Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession: First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

Characterization of patterns and variability in the dynamics of outdoor aquatic mesocosms: exploring the capabilities and challenges in data supporting aquatic system models

Aquatic mesocosms are complex test systems used within regulatory risk assessment of plant protection products. These model ecosystems allow researchers to capture interactions of multiple species under realistic environmental conditions. They enable assessment of direct and indirect effects of stressors at all trophic levels (i.e., from primary producers to secondary consumers) and impacts on ecosystem functions. Due to the limited ability to test the multitude of potential exposure scenarios, cross-linking aquatic mesocosm studies with virtual mesocosms, i.e., aquatic system models (ASMs), can serve to meet the demand for more environmental realism and ecological relevance in risk assessment. In this study, full control data sets from seven aquatic mesocosm studies conducted at a single test facility under GLP were analysed graphically and using descriptive statistics. Thereby, not only a comprehensive data base but also an insight into the species present, their dynamics over time, and variability in unchallenged mesocosms was observed. While consistency in dynamics could be discerned for physical and chemical parameters, variability was evident for several biological endpoints. This variability points to amplification of small differences over time as well as to stochastic processes. The outline of existing gaps and uncertainties in data leads to the estimation of what can be expected to be captured and predicted by ASMs.

Enhanced silica export in a future ocean triggers global diatom decline

Diatoms account for up to 40% of marine primary production^1,2 and require silicic acid to grow and build their opal shell³. On the physiological and ecological level, diatoms are thought to be resistant to, or even benefit from, ocean acidification^4–6. Yet, global-scale responses and implications for biogeochemical cycles in the future ocean remain largely unknown. Here we conducted five in situ mesocosm experiments with natural plankton communities in different biomes and find that ocean acidification increases the elemental ratio of silicon (Si) to nitrogen (N) of sinking biogenic matter by 17 ± 6 per cent under \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{p}}_{{{\rm{CO}}}_{2}}$$\end{document}pCO2 conditions projected for the year 2100. This shift in Si:N seems to be caused by slower chemical dissolution of silica at decreasing seawater pH. We test this finding with global sediment trap data, which confirm a widespread influence of pH on Si:N in the oceanic water column. Earth system model simulations show that a future pH-driven decrease in silica dissolution of sinking material reduces the availability of silicic acid in the surface ocean, triggering a global decline of diatoms by 13–26 per cent due to ocean acidification by the year 2200. This outcome contrasts sharply with the conclusions of previous experimental studies, thereby illustrating how our current understanding of biological impacts of ocean change can be considerably altered at the global scale through unexpected feedback mechanisms in the Earth system.

Mesocosm experiments in different biomes show that future ocean acidification will slow down the dissolution of biogenic silica, decreasing silicic acid availability in the surface ocean and triggering a global decline of diatoms as revealed by Earth system model simulations.

Ocean acidification induces distinct metabolic responses in subtropical zooplankton under oligotrophic conditions and after simulated upwelling

Ocean acidification (OA) is one of the most critical anthropogenic threats to marine ecosystems. While significant ecological responses of plankton communities to OA have been revealed mainly by small-scale laboratory approaches, the interactive effect of OA-related changes on zooplankton metabolism and their biogeochemical implications in the natural environment still remains less well understood. Here, we explore the responses of zooplankton respiration and ammonium excretion, two key processes in the nutrient cycling, to high pCO₂ levels in a 9-week in situ mesocosm experiment conducted during the autumn oligotrophic season in the subtropical northeast Atlantic. By simulating an upwelling event halfway through the study, we further evaluated the combined effects of OA and nutrient availability on the physiology of micro-and mesozooplankton. OA conditions generally resulted in a reduction in the biomass-specific metabolic and enzymatic rates, particularly in the mesozooplankton community. The situation reversed after the nutrient-rich deep-water addition, which initially promoted a diatom bloom and increased heterotrophic activities in all mesocosms. Under high pCO₂ conditions (>800 μatm), however, the nutrient fertilization triggered the proliferation of the harmful alga Vicicitus globosus, with important consequences for the metabolic performance of the two zooplankton size classes. Here, the zooplankton contribution to the remineralization of organic matter and nitrogen regeneration dropped by 30% and 24%, respectively, during the oligotrophic period, and by 40% and 70% during simulated upwelling. Overall, our results indicate a potential reduction in the biogeochemical role of zooplankton under future ocean conditions, with more evident effects on the large mesozooplankton and during high productivity events.

