply a resilient microbiome? To answer these questions, we applied our novel Eco-Evo Mandala to bacterioplankton data from four habitats within the Great Barrier Reef, to explore how patterns in community structure, function and genetics signal habitat-specific organization and departures from theoretical optimality. The Mandala revealed communities departing from optimality in habitat-specific ways, mostly along structural and functional traits related to bacterioplankton abundance and interaction distributions (reflected by ϵ and λ as power law and exponential distribution parameters), which are not linearly associated with each other. River and reef communities were similar in their relatively low abundance and interaction disorganization (low ϵ and λ) due to their protective structured habitats. On the contrary, lagoon and estuarine inshore reefs appeared the most disorganized due to the ocean temperature and biogeochemical stress. Phylogenetic distances (D) were minimally informative in characterizing bacterioplankton organization. However, dominant populations, such as Proteobacteria, Bacteroidetes, and Cyanobacteria, were largely responsible for community patterns, being generalists with a large functional gene repertoire (high D) that increases resilience. The relative balance of these populations was found to be habitat-specific and likely related to systemic environmental stress. The position on the Mandala along the three fundamental traits, as well as fluctuations in this ecological state, conveys information about the microbiome's health (and likely ecosystem health considering bacteria-based multitrophic dependencies) as divergence from the expected relative optimality. The Eco-Evo Mandala emphasizes how habitat and the microbiome's interaction network topology are first- and second-order factors for ecosystem health evaluation over taxonomic species richness. Unhealthy microbiome communities and unbalanced microbes are identified not by macroecological indicators but by mapping their impact on the collective proportion and distribution of interactions, which regulates the microbiome's ecosystem function.

Coupling high frequency monitoring and bioassay experiments to investigate a harmful algal bloom in the Bay of Seine (French-English Channel)

Coastal ecosystems are increasingly threatened by eutrophication and dystrophy. In this context, the full pattern of a bloom dominated by the dinoflagellate, Lepidodinium chlorophorum, was investigated by a high frequency monitoring buoy equipped with sensors allowing nutrients and photosynthesis measurements. An increase of the N/P ratio affected phytoplankton physiology leading to bloom collapse with a slight oxygen depletion. In parallel, enrichment experiments were performed on the natural bloom population. After 5 days of incubation the community structure, using flow cytometry and several physiological parameters were analysed. The data reveal a potential N and P co-limitation and a decoupling between primary production and productivity in fully enriched conditions. Under unbalanced N/P inputs, high level of alkaline phosphatase activity and transparent exopolymeric particle production, which favour phytoplankton sedimentation, were observed. Nutrient inputs and their stoichiometry control phytoplankton growth, the community structure, physiological regulations, the fate of the bloom and consequences.

Diverse harmful microalgal community assemblages in the Johor Strait and the environmental effects on its community dynamics

Coastal ecosystems are often subjected to anthropogenic disturbances that lead to water quality deterioration and an increase in harmful algal bloom (HAB) events. Using the next-generation molecular tool of 18S rDNA metabarcoding, we examined the community assemblages of HAB species in the Johor Strait, Malaysia between May 2018 and September 2019, covering 19 stations across the strait. The molecular operational taxonomic units (OTUs) of HAB taxa retrieved from the dataset (n = 194) revealed a much higher number of HAB taxa (26 OTUs) than before, with 12 taxa belong to new records in the strait. As revealed in the findings of this study, the diversity and community structure of HAB taxa varied significantly over time and space. The most common and abundant HAB taxa in the strait (frequency of occurrence >70%) comprised Heterosigma akashiwo, Fibrocapsa japonica, Pseudo-nitzschia pungens, Dinophysis spp., Gymnodinium catenatum, Alexandrium leei, and A. tamiyavanichii. Also, our results demonstrated that the HAB community assemblages in the strait were dependent on the interplay of environmental variables that influence by the monsoonal effects. Different HAB taxa, constitute various functional types, occupied and prevailed in different environmental niches across space and time, leading to diverse community assemblages and population density. This study adds to the current understandings of HAB dynamics and provides a robust overview of temporal-spatial changes in HAB community assemblages along the environmental gradients in a tropical eutrophic coastal ecosystem.

The genetic and ecophysiological diversity of Microcystis

Microcystis is a cyanobacterium that forms toxic blooms in freshwater ecosystems around the world. Biological variation among taxa within the genus is apparent through genetic and phenotypic differences between strains and via the spatial and temporal distribution of strains in the environment, and this fine-scale diversity exerts strong influence over bloom toxicity. Yet we do not know how varying traits of Microcystis strains govern their environmental distribution, the tradeoffs and links between these traits, or how they are encoded at the genomic level. Here we synthesize current knowledge on the importance of diversity within Microcystis and on the genes and traits that likely underpin ecological differentiation of taxa. We briefly review spatial and environmental patterns of Microcystis diversity in the field and genetic evidence for cohesive groups within Microcystis. We then compile data on strain-level diversity regarding growth responses to environmental conditions and explore evidence for variation of community interactions across Microcystis strains. Potential links and tradeoffs between traits are identified and discussed. The resulting picture, while incomplete, highlights key knowledge gaps that need to be filled to enable new models for predicting strain-level dynamics, which influence the development, toxicity and cosmopolitan nature of Microcystis blooms.

