Dynamics and drivers of phytoplankton richness and composition along productivity gradient

Environmental and hydrological synergies shaping phytoplankton diversity in the Hetao irrigation district

Phytoplankton are crucial primary producers in freshwater ecosystems, driving matter and energy flow across trophic levels, essential for biodiversity and ecological balance. Most research emphasizes environmental factors shaping their diversity, while the role of hydrological connectivity remains poorly understood. This study collected 81 phytoplankton samples from the Hetao Irrigation District along a gradient from upstream to downstream and utilized high-throughput sequencing to evaluate the spatial distribution patterns of phytoplankton diversity. The study analyzed the impacts of environmental factors, hydrological connectivity (water surface ratio, Wp), and human activities (land-use intensity, LUI) on phytoplankton diversity. The results revealed that the phytoplankton community comprised 9 phyla, 158 families, 378 genera, and 1189 species. There were significant differences in phytoplankton diversity among different water bodies, with a gradual increase in phytoplankton diversity from west to east across the five major irrigation areas. Lake Ulansuhai had relatively low phytoplankton diversity. The ASV number, Chao1 index, and ACE index showed significant positive correlations with dissolved oxygen (DO), pH, and water temperature (WT). The Shannon index and Pielou'e evenness (Pielou_e) index showed significant positive correlations with the water surface ratio (Wp). The partial least squares model indicated that environmental factors directly influenced phytoplankton diversity. Hydrological connectivity indirectly affected phytoplankton diversity by altering environmental factors. We emphasize that hydrological connectivity is as important as environmental factors in driving phytoplankton diversity in the Hetao Irrigation District. This study provides key insights for water quality assessment and biodiversity conservation in the region.

Unveiling Phytoplankton Diversity: Taxonomy, Functional Groups, and Environmental Drivers in North China Lakes

Functional groups (FGs) represent a classification scheme designed to study the ecological adaptations of phytoplankton. However, FG dynamics studies in phytoplankton are often conducted independent of taxonomic studies, so the factors influencing community dynamics have not been sufficiently investigated or compared between the two classification systems. In this study, we compared the intricate relationship between taxonomic and FG compositions in North China lakes and delve into the key environmental drivers shaping phytoplankton community dynamics. This investigation revealed that taxonomic and FG classifications exhibit high qualitative and quantitative similarities in the community structure. Environmental drivers had a stronger influence on the FG structure than taxonomic composition, indicating that the FG classification does not result in the loss of ecological information regarding the community structure, even with the reduced number of grouping units. Indeed, it was evident that FGs contained a larger quantity of ecological information. These conclusions were further verified using lakes in eastern China. Additionally, we found that climatic-geographical factors usually exerted indirect influences, by altering water chemistry, while water chemical factors had more direct and stronger influences. The combined effects of both types of environmental factors had a greater impact on the phytoplankton FG structure than on taxonomic composition. In conclusion, we believe that an in-depth study of FGs will better focus on the ecological characteristics of phytoplankton, while also avoiding the need for extensive species identification.

Significant Effects of Associated Microorganisms on the Community of Photosynthetic Picoeukaryotes

Photosynthetic picoeukaryotes (PPEs) form associations with other microorganisms. However, whether and how the associated microbes affect PPE communities remain unknown. We used flow cytometric cell sorting combined with parallel high-throughput sequencing of the 18S and 16S rRNA genes to simultaneously investigate PPEs and their associated microbial communities in the Yangtze-connected Lake Dongting. The lake harbors a great diversity of PPEs. PPE communities exhibited significant temporal rather than spatial variations. Two distinct PPE taxa affiliated with Discostella nipponica and Poterioochromonas malhamensis were dominant during winter/spring and summer, respectively. Parallel high-throughput sequencing revealed a great diversity of associated bacteria and non-pigmented eukaryotes (NPEs) in PPEs sorts. Proteobacteria, Actinobacteria, Bacteroidetes, and Cyanobacteria among the associated bacteria and fungi among the associated NPEs were dominant. PPEs were more apparently associated with bacteria than with NPEs. The co-occurrence network of PPEs and associated microbes formed five major modules, which exhibited distinct temporal patterns, being specific to a certain period. Variations in PPEs communities were significantly correlated with both environmental factors and associated microbial communities. In variation partitioning analysis, the associated bacteria explained the greatest variations in PPE communities, and associated bacteria and NPEs co-explained a large portion of environmental effects on PPE communities. Our results highlight the significance of associated microbes in shaping PPE communities.