Nutrient Alteration Drives the Impacts of Seawater Acidification on the Bloom-Forming Dinoflagellate Karenia mikimotoi

Seawater acidification and nutrient alteration are two dominant environmental factors in coastal environments that influence the dynamics and succession of marine microalgae. However, the impacts of their combination have seldom been recorded. A simulated experimental system was set up to mimic the effects of elevated acidification on a bloom-forming dinoflagellate, Karenia mikimotoi, exposed to different nutrient conditions, and the possible mechanism was discussed. The results showed that acidification at different pH levels of 7.6 or 7.4 significantly influenced microalgal growth (p<0.05) compared with the control at pH 8.0. Mitochondria, the key sites of aerobic respiration and energy production, were impaired in a pH-dependent manner, and a simultaneous alteration of reactive oxygen species (ROS) production occurred. Cytochrome c oxidase (COX) and citrate synthase (CS), two mitochondrial metabolism-related enzymes, were actively induced with acidification exposure, suggesting the involvement of the mitochondrial pathway in coping with acidification. Moreover, different nutrient statuses indicated by various N:P ratios of 7:1 (N limitation) and 52:1 (P limitation) dramatically altered the impacts of acidification compared with those exposed to an N:P ratio of 17:1 (control), microalgal growth at pH 7.4 was obviously accelerated with the elevation of the nutrient ratio compared to that at pH 8.1 (p<0.05), and nutrient limitations seemed beneficial for growth in acidifying conditions. The production of alkaline phosphatase (AP) and acid phosphatase (AcP), an effective index indicating the microalgal growth status, significantly increased at the same time (p<0.05), which further supported this speculation. However, nitrate reductase (NR) was slightly inhibited. Hemolytic toxin production showed an obvious increase as the N:P ratio increased when exposed to acidification. Taken together, mitochondrial metabolism was suspected to be involved in the process of coping with acidification, and nutrient alterations, especially P limitation, could effectively alleviate the negative impacts induced by acidification. The obtained results might be a possible explanation for the competitive fitness of K. mikimotoi during bloom development.

Short term response of plankton community to nutrient enrichment in central eastern Arabian Sea: Elucidation through mesocosm experiments

Oligotrophic waters (OW), generally favour longer food chain facilitated by the microbial loop. In such ecosystems, physical mixing (e.g. upwelling, and winter convection) inject nutrients and propagules from subsurface to the photic zone. Such events are expected to alter the food chain through shifts in the plankton community. Mesocosm experiments were carried out to evaluate the influence of nutrient enrichment from the deep (100-150 m) on the surface plankton community for the first time in the Arabian Sea, through custom-designed enclosures in OW of the central-eastern Arabian Sea (CEAS). Surface water was characterized by low nutrients and phytoplankton biomass (chlorophyll-a of <0.2 μg m^-3) and upon nutrient enrichment yielded differing response. Higher abundance of picophytoplankton, bacteria and protists was noticed at a depth of ~100 m than at surface. The inoculation of such a population to the surface, resulted in a significant enhancement of autotrophic (picophytoplankton) and heterotrophic (bacteria and protists) populations. However, significant changes in the abundance of larger plankton was not evident till three days of incubation. Even though autotrophic picophytoplankton responded positively, a distinct increase in chlorophyll-a was not evident. This study points out that the lack of sufficient viable microphytoplankton propagules, neither at the surface nor at the depth (inoculum) are the possible reasons for the lack of their distinct positive response. These experiments suggest the dominance of microbial community response to physical mixing in the OW regions of the Arabian Sea and the importance of propagule diversity. The insights from this experiment will serve as a precursor for appropriate modifications in ocean modelling and forecasting studies and help in building global environmental management tools.