Effects of Grazing and Nutrients on Phytoplankton Blooms and Microplankton Assemblage Structure in Four Temperate Lakes Spanning a Eutrophication Gradient

Phytoplankton assemblage dynamics are sensitive to biotic and abiotic factors, as well as anthropogenic stressors such as eutrophication, and thus are likely to vary between lakes of differing trophic state. We selected four lakes in Washington State, USA, ranging from oligo- to hypereutrophic, to study the separate and interactive effects of enhanced nutrient availability and zooplankton grazing on phytoplankton net growth rates and overall microplankton (phytoplankton and microzooplankton) assemblage structure. We collected water quality and plankton samples monthly in each lake from May to October 2014, and also conducted laboratory incubation experiments using ambient plankton assemblages from each lake with amendments of zooplankton grazers (5× ambient densities) and nutrients (Nitrogen + Phosphorus) in June, August, and October. In each set of monthly experiments, nested two-way ANOVAs were used to test the effects of enhanced grazers and nutrients on net chlorophyll a-based phytoplankton growth rates. Nested PERMANOVAs were used to test the effects of each factor on microplankton assemblage structure. Enhanced grazing reduced phytoplankton net growth in oligotrophic Cle Elum Lake and oligo-mesotrophic Lake Merwin in August (p < 0.001) and Merwin again in October (p < 0.05), while nutrient enhancement increased phytoplankton net growth in Lake Merwin in June (p < 0.01). Changes in microplankton assemblage composition were not detected as a result of either factor, but they were significantly different between sites (p < 0.001) during each month, and varied by month within each lake. Significant effects of both enhanced grazers and nutrients were detected in systems of low, but not high, trophic state, although this varied by season. We suggest that it is critical to consider trophic state when predicting the response of phytoplankton to bottom-up and top-down factors in lakes.

Phytoplankton community structuring and succession in a competition-neutral resource landscape

Phytoplankton community composition and succession affect aquatic food webs and biogeochemistry. Resource competition is commonly viewed as an important governing factor for community structuring and this perception is imbedded in modern ecosystem models. Quantitative consideration of the physical spacing between phytoplankton cells, however, suggests that direct competition for growth-limiting resources is uncommon. Here we describe how phytoplankton size distributions and temporal successions are compatible with a competition-neutral resource landscape. Consideration of phytoplankton-herbivore interactions with proportional feeding size ranges yields small-cell dominated size distributions consistent with observations for stable aquatic environments, whereas predator-prey temporal lags and blooming physiologies shift this distribution to larger mean cell sizes in temporally dynamic environments. We propose a conceptual mandala for understanding phytoplankton community composition where species successional series are initiated by environmental disturbance, guided by the magnitude of these disturbances and nutrient stoichiometry, and terminated with the return toward a 'stable solution'. Our conceptual mandala provides a framework for interpreting and modeling the environmental structuring of natural phytoplankton populations.

Integrative omics identification, evolutionary and structural analysis of low affinity nitrate transporters in diatoms, diNPFs

Diatoms are one of the major and most diverse groups of phytoplankton, with chimeric genomes harbouring a combination of genes of bacterial, animal and plant origin. They have developed sophisticated mechanisms to face environmental variations. In marine environments, nutrients concentration shows significant temporal and spatial variability, influencing phytoplankton growth. Among nutrients, nitrogen, present at micromolar levels, is often a limiting resource. Here, we report a comprehensive characterization of the Nitrate Transporter 1/Peptide Transporter Family (NPF) in diatoms, diNPFs. NPFs are well characterized in many organisms where they recognize a broad range of substrates, ranging from short-chained di- and tri-peptides in bacteria, fungi and mammals to a wide variety of molecules including nitrate in higher plants. Scarce information is available for diNPFs. We integrated-omics, phylogenetic, structural and expression analyses, to infer information on their role in diatoms. diNPF genes diverged to produce two distinct clades with strong sequence and structural homology with either bacterial or plant NPFs, with different predicted sub-cellular localization, suggesting that the divergence resulted in functional diversification. Moreover, transcription analysis of diNPF genes under different laboratory and environmental growth conditions suggests that diNPF diversification led to genetic adaptations that might contribute to diatoms ability to flourish in diverse environmental conditions.

Niche differentiation of Dinophysis acuta and D. acuminata in a stratified fjord

Dinophysis acuta and D. acuminata are associated with lipophilic toxins in Southern Chile. Blooms of the two species coincided during summer 2019 in a highly stratified fjord system (Puyuhuapi, Chilean Patagonia). High vertical resolution measurements of physical parameters were carried out during 48 h sampling to i) explore physiological status (e.g., division rates, toxin content) and ii) illustrate the fine scale distribution of D. acuta and D. acuminata populations with a focus on water column structure and co-occurring plastid-bearing ciliates. The species-specific resources and regulators defining the realized niches (sensu Hutchinson) of the two species were identified. Differences in vertical distribution, daily vertical migration and in situ division rates (with record values, 0.76 d^-1, in D. acuta), in response to the environmental conditions and potential prey availability, revealed their niche differences. The Outlying Mean Index (OMI) analysis showed that the realized niche of D. acuta (cell maximum 7 × 10³ cells L^-1 within the pycnocline) was characterized by sub-surface estuarine waters (salinity 23 - 25), lower values of turbulence and PAR, and a narrow niche breath. In contrast, the realized niche of D. acuminata (cell maximum 6.8 × 10³ cells L^-1 just above the pycnocline) was characterized by fresher (salinity 17 - 20) outflowing surface waters, with higher turbulence and light intensity and a wider niche breadth. Results from OMI and PERMANOVA analyses of co-occurring microplanktonic ciliates were compatible with the hypothesis of species such as those from genera Pseudotontonia and Strombidium constituting an alternative ciliate prey to Mesodinium. The D. acuta cell maximum was associated with DSP (OA and DTX-1) toxins and pectenotoxins; that of D. acuminata only with pectenotoxins. Results presented here contribute to a better understanding of the environmental drivers of species-specific blooms of Dinophysis and management of their distinct effects in Southern Chile.