Water depth and land-use intensity indirectly determine phytoplankton functional diversity and further regulate resource use efficiency at a multi-lake scale

Biodiversity-ecosystem functioning relationships under multiple pressures have recently been the subject of broad studies. For the key primary producer in aquatic ecosystems, phytoplankton, several studies have focused on trait-based functional diversity (FD) and the related functioning (e.g., resource use efficiency, RUE), and their linkages. However, investigations of the effects of environmental factors at different levels (e.g., land use, lake morphometry, climate and nutrients) on FD and RUE are sparse. We developed a data-driven-model framework to simultaneously elucidate the effects of multiple drivers on FD (functional diversity based on dendrograms, FDc and functional richness, FRic) and RUE (of nitrogen and phosphorus) of phytoplankton based on data from 68 Yunnan-Guizhou Plateau lakes, Southwest China. We found that the concentration of total phosphorus, which is mainly affected by land-use intensity and influenced by water depth, was the primary (positive) driver of changes in both FDc and FRic, while RUE was mainly explained by phytoplankton FD (i.e., FRic). These results indicate that water depth and land-use intensity influence indirectly phytoplankton FD and further regulate RUE. Moreover, nonlinear correlations of RUE with FRic were found, which may be caused by interspecific competition and niche differentiation of the phytoplankton community related to nutrient levels. Our finding may help managers to set trade-off targets between FD and RUE in lake ecosystems except for extremely polluted ones, in which the thresholds derived from the Bayesian network, of total phosphorus, total nitrogen and land-use intensity were approximately 0.04 mg/L, 0.50 mg/L and 244 (unitless), respectively. The probability of meeting the RUE objectives was lower in shallow lakes than in deep lakes, but for FRic the opposite was observed.

The synergistic effect of rising temperature and declining light boosts the dominance of bloom-forming cyanobacteria in spring

Global warming and eutrophication result in rising temperature and declining underwater light, respectively, which affect the shift of the phytoplankton community in spring. However, knowledge of how temperature and light synergistically impact phytoplankton community shifts and cyanobacterial dominance is limited. In this study, we performed a long-term data analysis and an outdoor mesocosm experiment to detect the synergistic effect of temperature and light on shift of phytoplankton community and dominance of bloom-forming cyanobacteria in Lake Taihu, China. The results showed that cyanobacterial biomass was boosted alone and jointly by increased temperature and decreased light levels (sunshine hours and light intensity), and the interaction might be more important than temperature or light levels independently. Chlorophyta biomass was driven by the joint effect of temperature and light levels. Bacillariophyta biomass was mainly affected by light levels, and decreased with declining light levels. Our results emphasize that the interactions of temperature and light have an important impact on the shift of the phytoplankton community in spring. Increasing temperature and declining underwater light boosted the flourishing of cyanobacteria, especially Microcystis, and were adverse to the development of diatoms in spring. Our findings contribute to an increased understanding of the effects of temperature and light on phytoplankton composition shifts and the development of cyanobacterial dominance in spring.