Exploring biogeochemical and ecological redundancy in phytoplankton communities in the global ocean

Climate-change-induced alterations of oceanic conditions will lead to the ecological niches of some marine phytoplankton species disappearing, at least regionally. How will such losses affect the ecosystem and the coupled biogeochemical cycles? Here, we couch this question in terms of ecological redundancy (will other species be able to fill the ecological roles of the extinct species) and biogeochemical redundancy (can other species replace their biogeochemical roles). Prior laboratory and field studies point to a spectrum in the degree of redundancy. We use a global three-dimensional computer model with diverse planktonic communities to explore these questions further. The model includes 35 phytoplankton types that differ in size, biogeochemical function and trophic strategy. We run two series of experiments in which single phytoplankton types are either partially or fully eliminated. The niches of the targeted types were not completely reoccupied, often with a reduction in the transfer of matter from autotrophs to heterotrophs. Primary production was often decreased, but sometimes increased due to reduction in grazing pressure. Complex trophic interactions (such as a decrease in the stocks of a predator's grazer) led to unexpected reshuffling of the community structure. Alterations in resource utilization may cause impacts beyond the regions where the type went extinct. Our results suggest a lack of redundancy, especially in the 'knock on' effects on higher trophic levels. Redundancy appeared lowest for types on the edges of trait space (e.g. smallest) or with unique competitive strategies. Though highly idealized, our modelling findings suggest that the results from laboratory or field studies often do not adequately capture the ramifications of functional redundancy. The modelled, often counterintuitive, responses-via complex food web interactions and bottom-up versus top-down controls-indicate that changes in planktonic community will be key determinants of future ocean global change ecology and biogeochemistry.

A review of mesocosm experiments on heavy metals in marine environment and related issues of emerging concerns

Mesocosms are real-world environmental science tools for bridging the gap between laboratory-scale experiments and actual habitat studies on ecosystem complexities. These experiments are increasingly being applied in understanding the complex impacts of heavy metals, ocean acidification, global warming, and oil spills. The insights of the present review indicate how metals and metal-bound activities impact on various aspects of ecological complexities like prey predator cues, growth, embryonic development, and reproduction. Plankton and benthos are used more often over fish and microbes owing to their smaller size, faster reproduction, amenability, and repeatability during mesocosm experiments. The results of ocean acidification reveal calcification of plankton, corals, alteration of pelagic structures, and plankton blooms. The subtle effect of oil spills is amplified on sediment microorganisms, primary producers, and crustaceans. An overview of the mesocosm designs over the years indicates that gradual changes have evolved in the type, size, design, composition, parameters, methodology employed, and the outputs obtained. Most of the pelagic and benthic mesocosm designs involve consideration of interactions within the water columns, between water and sediments, trophic levels, and nutrient rivalry. Mesocosm structures are built considering physical processes (tidal currents, turbulence, inner cycling of nutrients, thermal stratification, and mixing), biological complexities (population, community, and ecosystem) using appropriate filling containers, and sampling facilities that employ inert materials. The principle of design is easy transportation, mooring, deployment, and free floating structures besides addressing the unique ecosystem-based science problems. The evolution of the mesocosm tools helps in understanding further advancement of techniques and their applications in marine ecosystems.

Coccolithophore community response to ocean acidification and warming in the Eastern Mediterranean Sea: results from a mesocosm experiment

Mesocosm experiments have been fundamental to investigate the effects of elevated CO₂ and ocean acidification (OA) on planktic communities. However, few of these experiments have been conducted using naturally nutrient-limited waters and/or considering the combined effects of OA and ocean warming (OW). Coccolithophores are a group of calcifying phytoplankton that can reach high abundances in the Mediterranean Sea, and whose responses to OA are modulated by temperature and nutrients. We present the results of the first land-based mesocosm experiment testing the effects of combined OA and OW on an oligotrophic Eastern Mediterranean coccolithophore community. Coccolithophore cell abundance drastically decreased under OW and combined OA and OW (greenhouse, GH) conditions. Emiliania huxleyi calcite mass decreased consistently only in the GH treatment; moreover, anomalous calcifications (i.e. coccolith malformations) were particularly common in the perturbed treatments, especially under OA. Overall, these data suggest that the projected increase in sea surface temperatures, including marine heatwaves, will cause rapid changes in Eastern Mediterranean coccolithophore communities, and that these effects will be exacerbated by OA.