Seasonal plankton succession is in accordance with phycotoxin occurrence in Disko Bay, West Greenland

Harmful algal blooms (HABs) are occurring more frequently in the world's oceans, probably as a consequence of climate change. HABs have not been considered a serious concern in the Arctic, even though the Arctic warms faster than any other region. While phycotoxins and toxin-producing phytoplankton have been found in Arctic waters on several occasions, there is a lack of information on seasonal succession of species and whether the occurrence of harmful species correlates with the presence of their respective phycotoxins. Hence, there is no baseline to assess future changes of HABs in this area. Here, we investigated two periods, from winter to spring and from the spring bloom until summer, in Disko Bay, West Greenland and followed the succession of toxins and their producers using metabarcoding, as well as analyses of particulate and dissolved toxins. We observed a typical seasonal succession with a spring bloom dominated by diatoms, followed by dinoflagellates in summer, with the two most important potentially toxic taxa found being Pseudo-nitzschia spp. and Alexandrium ostenfeldii. The Pseudo-nitzschia spp. peak correlated with a clear increase in particulate domoic acid, reaching 0.05 pg/L. Presence of Alexandrium ostenfeldii could be linked to an increase in spirolides, up to 56.4 pg/L in the particulate phase. Generally, the majority of detected dissolved toxins followed the succession pattern of the particulate toxins with a delay in time. Our results further show that Arctic waters are a suitable habitat for various toxin producers and that the strong seasonality of this environment is reflected by changing abundances of different toxins that pose a potential threat to the ecosystem and its beneficiaries.

Monoculture-based consumer-resource models predict species dominance in mixed batch cultures of dinoflagellates

Global change will disturb the frequency, scale and distribution of harmful algal blooms (HABs), but we are unable to predict future HABs due to our limited understanding of how physicochemical changes in the environment affect interspecific competition between dinoflagellates. Trait-based mechanistic modelling is an important tool to unravel and quantify various direct and indirect interactions between species. The present study explores whether MacArthur's consumer-resource model can be used as a viable base model to predict dinoflagellate growth in closed multispecies systems. To this end, two batch culture experiments (294 cultures in total) with monocultures and multispecies cultures of Alexandrium minutum, Prorocentrum lima, P. micans, Protoceratium reticulatum and Scrippsiella trochoidea were performed. Despite changes to the relative (different nitrate concentrations) and absolute nutrient availability (dilutions of L1 medium), P. micans outcompeted all other species in mixed cultures. Consumer-resource modelling parameterized using monoculture growth correctly predicted this species dominance (R² between 0.80 and 0.95). Parameter estimates revealed that P. micans had a faster uptake of nitrogen when compared to its competitors, but did not differ in resource efficiency and natural mortality rate. Yet, while the model accurately predicted community dynamics during the growth phase, it was not able to predict their dynamics beyond the point of quiescence. Consumer-resource modelling was shown to differentiate the roles of resource assimilation, resource efficiency, and natural mortality rates in batch culture experiments with minimal data requirements beyond common measurements. The results suggest that consumer-resource models provide a promising basis for trait-based modelling of interspecific competition between (harmful) algae.

Interannual variability of Emiliania huxleyi blooms in the Barents Sea: In situ data 2014-2018

Using in situ data of 2014-2018, the summers Emiliania huxleyi blooms in the Barents Sea were studied. The blooms were recorded in the upper mixed layer in July and August every year, during which they spread to cover large areas and were associated with Atlantic waters. The E. huxleyi abundance revealed interannual variability, with the highest values (up to 12 × 10⁶ cells/L) in July 2016. Bloom is characterized by a sharp seasonal thermocline, water surface temperature of about 7.14-11.7 °C, low silicate (0.45 ± 0.08 μM) and nitrogen (0.74 ± 0.16 μM) concentration, high phosphorus concentration (0.09 ± 0.01 μM) and nitrogen to phosphorus ratio significantly below the Redfield ratio. Data confirming the hypothesis of limiting the growth of diatoms by nitrogen concentration are presented. When E. huxleyi bloomed, its biomass exceeded 70% of the total phytoplankton biomass, species diversity was low, and diatoms were practically absent, and dinoflagellates were usually represented by large species.

Fifty years of ecological changes: Regime shifts and drivers in a coastal Mediterranean lagoon during oligotrophication

Thau lagoon is a large Mediterranean coastal lagoons and it supports traditional shellfish farming activities. It has been subject to eutrophication leading to major anoxic events associated with massive mortalities of shellfish stocks. Since the 1970s, improvements have been made to wastewater treatment systems, which have gradually led to oligotrophication of the lagoon. The aim of our study was to determine how the decrease in nutrient inputs resulted in major ecological changes in Thau lagoon, by analysing five decades of time-series (1970-2018) of observations on pelagic and benthic autotrophic communities. We were able to identify two periods during the oligotrophication process. Period 1 (1970-1992) was considered a eutrophic period, characterised by the shift from seagrass dominance to dominance of red macroalgae. Period 2 (1993-2018), characterised by improved eutrophication status, was further divided into three: a transition phase (1993-2003) during which the water column continued to recover but the benthic community lagged behind in recovery and in partial resilience; a regime shift (2003-2006), after which the water column became oligotrophic and seagrass began to recover (2007-2018). Considering anoxia crises as indicators of ecosystem resilience and resistance, we used a generalised linear model to analyse meteorological and environmental data with the aim of identifying the triggers of summer anoxia over the study period. Among the meteorological variables studied, air temperature had the strongest positive effect, followed by the period and wind intensity (both negative effects) and by rainfall in July (positive effect). The risk of triggering anoxia was lower in period 2, evidence for the increasing resistance of the ecosystem to climatic stress throughout the oligotrophication process. At the ecosystem scale and in the long term perspective, the ecological gains related to oligotrophication are especially important in the context of climate change, with more frequent and severe heat waves predicted.