Occurrence and distribution of antibiotic resistant bacteria and genes in the Fuhe urban river and its driving mechanism

Antibiotic resistance genes (ARGs) in urban rivers can affect human health via the food chain and human pathogenic bacteria diffusion. Sediment can be a sink for ARGs, causing second sources of ARG contamination through diffusion. Therefore, we evaluated the effects of total petroleum hydrocarbons (TPHs) and phytoplankton on the distribution of the ARGs in the sediment and water of Fuhe river in Baoding city, China. The ARGs and human pathogenic bacteria in urban river were analyzed, and the phytoplankton and bacterial abundance, TPH, and physicochemical parameters ranked using the partial least squares path modelling (PLS-PM) and aggregated boosted tree (ABT) analysis. The main ARGs in Fuhe river sediment were sulfonamide and tetracycline resistance genes, with sul2 exhibiting the highest level. The main human pathogenic bacteria in the pathogens pool were Clostridium, Bacillus and Burkholderiaceae, with Clostridium demonstrating a positive correlation with SulAfolP01. The PLS-PM analysis confirmed that, among the multiple drivers, water physicochemical factors, TPH, phytoplankton, and heavy metals positively and directly affected the ARG profiles in sediment while sediment heavy metals and bacterial communities did the similar effect. These factors (nutrient factors, heavy metals, and TPH) in water and sediment posed the opposite total effect on ARGs in the sediment, suggesting medium factors should have a conclusive effect on the distribution of ARGs in the sediment. The ABT analysis showed that dissolved oxygen (DO), total nitrogen (TN) and Chlorophyta were the most important factors affecting the ARGs distribution in the water, while TN affected the distribution of the genes in the sediment.

A pilot experiment to assess the efficiency of pharmaceutical plant wastewater treatment and the decreasing effluent toxicity to periphytic biofilms

Pharmaceutical industry effluents are complex and highly variable in time. Assessing the efficiency of a pharmaceutical industry wastewater treatment plant (WWTP) and the resulting decrease in effluent toxicity and ecological risk is thus not straightforward. We set up an original in situ pilot directly connected to a pharmaceutical WWTP to monitor the chronic toxicity of successive effluents using natural periphytic biofilms. Their structural and functional responses to effluent exposure were assessed by combining (i) a molecular approach to characterize the bacterial and diatom diversity and (ii) functional measurements of photosynthetic and enzyme activities. Effluent contamination by pharmaceuticals strongly decreased after the quaternary treatment (activated carbon). Most of the structural biological characteristics improved with cumulative WWTP treatment (bacterial diversity, microbial genetic structure, and biological diatom index), showing community recovery along the treatment process. However, functional parameters did not show clear links with treatment steps, suggesting that microbial activities were not solely driven by pharmaceuticals produced during the experimental period. Operationally, this type of pilot system offers a useful tool for biomonitoring approaches and offers new approaches for industrial managers to assess the ecological risk of production effluents in receiving water.

Long-term dynamics and drivers of phytoplankton biomass in eutrophic Lake Taihu

Understanding the relative effect sizes of climate-related environmental variables and nutrients on the high annual variation in the phytoplankton biomass in eutrophic lakes is important for lake management efforts. In this study, we used a data set of phytoplankton dynamics in eutrophic Lake Taihu that cover more than two decades (1993-2015) to show the variation in and the drivers of phytoplankton biomass under complex, fluctuating environmental conditions. Our results showed that the phytoplankton biomass increased slowly over the studied period despite the recent decrease in nutrient levels. The distribution of the phytoplankton biomass expanded spatially towards the central lake region, and seasonally towards the autumn and winter. Nutrients were still the primary predictors of the long-term phytoplankton biomass trend. The effect size of climate-related variables was also high and close to the effect size of nutrients. Among the climate-related variables, wind speed and underwater available light were more important predictors than temperature. The biomass of the phytoplankton taxonomic groups showed different responses to the environmental variables based on their niches. However, the compensatory dynamics affecting biomass were weak at phylum level, and synchronous dynamics drove the variation in total biomass. Our findings highlight the effect of climate-related variables on the phytoplankton biomass in Lake Taihu, which has experienced high nutrient loadings and concentrations for more than two decades. Therefore, changes in climate-related variables, such as wind speed and underwater available light, should be considered when evaluating the amount that nutrients should be reduced in Lake Taihu for future lake management.