The impacts of ocean acidification on marine trace gases and the implications for atmospheric chemistry and climate

Surface ocean biogeochemistry and photochemistry regulate ocean–atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO₂) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N₂O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes.

Ocean acidification increases domoic acid contents during a spring to summer succession of coastal phytoplankton

Enrichment of the oceans with CO₂ may be beneficial for some marine phytoplankton, including harmful algae. Numerous laboratory experiments provided valuable insights into the effects of elevated pCO₂ on the growth and physiology of harmful algal species, including the production of phycotoxins. Experiments close to natural conditions are the next step to improve predictions, as they consider the complex interplay between biotic and abiotic factors that can confound the direct effects of ocean acidification. We therefore investigated the effect of ocean acidification on the occurrence and abundance of phycotoxins in bulk plankton samples during a long-term mesocosm experiment in the Gullmar Fjord, Sweden, an area frequently experiencing harmful algal blooms. During the experimental period, a total of seven phycotoxin-producing harmful algal genera were identified in the fjord, and in accordance, six toxin classes were detected. However, within the mesocosms, only domoic acid and the corresponding producer Pseudo-nitzschia spp. was observed. Despite high variation within treatments, significantly higher particulate domoic acid contents were measured in the mesocosms with elevated pCO₂. Higher particulate domoic acid contents were additionally associated with macronutrient limitation. The risks associated with potentially higher phycotoxin levels in the future ocean warrants attention and should be considered in prospective monitoring strategies for coastal marine waters.

Ecological and functional consequences of coastal ocean acidification: Perspectives from the Baltic-Skagerrak System

Ocean temperatures are rising; species are shifting poleward, and pH is falling (ocean acidification, OA). We summarise current understanding of OA in the brackish Baltic-Skagerrak System, focussing on the direct, indirect and interactive effects of OA with other anthropogenic drivers on marine biogeochemistry, organisms and ecosystems. Substantial recent advances reveal a pattern of stronger responses (positive or negative) of species than ecosystems, more positive responses at lower trophic levels and strong indirect interactions in food-webs. Common emergent themes were as follows: OA drives planktonic systems toward the microbial loop, reducing energy transfer to zooplankton and fish; and nutrient/food availability ameliorates negative impacts of OA. We identify several key areas for further research, notably the need for OA-relevant biogeochemical and ecosystem models, and understanding the ecological and evolutionary capacity of Baltic-Skagerrak ecosystems to respond to OA and other anthropogenic drivers.

Enhanced transfer of organic matter to higher trophic levels caused by ocean acidification and its implications for export production: A mass balance approach

Ongoing acidification of the ocean through uptake of anthropogenic CO₂ is known to affect marine biota and ecosystems with largely unknown consequences for marine food webs. Changes in food web structure have the potential to alter trophic transfer, partitioning, and biogeochemical cycling of elements in the ocean. Here we investigated the impact of realistic end-of-the-century CO₂ concentrations on the development and partitioning of the carbon, nitrogen, phosphorus, and silica pools in a coastal pelagic ecosystem (Gullmar Fjord, Sweden). We covered the entire winter-to-summer plankton succession (100 days) in two sets of five pelagic mesocosms, with one set being CO₂ enriched (~760 μatm pCO₂) and the other one left at ambient CO₂ concentrations. Elemental mass balances were calculated and we highlight important challenges and uncertainties we have faced in the closed mesocosm system. Our key observations under high CO₂ were: (1) A significantly amplified transfer of carbon, nitrogen, and phosphorus from primary producers to higher trophic levels, during times of regenerated primary production. (2) A prolonged retention of all three elements in the pelagic food web that significantly reduced nitrogen and phosphorus sedimentation by about 11 and 9%, respectively. (3) A positive trend in carbon fixation (relative to nitrogen) that appeared in the particulate matter pool as well as the downward particle flux. This excess carbon counteracted a potential reduction in carbon sedimentation that could have been expected from patterns of nitrogen and phosphorus fluxes. Our findings highlight the potential for ocean acidification to alter partitioning and cycling of carbon and nutrients in the surface ocean but also show that impacts are temporarily variable and likely depending upon the structure of the plankton food web.