Functional significance of phylogeographic structure in a toxic benthic marine microbial eukaryote over a latitudinal gradient along the East Australian Current

Genetic diversity in marine microbial eukaryotic populations (protists) drives their ecological success by enabling diverse phenotypes to respond rapidly to changing environmental conditions. Despite enormous population sizes and lack of barriers to gene flow, genetic differentiation that is associated with geographic distance, currents, and environmental gradients has been reported from planktonic protists. However, for benthic protists, which have reduced dispersal opportunities, phylogeography and its phenotypic significance are little known. In recent years, the East Australian Current (EAC) has intensified its southward flow, associated with the tropicalization of temperate waters. Benthic harmful algal species have been increasingly found in south-eastern Australia. Yet little is known about the potential of these species to adapt or extend their range in relation to changing conditions. Here, we examine genetic diversity and functional niche divergence in a toxic benthic dinoflagellate, Ostreopsis cf. siamensis, along a 1,500 km north-south gradient in southeastern Australia. Sixty-eight strains were established from eight sampling sites. The study revealed long-standing genetic diversity among strains established from the northern-most sites, along with large phenotypic variation in observed physiological traits such as growth rates, cell volume, production of palytoxin-like compounds, and photophysiological parameters. Strains from the southern populations were more uniform in both genetic and functional traits, and have possibly colonized their habitats more recently. Our study reports significant genetic and functional trait variability in a benthic harmful algal species, indicative of high adaptability, and a possible climate-driven range extension. The observed high trait variation may facilitate development of harmful algal blooms under dynamic coastal environmental conditions.

The impact of urea on toxic diatoms - Potential effects of fertilizer silo breakdown on a Pseudo-nitzschia bloom

In spring 2016, two silos containing liquid nitrogen-containing fertilizer collapsed on a harbor in Fredericia, Denmark. More than 2,750 tons of fertilizer spilled into inner Danish waters. A bloom of Pseudo-nitzschia occurred approximately one month after the incident. The bloom caused a 5-week quarantine of numerous mussel-harvesting areas along the eastern coast of Jutland. The levels of domoic acid measured up to 49 mg kg^-1 in mussel meat after the bloom. In the months following the event, the species diversity of phytoplankton was low, while the abundance was high comprising few dominant species including Pseudo-nitzschia. The main part of the liquid nitrogen-containing compound was urea, chemically produced for agricultural use. To investigate the potential impact of urea on Pseudo-nitzschia, four strains, including one strain of P. delicatissima, two of P. seriata and one of P. obtusa, were exposed each to three concentrations of urea in a batch culture experiment: 10 μM, 20 μM and 100 μM N urea, and for comparison one concentration of nitrate (10 μM). Nitrate, ammonium, and urea were metabolized at different rates. Pseudo-nitzschia obtusa produced domoic acid and grew best at low urea concentrations. Both P. seriata strains had a positive correlation between urea concentration and growth rate, and the highest growth rate in the nitrate treatment. One strain of P. seriata produced domoic acid peaking at low N loads (10 µM N urea and 10 µM N nitrate). In conclusion, the ability to adapt to the available nitrogen source and retain a high growth rate was exceedingly varying and not only species-specific but also strain specific.

Functional Genomics Differentiate Inherent and Environmentally Influenced Traits in Dinoflagellate and Diatom Communities

Dinoflagellates and diatoms are among the most prominent microeukaryotic plankton groups, and they have evolved different functional traits reflecting their roles within ecosystems. However, links between their metabolic processes and functional traits within different environmental contexts warrant further study. The functional biodiversity of dinoflagellates and diatoms was accessed with metatranscriptomics using Pfam protein domains as proxies for functional processes. Despite the overall geographic similarity of functional responses, abiotic (i.e., temperature and salinity; ~800 Pfam domains) and biotic (i.e., taxonomic group; ~1500 Pfam domains) factors influencing particular functional responses were identified. Salinity and temperature were identified as the main drivers of community composition. Higher temperatures were associated with an increase of Pfam domains involved in energy metabolism and a decrease of processes associated with translation and the sulfur cycle. Salinity changes were correlated with the biosynthesis of secondary metabolites (e.g., terpenoids and polyketides) and signal transduction processes, indicating an overall strong effect on the biota. The abundance of dinoflagellates was positively correlated with nitrogen metabolism, vesicular transport and signal transduction, highlighting their link to biotic interactions (more so than diatoms) and suggesting the central role of species interactions in the evolution of dinoflagellates. Diatoms were associated with metabolites (e.g., isoprenoids and carotenoids), as well as lysine degradation, which highlights their ecological role as important primary producers and indicates the physiological importance of these metabolic pathways for diatoms in their natural environment. These approaches and gathered information will support ecological questions concerning the marine ecosystem state and metabolic interactions in the marine environment.