Food web changes under ocean acidification promote herring larvae survival

Ocean acidification-the decrease in seawater pH due to rising CO₂ concentrations-has been shown to lower survival in early life stages of fish and, as a consequence, the recruitment of populations including commercially important species. To date, ocean-acidification studies with fish larvae have focused on the direct physiological impacts of elevated CO₂, but largely ignored the potential effects of ocean acidification on food web interactions. In an in situ mesocosm study on Atlantic herring (Clupea harengus) larvae as top predators in a pelagic food web, we account for indirect CO₂ effects on larval survival mediated by changes in food availability. The community was exposed to projected end-of-the-century CO₂ conditions (~760 µatm pCO₂) over a period of 113 days. In contrast with laboratory studies that reported a decrease in fish survival, the survival of the herring larvae in situ was significantly enhanced by 19 ± 2%. Analysis of the plankton community dynamics suggested that the herring larvae benefitted from a CO₂-stimulated increase in primary production. Such indirect effects may counteract the possible direct negative effects of ocean acidification on the survival of fish early life stages. These findings emphasize the need to assess the food web effects of ocean acidification on fish larvae before we can predict even the sign of change in fish recruitment in a high-CO₂ ocean.

Rapid evolution of highly variable competitive abilities in a key phytoplankton species

Climate change challenges plankton communities, but evolutionary adaptation could mitigate the potential impacts. Here, we tested with the phytoplankton species Emiliania huxleyi whether adaptation to a stressor under laboratory conditions leads to equivalent fitness gains in a more natural environment. We found that fitness advantages that had evolved under laboratory conditions were masked by pleiotropic effects in natural plankton communities. Moreover, new genotypes with highly variable competitive abilities evolved on timescales significantly shorter than climate change.

Simulated ocean acidification reveals winners and losers in coastal phytoplankton

The oceans absorb ~25% of the annual anthropogenic CO2 emissions. This causes a shift in the marine carbonate chemistry termed ocean acidification (OA). OA is expected to influence metabolic processes in phytoplankton species but it is unclear how the combination of individual physiological changes alters the structure of entire phytoplankton communities. To investigate this, we deployed ten pelagic mesocosms (volume ~50 m3) for 113 days at the west coast of Sweden and simulated OA (pCO2 = 760 μatm) in five of them while the other five served as controls (380 μatm). We found: (1) Bulk chlorophyll a concentration and 10 out of 16 investigated phytoplankton groups were significantly and mostly positively affected by elevated CO2 concentrations. However, CO2 effects on abundance or biomass were generally subtle and present only during certain succession stages. (2) Some of the CO2-affected phytoplankton groups seemed to respond directly to altered carbonate chemistry (e.g. diatoms) while others (e.g. Synechococcus) were more likely to be indirectly affected through CO2 sensitive competitors or grazers. (3) Picoeukaryotic phytoplankton (0.2-2 μm) showed the clearest and relatively strong positive CO2 responses during several succession stages. We attribute this not only to a CO2 fertilization of their photosynthetic apparatus but also to an increased nutrient competitiveness under acidified (i.e. low pH) conditions. The stimulating influence of high CO2/low pH on picoeukaryote abundance observed in this experiment is strikingly consistent with results from previous studies, suggesting that picoeukaryotes are among the winners in a future ocean.