Functional trait thermal acclimation differs across three species of mid-Atlantic harmful algae

Characterizing the thermal niche of harmful algae is crucial for understanding and projecting the effects of future climate change on harmful algal blooms. The effects of 6 different temperatures (18-32 °C) on the growth, photophysiology, and toxicity were examined in the dinoflagellate Karlodinium veneficum, and the raphidophytes, Heterosigma akashiwo and Chattonella subsalsa from the Delaware Inland Bays (DIB). K. veneficum and H. akashiwo had skewed unimodal growth patterns, with temperature optima (Topt) at 28.6 and 27.3 °C respectively and an upper thermal niche limit of 32 °C. In contrast, C. subsalsa growth increased linearly with temperature, suggesting Topt and upper thermal boundaries >32 °C. K. veneficum photosystem II (PSII) photochemical efficiency remained stable across all temperatures, while H. akashiwo PSII efficiency declined at higher temperature and C. subsalsa was susceptible to low temperature (~18 °C) photoinactivation. Cell toxicity thermal response was species-specific such that K. veneficum toxicity increased with temperature above Topt. Raphidophyte toxicity peaked at 25-28 °C and was in close agreement with Topt for growth in H. akashiwo but below C. subsalsa maximal growth. The mode of toxicity was markedly different between the dinoflagellate and the raphidophytes such that K. veneficum had greater hemolytic activity while the raphidophytes had pronounced fish gill cell toxicity. These results and patterns of natural abundance for these algae in the DIB suggest that continued ocean warming may contribute to C. subsalsa bloom formation while possibly promoting highly toxic blooms of K. veneficum.

Predicting fish kills and toxic blooms in an intensive mariculture site in the Philippines using a machine learning model

Harmful algal blooms (HABs) that produce toxins and those that lead to fish kills are global problems that appear to be increasing in frequency and expanding in area. One way to help mitigate their impacts on people's health and livelihoods is to develop early-warning systems. Models to predict and manage HABs typically make use of complex multi-model structures incorporating satellite imagery and frequent monitoring data with different levels of detail into hydrodynamic models. These relatively more sophisticated methods are not necessarily applicable in countries like the Philippines. Empirical statistical models can be simpler alternatives that have also been successful for HAB forecasting of toxic blooms. Here, we present the use of the random forest, a machine learning algorithm, to develop an early-warning system for the prediction of two different types of HABs: fish kill and toxic bloom occurrences in Bolinao-Anda, Philippines, using data that can be obtained from in situ sensors. This site features intensive and extensive mariculture activities, as well as a long history of HABs. Data on temperature, salinity, dissolved oxygen, pH and chlorophyll from 2015 to 2017 were analyzed together with shellfish ban and fish kill occurrences. The random forest algorithm performed well: the fish kill and toxic bloom models were 96.1% and 97.8% accurate in predicting fish kill and shellfish ban occurrences, respectively. For both models, the most important predictive variable was a decrease in dissolved oxygen. Fish kills were more likely during higher salinity and temperature levels, whereas the toxic blooms occurred more at relatively lower salinity and higher chlorophyll conditions. This demonstrates a step towards integrating information from data that can be obtained through real-time sensors into a an early-warning system for two different types of HABs. Further testing of these models through times and different areas are recommended.

Detection of the phycotoxin pectenotoxin-2 in waters around King George Island, Antarctica

In order to set a base line for the observation of planktonic community changes due to global change, protistan plankton sampling in combination with phycotoxin measurements and solid phase adsorption toxin tracking (SPATT) was performed in two bays of King George Island (KGI) in January 2013 and 2014. In addition, SPATT sampling was performed in Potter Cove during a one-year period from January 2014 until January 2015. Known toxigenic taxa were not firmly identified in plankton samples but there was microscopical evidence for background level presence of Dinophysis spp. in the area. This was consistent with environmental conditions during the sampling periods, especially strong mixing of the water column and low water temperatures that do not favor dinoflagellate proliferations. Due to the lack of significant abundance of thecate toxigenic dinoflagellate species in microplankton samples, no phycotoxins were found in net tow samples. In contrast, SPATT sampling revealed the presence of dissolved pectenotoxin-2 (PTX-2) and its hydrolyzed form PTX-2 seco acid in both bays and during the entire one-year sampling period. The presence of dissolved PTX in coastal waters of KGI is strong new evidence for the presence of PTX-producing species, i.e., dinoflagellates of the genus Dinophysis in the area. The presence of phycotoxins and their respective producers, even at the low background concentrations found in this study, may be the seed of possible proliferations of these species under changing environmental conditions. Furthermore, phycotoxins can be used as chemotaxonomic markers for a very specific group of plankton thus allowing to track the presence of this group over time.

The relationship between toxic phytoplankton species occurrence and environmental and meteorological factors along the Eastern Adriatic coast

In this study, the time series of toxic phytoplankton species collected between 2004 and 2018 from the Northern Adriatic, Šibenik Bay, and Mali Ston Bay was analyzed in relation to environmental (temperature, salinity, water column stability, and river flow) and meteorological parameters (precipitation and wind). Because of the mostly non-linear relation between biotic and abiotic parameters, self-organizing maps (SOM) were used to identify these relationships. SOM analysis distinguished species of the genus Dinophysis from Gonyaulax spinifera and Lingulodinium polyedrum species, which better tolerate wind-induced disturbance. Among the Dinophysis species, Dinophysis fortii, Dinophysis tripos, and Dinophysis acuta preferred higher precipitation rate and river flow in addition to optimal temperatures. The abundances of Alexandrium species, which occurred more frequently in estuarine areas, were associated with river flow and maximum stable water column. Regardless of the ecological preferences of individual harmful algae, freshwater inflow-caused stratification is present in all clusters of environmental conditions associated with increased abundances of harmful algae in the SOM analysis. It is highly likely that stratification represents an important factor for the development and maintenance of HABs. The non-linear relationship between the NAO index and rainfall was noted, of which the most important for the development of harmful algae is the proportional correlation between the positive phase of the NAO index and higher rainfall, especially in winter and spring. Such conditions are conducive to the development of harmful algae because, with the increase in temperature accompanying the positive phase of the NAO index, increased rainfall further stimulates their growth. This can be achieved either through nutrient yields or through higher freshwater inflow that further stabilizes the water column.

Uptake of Inorganic and Organic Nitrogen Sources by Dinophysis acuminata and D. acuta

Dinoflagellate species of Dinophysis are obligate mixotrophs that require light, nutrients, and prey for sustained growth. Information about their nitrogenous nutrient preferences and their uptake kinetics are scarce. This study aimed to determine the preferred nitrogen sources in cultures of D. acuminata and D. acuta strains from the Galician Rías Baixas (NW Spain) and to compare their uptake kinetics. Well-fed versus starved cultures of D. acuminata and D. acuta were supplied with N¹⁵ labeled inorganic (nitrate, ammonium) and organic (urea) nutrients. Both species showed a preference for ammonium and urea whereas uptake of nitrate was negligible. Uptake rates by well-fed cells of D. acuminata and D. acuta were 200% and 50% higher, respectively, than by starved cells. Uptake of urea by D. acuminata was significantly higher than that of ammonium in both nutritional conditions. In contrast, similar uptake rates of both compounds were observed in D. acuta. The apparent inability of Dinophysis to take up nitrate suggests the existence of incomplete nitrate-reducing and assimilatory pathways, in line with the paucity of nitrate transporter homologs in the D. acuminata reference transcriptome. Results derived from this study will contribute to understand Harmful Algal Blooms succession and differences in the spatio-temporal distribution of the two Dinophysis species when they co-occur in stratified scenarios.

Effects of small-scale turbulence on two species of Dinophysis

Dinoflagellate species of Dinophysis, in particular D. acuminata and D. acuta, produce lipophilic toxins that pose a threat to human health when concentrated in shellfish and jeopardize shellfish exploitations in western Europe. In northwestern Iberia, D. acuminata has a long growing season, from spring to early autumn, and populations develop as soon as shallow stratification forms when the upwelling season begins. In contrast, D. acuta blooms in late summer, when the depth of the pycnocline is maximal and upwelling pulses are moderate. In situ observations on the hydrodynamic regimes during the two windows of opportunity for Dinophysis species led us to hypothesize that D. acuta should be more sensitive to turbulence than D. acuminata. To test this hypothesis, we studied the response of D. acuminata and D. acuta to three realistic turbulence levels low (LT), ε ≈ 10^-6 m² s^-3; medium (MT), ε ≈ 10^-5  m² s^-3 and high (HT), ε ≈ 10^-4 m² s^-3generated by Turbogen, a highly reproducible, computer-controlled system. Cells of both species exposed to LT and MT grew at rates similar to the controls. Marked differences were found in the response to HT: D. acuminata grew slowly after an initial lag phase, whereas D. acuta cell numbers declined. Results from this study support the hypothesis that turbulence may play a role in shaping the spatio-temporal distribution of individual species of Dinophysis. We also hypothesize that, in addition to cell disturbance affecting division, sustained high shear generated by microturbulence may cause a decline in Dinophysis numbers due to decreased densities of ciliate prey.

Free-living dinoflagellates of the central Red Sea, Saudi Arabia: Variability, new records and potentially harmful species

The diversity of free-living dinoflagellates in the coastal areas of the central Red Sea, Saudi Arabia, was studied from April 2016 to March 2017. A total of 106 dinoflagellates belonging to 36 genera, 20 families and 7 orders were identified and characterized using light microscopy. Of these, 47 taxa were potentially harmful, and 60 taxa were recorded for the first time from the Red Sea. The unexpectedly high species diversity, including new records, was due to the benthic species. The monthly variability of planktonic species records exhibited negative correlations with temperature and salinity, although in most cases, the links between them were insignificant. Subsequently, the dinoflagellates checklist for the entire Red Sea was updated and showed that there were currently 395 taxa and 66 genera. The results of this study provide a solid foundation for future studies of dinoflagellate biodiversity in the Red Sea, particularly for benthic and harmful species.

Drivers of phytoplankton blooms in the northeastern Black Sea

In order to understand of the processes controlling phytoplankton successions in the NE Black Sea, long-term data series are needed. We compiled 15 years (2002-2017) of measurements from which the existence emerges of a tight link between phytoplankton species dominance and nutrients concentrations. The latter is strongly influenced by wind direction. The link between algal dominance and nutrients is mediated by the growth strategy adopted by algal species. In spring, when nutrients are abundant, small diatoms such as Pseudo-nitzschia pseudodelicatissima, with a "rapid growth strategy", prevail. In late spring and early summer, when N is low and P and Si are high, coccolithophorids such as Emiliania huxhleyi dominate, thanks to an "affinity growth strategy". Large diatoms, especially Pseudosolenia calcar-avis, dominate in summer and autumn, when their "storage growth strategy" allows the exploitation of discontinuous upwelling of nutrients. These seasonal changes of dominant species influence the structure of the food web.