Community barcoding reveals little effect of ocean acidification on the composition of coastal plankton communities: Evidence from a long-term mesocosm study in the Gullmar Fjord, Skagerrak

The acidification of the oceans could potentially alter marine plankton communities with consequences for ecosystem functioning. While several studies have investigated effects of ocean acidification on communities using traditional methods, few have used genetic analyses. Here, we use community barcoding to assess the impact of ocean acidification on the composition of a coastal plankton community in a large scale, in situ, long-term mesocosm experiment. High-throughput sequencing resulted in the identification of a wide range of planktonic taxa (Alveolata, Cryptophyta, Haptophyceae, Fungi, Metazoa, Hydrozoa, Rhizaria, Straminipila, Chlorophyta). Analyses based on predicted operational taxonomical units as well as taxonomical compositions revealed no differences between communities in high CO2 mesocosms (~ 760 μatm) and those exposed to present-day CO2 conditions. Observed shifts in the planktonic community composition were mainly related to seasonal changes in temperature and nutrients. Furthermore, based on our investigations, the elevated CO2 did not affect the intraspecific diversity of the most common mesozooplankter, the calanoid copepod Pseudocalanus acuspes. Nevertheless, accompanying studies found temporary effects attributed to a raise in CO2. Differences in taxa composition between the CO2 treatments could, however, only be observed in a specific period of the experiment. Based on our genetic investigations, no compositional long-term shifts of the plankton communities exposed to elevated CO2 conditions were observed. Thus, we conclude that the compositions of planktonic communities, especially those in coastal areas, remain rather unaffected by increased CO2.

Abundance of the iron containing biomolecule, heme b, during the progression of a spring phytoplankton bloom in a mesocosm experiment

Concentrations of heme b were determined in a mesocosm experiment situated in Gullmar Fjord off Sweden. The mesocosm experiment lasted for ca. one hundred days and was characterised by the growth of a primary nutrient replete and a secondary nutrient deplete phytoplankton bloom. Heme b varied between 40 ± 10 pmol L^-1 in the prebloom period up to a maximum of 700 ± 400 pmol L^-1 just prior to the time of the primary chlorophyll a maximum. Thereafter, heme b concentrations decreased again to an average of 120 ± 60 pmol L^-1. When normalised to total particulate carbon, heme b was most abundant during the initiation of the nutrient replete spring bloom, when ratios reached 52 ± 24 μmol mol^-1; ten times higher than values observed both pre and post the primary bloom. Concentrations of heme b correlated with those of chlorophyll a. Nevertheless, differences were observed in the relative concentrations of the two parameters, with heme b concentrations increasing relative to chlorophyll a during the growth of the primary bloom, decreasing over the period of the secondary bloom and increasing again through the latter period of the experiment. Heme b abundance was therefore influenced by nutrient concentrations and also likely by changing community composition. In half of the mesocosms, pCO₂ was elevated and maintained at ca.1000 μatm, however we observed no significant differences between heme b in plus or ambient pCO₂ mesocosms, either in absolute terms, or relative to total particulate carbon and chlorophyll a. The results obtained in this study contribute to our understanding of the distribution of this significant component of the biogenic iron pool, and provide an iron replete coastal water end member that aids the interpretation of the distributions of heme b in more iron deplete open ocean waters.

Ocean acidification effects on mesozooplankton community development: Results from a long-term mesocosm experiment

Ocean acidification may affect zooplankton directly by decreasing in pH, as well as indirectly via trophic pathways, where changes in carbon availability or pH effects on primary producers may cascade up the food web thereby altering ecosystem functioning and community composition. Here, we present results from a mesocosm experiment carried out during 113 days in the Gullmar Fjord, Skagerrak coast of Sweden, studying plankton responses to predicted end-of-century pCO2 levels. We did not observe any pCO2 effect on the diversity of the mesozooplankton community, but a positive pCO2 effect on the total mesozooplankton abundance. Furthermore, we observed species-specific sensitivities to pCO2 in the two major groups in this experiment, copepods and hydromedusae. Also stage-specific pCO2 sensitivities were detected in copepods, with copepodites being the most responsive stage. Focusing on the most abundant species, Pseudocalanus acuspes, we observed that copepodites were significantly more abundant in the high-pCO2 treatment during most of the experiment, probably fuelled by phytoplankton community responses to high-pCO2 conditions. Physiological and reproductive output was analysed on P. acuspes females through two additional laboratory experiments, showing no pCO2 effect on females' condition nor on egg hatching. Overall, our results suggest that the Gullmar Fjord mesozooplankton community structure is not expected to change much under realistic end-of-century OA scenarios as used here. However, the positive pCO2 effect detected on mesozooplankton abundance could potentially affect biomass transfer to higher trophic levels in the future.

Effects of ocean acidification on primary production in a coastal North Sea phytoplankton community

We studied the effect of ocean acidification (OA) on a coastal North Sea plankton community in a long-term mesocosm CO₂-enrichment experiment (BIOACID II long-term mesocosm study). From March to July 2013, 10 mesocosms of 19 m length with a volume of 47.5 to 55.9 m³ were deployed in the Gullmar Fjord, Sweden. CO₂ concentrations were enriched in five mesocosms to reach average CO₂ partial pressures (pCO₂) of 760 μatm. The remaining five mesocosms were used as control at ambient pCO₂ of 380 μatm. Our paper is part of a PLOS collection on this long-term mesocosm experiment. Here, we here tested the effect of OA on total primary production (PP_T) by performing ¹⁴C-based bottle incubations for 24 h. Furthermore, photoacclimation was assessed by conducting ¹⁴C-based photosynthesis-irradiance response (P/I) curves. Changes in chlorophyll a concentrations over time were reflected in the development of PP_T, and showed higher phytoplankton biomass build-up under OA. We observed two subsequent phytoplankton blooms in all mesocosms, with peaks in PP_T around day 33 and day 56. OA had no significant effect on PP_T, except for a marginal increase during the second phytoplankton bloom when inorganic nutrients were already depleted. Maximum light use efficiencies and light saturation indices calculated from the P/I curves changed simultaneously in all mesocosms, and suggest that OA did not alter phytoplankton photoacclimation. Despite large variability in time-integrated productivity estimates among replicates, our overall results indicate that coastal phytoplankton communities can be affected by OA at certain times of the seasonal succession with potential consequences for ecosystem functioning.

Influence of ocean acidification on plankton community structure during a winter-to-summer succession: An imaging approach indicates that copepods can benefit from elevated CO2 via indirect food web effects

Plankton communities play a key role in the marine food web and are expected to be highly sensitive to ongoing environmental change. Oceanic uptake of anthropogenic carbon dioxide (CO2) causes pronounced shifts in marine carbonate chemistry and a decrease in seawater pH. These changes-summarized by the term ocean acidification (OA)-can significantly affect the physiology of planktonic organisms. However, studies on the response of entire plankton communities to OA, which also include indirect effects via food-web interactions, are still relatively rare. Thus, it is presently unclear how OA could affect the functioning of entire ecosystems and biogeochemical element cycles. In this study, we report from a long-term in situ mesocosm experiment, where we investigated the response of natural plankton communities in temperate waters (Gullmarfjord, Sweden) to elevated CO2 concentrations and OA as expected for the end of the century (~760 μatm pCO2). Based on a plankton-imaging approach, we examined size structure, community composition and food web characteristics of the whole plankton assemblage, ranging from picoplankton to mesozooplankton, during an entire winter-to-summer succession. The plankton imaging system revealed pronounced temporal changes in the size structure of the copepod community over the course of the plankton bloom. The observed shift towards smaller individuals resulted in an overall decrease of copepod biomass by 25%, despite increasing numerical abundances. Furthermore, we observed distinct effects of elevated CO2 on biomass and size structure of the entire plankton community. Notably, the biomass of copepods, dominated by Pseudocalanus acuspes, displayed a tendency towards elevated biomass by up to 30-40% under simulated ocean acidification. This effect was significant for certain copepod size classes and was most likely driven by CO2-stimulated responses of primary producers and a complex interplay of trophic interactions that allowed this CO2 effect to propagate up the food web. Such OA-induced shifts in plankton community structure could have far-reaching consequences for food-web interactions, biomass transfer to higher trophic levels and biogeochemical cycling of marine ecosystems.