Plankton community assessment in anthropogenic-impacted oligotrophic coastal regions

Microbial planktonic communities are critical components of marine biogeochemical pathways. Despite this, there is still limited knowledge on the dynamics of this group in warm and oligotrophic waters. We used high-throughput sequencing to characterise the bacterial (16S rRNA) and eukaryotic (18S rRNA) microbial plankton communities in two regions under the influence of anthropogenic impacts (a port and sewage outflow) and a coastal region with no direct anthropogenic disturbances in the central Red Sea. Overall, bacterial and eukaryotic components responded in a similar way to the environmental conditions. Community composition and structure were more sensitive than alpha diversity measures to environmental impacts. With the exception of eukaryotes, for which the number of OTU differed significantly between sampling periods in all the regions, environmental changes associated with anthropogenic pressures seem to be better reflected by variations in the relative dominance of microbial groups. For example, elevated proportional abundances of nitrifying and sewage-/faecal-related bacteria at the impacted sites were observed compared with the coastal region. The recently developed microgAMBI also appeared to correlate well with the level of anthropogenic impact the regions experienced, showing the potential to be applied in oligotrophic waters.

Winter picoplankton diversity in an oligotrophic marginal sea

Marine picoplankton, unicellular organisms with cell sizes up to 3 μm in diameter, numerically dominate marine ecosystems, encompassing Archaea, Bacteria, Eukarya (protists and fungi) as well as viruses. Autotrophic and heterotrophic picoplankton abundance and community composition with a focus on picoeukaryotes (PEs) were investigated in the winter of 2016 at three stations along a coast-to-offshore transect in the southern Adriatic Sea. Abundances were estimated by flow cytometry, while community composition by Illumina High-Throughput Sequencing (HTS) of 16S and 18S rRNA genes. The photosynthetic picoplankton diversity was also investigated by High-Performance Liquid Chromatography (HPLC) of liposoluble pigments. Heterotrophic bacteria and cyanobacteria (Prochlorococcus and Synechococcus) accounted for up to 7 × 10⁵; 2.3 × 10⁴ and 2.5 × 10⁴ cells mL^-1, respectively, while photosynthetic picoeukaryotes peaked with 3 × 10³ cells mL^-1. Prokaryotes, as revealed by HTS were dominated by Alphaproteobacteria (mainly SAR11, 44.91% of total 16S sequence reads), followed by Gammaproteobacteria (Oceanospirillales and Pseudomonadales, 14.96%), Bacteroidetes (mainly Flavobacteriales, 13%), Cyanobacteria (Prochlorococcus and Synechococcus, 9.52%), Marinimicrobia (SAR406, 7.97%), Deltaproteobacteria (SAR324, 3.83%), Actinobacteria (2.24%) and Chloroflexi (SAR202, 1.90%). Photosynthetic pigment concentrations were very low (12.12 μgL^-1 at the most) and taxonomic pigments could be attributed to Prochlorococcus, Synechococcus, Prymnesiophyceae, Bacillariophyceae, Chrysophyceae, and Prasinophyceae. HTS data revealed that PEs were dominated by heterotrophs, such as Syndiniophyceae, parasitic dinoflagellates (79.67% of total 18S sequence reads), Dinophyceae (8.7%) and the radiolarians Collodaria belonging to Sphaerozoidae (22.1%) and Spumellaria (5.0%). On the other hand, photoautotrophs, including Chlorophyta (Mamiellophyceae, Prasinophyceae, Trebouxiophyceae, and Ulvophyceae), Stramenopiles (Bacillariophyta, Chrysophyceae, Dictyochophyceae, Pelagophyceae), photoautotrophic Cryptophyta and some Haptophyta (Prymnesiophyceae), did not exceed 5% of total sequence reads. This study provides the first snapshot of the PEs diversity in oligotrophic euphotic waters of the southern Adriatic Sea, hence setting the stage for large-scale surveying and characterization of the eukaryotic diversity in the entire basin.

Environmental factors influencing the distribution and abundance of Alexandrium catenella in Kachemak bay and lower cook inlet, Alaska

Despite the long history of paralytic shellfish poisoning (PSP) events in Alaska, little is known about the seasonal distribution and abundance of the causative organism, Alexandrium, or the environmental factors that govern toxic bloom development. To address this issue, a five year study (2012-2017) was undertaken in Kachemak Bay and lower Cook Inlet Alaska to determine how the occurrence of Alexandrium catenella, the dominant PSP-causing Alexandrium species, was influenced by temperature, salinity, nutrient concentrations, and other environmental factors. Cell concentrations from 572 surface water samples were estimated using quantitative PCR. Monthly sampling revealed a seasonal pattern of A. catenella bloom development that was positively correlated with water temperature. Prevailing salinity conditions did not significantly affect abundance, nor was nutrient limitation a direct factor. Elevated cell concentrations were detected in 35 samples from Kachemak Bay (100-3050 cell eq. L^-1) while a maximum abundance of 67 cell eq. L^-1 was detected in samples from lower Cook Inlet sites. Monitoring data showed average water temperatures in Kachemak Bay increased by ∼2 °C over the course of the study and were accompanied by an increase in Alexandrium abundance. Based on these findings, 7-8 °C appears to represent a temperature threshold for significant bloom development in Kachemak Bay, with the greatest risk of shellfish toxicity occurring when temperatures exceed 10-12 °C. The role of temperature is further supported by time series data from the Alaska Coastal Current (station GAK1), which showed that summertime shellfish toxicity events in Kachemak Bay generally followed periods of anomalously high winter water temperatures. These data indicate monitoring changes in water temperatures may be used as an early warning signal for subsequent development of shellfish toxicity in Kachemak Bay.