pCO2 effects on species composition and growth of an estuarine phytoplankton community

The effects of ongoing changes in ocean carbonate chemistry on plankton ecology have important implications for food webs and biogeochemical cycling. However, conflicting results have emerged regarding species-specific responses to pCO2 enrichment and thus community responses have been difficult to predict. To assess community level effects (e.g., production) of altered carbonate chemistry, studies are needed that capitalize on the benefits of controlled experiments but also retain features of intact ecosystems that may exacerbate or ameliorate the effects observed in single-species or single cohort experiments. We performed incubations of natural plankton communities from Narragansett Bay, RI, USA in winter at ambient bay temperatures (5-13 °C), light and nutrient concentrations under three levels of controlled and constant CO2 concentrations, simulating past, present and future conditions at mean pCO2 levels of 224, 361, and 724 μatm respectively. Samples for carbonate analysis, chlorophyll a, plankton size-abundance, and plankton species composition were collected daily and phytoplankton growth rates in three different size fractions (<5, 5-20, and >20 μm) were measured at the end of the 7-day incubation period. Community composition changed during the incubation period with major increases in relative diatom abundance, which were similar across pCO2 treatments. At the end of the experiment, 24-hr growth responses to pCO2 levels varied as a function of cell size. The smallest size fraction (<5 μm) grew faster at the elevated pCO2 level. In contrast, the 5-20 μm size fraction grew fastest in the Present treatment and there were no significant differences in growth rate among treatments in the > 20 μm size fraction. Cell size distribution shifted toward smaller cells in both the Past and Future treatments but remained unchanged in the Present treatment. Similarity in Past and Future treatments for cell size distribution and growth rate (5-20 μm size fraction) illustrate non-monotonic effects of increasing pCO2 on ecological indicators and may be related to opposing physiological effects of high CO2 and low pH both within and among species. Interaction of these effects with other factors (e.g., nutrients, light, temperature, grazing, initial species composition) may explain variability among published studies. The absence of clear treatment-specific effects at the community level suggest that extrapolation of species-specific responses or experiments with only present day and future pCO2 treatments levels would produce misleading predictions of ocean acidification impacts on plankton production.

Low CO2 Sensitivity of Microzooplankton Communities in the Gullmar Fjord, Skagerrak: Evidence from a Long-Term Mesocosm Study

Ocean acidification is considered as a crucial stressor for marine communities. In this study, we tested the effects of the IPCC RPC6.0 end-of-century acidification scenario on a natural plankton community in the Gullmar Fjord, Sweden, during a long-term mesocosm experiment from a spring bloom to a mid-summer situation. The focus of this study was on microzooplankton and its interactions with phytoplankton and mesozooplankton. The microzooplankton community was dominated by ciliates, especially small Strombidium sp., with the exception of the last days when heterotrophic dinoflagellates increased in abundance. We did not observe any effects of high CO₂ on the community composition and diversity of microzooplankton. While ciliate abundance, biomass and growth rate were not affected by elevated CO₂, we observed a positive effect of elevated CO₂ on dinoflagellate abundances. Additionally, growth rates of dinoflagellates were significantly higher in the high CO₂ treatments. Given the higher Chlorophyll a content measured under high CO₂, our results point at mainly indirect effects of CO₂ on microzooplankton caused by changes in phytoplankton standing stocks, in this case most likely an increase in small-sized phytoplankton of <8 μm. Overall, the results from the present study covering the most important part of the growing season indicate that coastal microzooplankton communities are rather robust towards realistic acidification scenarios.