Imbalanced nutrient regimes increase Prymnesium parvum resilience to herbicide exposure

The toxigenic haptophyte Prymnesium parvum is a mixotrophic phytoplankter with an extensive historic record of forming nearly monospecific, high-biomass, ecosystem-disrupting blooms, and it has been responsible for major fish kills in brackish waters and aquaculture facilities in many regions of the world. Little is known about how this species responds to commonly occurring environmental contaminants, or how nutrient (nitrogen, phosphorus) pollution may interact with environmentally relevant pesticide exposures to affect this harmful algal species. Here, standard algal toxicity bioassays from pesticide hazard assessments were used along with modified erythrocyte lysis assays to evaluate how atrazine exposures, imbalanced nutrient supplies, and salinity interact to influence the growth and toxicity in P. parvum isolates from three different regions. In nutrient-replete media, P. parvum 96 h IC₅₀s ranged from 73.0 to 88.3 μg atrazine L^-1 at salinity 10 and from 118 to >200 μg atrazine μg L^-1 at salinity 20, and the response depended on the strain and the test duration. Relative hemolytic activity, used as an indication of toxicity, was a function of herbicide exposure, nutrient availability, salinity, geographic origin, and interactions among these factors. Highest levels of hemolytic activity were measured from a South Carolina strain in low-nitrogen media with high atrazine concentrations. Herbicide concentration was related to relative hemolytic activity, although a consistent relationship between growth phase and toxicity was not observed. Overall, these findings suggest that increasing chemical contamination is helping to promote ecosystem-disruptive, strongly mixotrophic algal blooms.

Realized niche analysis of phytoplankton communities involving HAB: Phaeocystis spp. as a case study

The link between harmful algal blooms, phytoplankton community dynamics and global environmental change is not well understood. To tackle this challenging question, a new method was used to reveal how phytoplankton communities responded to environmental change with the occurrence of an harmful algae, using the coastal waters of the eastern English Channel as a case study. The great interannual variability in the magnitude and intensity of Phaeocystis spp. blooms, along with diatoms, compared to the ongoing gradual decrease in anthropogenic nutrient concentration and rebalancing of nutrient ratios; suggests that other factors, such as competition for resources, may also play an important role. A realized niche approach was used with the Outlying Mean Index analysis and the dynamics of the species' realized subniches were estimated using the Within Outlying Mean Indexes calculations under low (L) and high (H) contrasting Phaeocystis spp. abundance. The Within Outlying Mean Indexes allows the decomposition of the realized niche into realized subniches, found within the subset of habitat conditions and constrained by a subset of a biotic factor. The two contrasting scenarios were characterized by significantly different subsets of environmental conditions and diatom species (BV-step analysis), and different seasonality in salinity, turbidity, and nutrients. The subset L environmental conditions were potentially favorable for Phaeocystis spp. but it suffered from competitive exclusion by key diatom species such as Skeletonema spp., Thalassiosira gravida, Thalassionema nitzschioides and the Pseudo-nitzchia seriata complex. Accordingly, these diatoms species occupied 81% of Phaeocystis spp.'s existing fundamental subniche. In contrast, the greater number of diatoms, correlated with the community trend, within subset H exerted a weaker biological constraint and favored Phaeocystis spp. realized subniche expansion. In conclusion, the results strongly suggest that both abiotic and biotic interactions should be considered to understand Phaeocystis spp. blooms with greater consideration of the preceeding diatoms. HABs needs must therefore be studied as part of the total phytoplankton community.

Microbial planktonic communities in the Red Sea: high levels of spatial and temporal variability shaped by nutrient availability and turbulence

The semi-enclosed nature of the Red Sea (20.2°N-38.5°N) makes it a natural laboratory to study the influence of environmental gradients on microbial communities. This study investigates the composition and structure of microbial prokaryotes and eukaryotes using molecular methods, targeting ribosomal RNA genes across different regions and seasons. The interaction between spatial and temporal scales results in different scenarios of turbulence and nutrient conditions allowing for testing of ecological theory that categorizes the response of the plankton community to these variations. The prokaryotic reads are mainly comprised of Cyanobacteria and Proteobacteria (Alpha and Gamma), with eukaryotic reads dominated by Dinophyceae and Syndiniophyceae. Periodic increases in the proportion of Mamiellophyceae and Bacillariophyceae reads were associated with alterations in the physical oceanography leading to nutrient increases either through the influx of Gulf of Aden Intermediate Water (south in the fall) or through water column mixing processes (north in the spring). We observed that in general dissimilarity amongst microbial communities increased when nutrient concentrations were higher, whereas richness (observed OTUs) was higher in scenarios of higher turbulence. Maximum abundance models showed the differential responses of dominant taxa to temperature giving an indication how taxa will respond as waters become warmer and more oligotrophic.

Nutrients structure changes impact the competition and succession between diatom and dinoflagellate in the East China Sea

Nutrients variations caused by anthropogenic activities alter phytoplankton community interactions, especially competition and succession between two algal species. East China Sea experiences annual successions of Skeletonema costatum and Prorocentrum donghaiense and large-scale blooms of P. donghaiense. In this study, the growth and competition responses of S. costatum and P. donghaiense to different inorganic nutrients structure were evaluated through field and indoors experiments to further understand the nutrients mechanism of these events. Results showed that low Si/N ratio (Si/N<1) and high N/P (>50) possibly accelerated P. donghaiense outbreak and decreased Si/N caused by low Si concentration could speed up S. costatum decay. Excessive DIN also accelerated blooms dominated by P. donghaiense (D_t up to -3) when S. costatum perished. Increased DIN loads from anthropogenic activities were possibly responsible for the changes in phytoplankton communities and dinoflagellate outbreak when Si concentration decreased as a result of governmental control efforts. With effective management practices for Si and P reductions worldwide, managers should be aware of the negative implications of unsuccessful management of system N loading because N may significantly alter the composition and ecosystem of phytoplankton communities.