Rising CO2 Levels Will Intensify Phytoplankton Blooms in Eutrophic and Hypertrophic Lakes

Harmful algal blooms threaten the water quality of many eutrophic and hypertrophic lakes and cause severe ecological and economic damage worldwide. Dense blooms often deplete the dissolved CO₂ concentration and raise pH. Yet, quantitative prediction of the feedbacks between phytoplankton growth, CO₂ drawdown and the inorganic carbon chemistry of aquatic ecosystems has received surprisingly little attention. Here, we develop a mathematical model to predict dynamic changes in dissolved inorganic carbon (DIC), pH and alkalinity during phytoplankton bloom development. We tested the model in chemostat experiments with the freshwater cyanobacterium Microcystis aeruginosa at different CO₂ levels. The experiments showed that dense blooms sequestered large amounts of atmospheric CO₂, not only by their own biomass production but also by inducing a high pH and alkalinity that enhanced the capacity for DIC storage in the system. We used the model to explore how phytoplankton blooms of eutrophic waters will respond to rising CO₂ levels. The model predicts that (1) dense phytoplankton blooms in low- and moderately alkaline waters can deplete the dissolved CO₂ concentration to limiting levels and raise the pH over a relatively wide range of atmospheric CO₂ conditions, (2) rising atmospheric CO₂ levels will enhance phytoplankton blooms in low- and moderately alkaline waters with high nutrient loads, and (3) above some threshold, rising atmospheric CO₂ will alleviate phytoplankton blooms from carbon limitation, resulting in less intense CO₂ depletion and a lesser increase in pH. Sensitivity analysis indicated that the model predictions were qualitatively robust. Quantitatively, the predictions were sensitive to variation in lake depth, DIC input and CO₂ gas transfer across the air-water interface, but relatively robust to variation in the carbon uptake mechanisms of phytoplankton. In total, these findings warn that rising CO₂ levels may result in a marked intensification of phytoplankton blooms in eutrophic and hypertrophic waters.     

Decreased photosynthesis and growth with reduced respiration in the model diatom Phaeodactylum tricornutum grown under elevated CO2 over 1800 generations

Studies on the long‐term responses of marine phytoplankton to ongoing ocean acidification (OA) are appearing rapidly in the literature. However, only a few of these have investigated diatoms, which is disproportionate to their contribution to global primary production. Here we show that a population of the model diatom Phaeodactylum tricornutum, after growing under elevated CO₂ (1000 μatm, HCL, pH_T: 7.70) for 1860 generations, showed significant differences in photosynthesis and growth from a population maintained in ambient CO₂ and then transferred to elevated CO₂ for 20 generations (HC). The HCL population had lower mitochondrial respiration, than did the control population maintained in ambient CO₂ (400 μatm, LCL, pH_T: 8.02) for 1860 generations. Although the cells had higher respiratory carbon loss within 20 generations under the elevated CO₂, being consistent to previous findings, they downregulated their respiration to sustain their growth in longer duration under the OA condition. Responses of phytoplankton to OA may depend on the timescale for which they are exposed due to fluctuations in physiological traits over time. This study provides the first evidence that populations of the model species, P. tricornutum, differ phenotypically from each other after having been grown for differing spans of time under OA conditions, suggesting that long‐term changes should be measured to understand responses of primary producers to OA, especially in waters with diatom‐dominated phytoplankton assemblages.     

Long-term culture at elevated atmospheric CO2 fails to evoke specific adaptation in seven freshwater phytoplankton species

The concentration of CO₂ in the atmosphere is expected to double by the end of the century. Experiments have shown that this will have important effects on the physiology and ecology of photosynthetic organisms, but it is still unclear if elevated CO₂ will elicit an evolutionary response in primary producers that causes changes in physiological and ecological attributes. In this study, we cultured lines of seven species of freshwater phytoplankton from three major groups at current (approx. 380 ppm CO₂) and predicted future conditions (1000 ppm CO₂) for over 750 generations. We grew the phytoplankton under three culture regimes: nutrient-replete liquid medium, nutrient-poor liquid medium and solid agar medium. We then performed reciprocal transplant assays to test for specific adaptation to elevated CO₂ in these lines. We found no evidence for evolutionary change. We conclude that the physiology of carbon utilization may be conserved in natural freshwater phytoplankton communities experiencing rising atmospheric CO₂ levels, without substantial evolutionary change.     

Glacier melting and stoichiometric implications for lake community structure: zooplankton species distributions across a natural light gradient

Glaciers around the globe are melting rapidly, threatening the receiving environments of the world's fresh water reservoirs with significant changes. The meltwater, carried by rivers, contains large amounts of suspended sediment particles, producing longitudinal gradients in the receiving lakes. These gradients may result in changes in the light : nutrient ratio that affect grazer performance by altering elemental food quality. Thus, glacial melting may induce a shift in the phytoplankton carbon : nutrient ratio and hence influence the dominance of herbivorous zooplankton through stoichiometric mechanisms. To test this hypothesis, we combined field and experimental data, taking advantage of a natural light intensity gradient caused by glacial clay input in a deep oligotrophic Patagonian lake. Across this gradient, we evaluated the abundances of two consumer taxa with different phosphorus requirements, the copepod Boeckella gracilipes and the cladoceran Daphnia commutata, using a six‐station transect along the lake. We found significant differences in light : nutrient ratio and stoichiometric food quality of the seston, together with a switch from dominance of P‐rich Daphnia in low carbon : nutrient stations to dominance of low‐P copepods in high carbon : nutrient stations. The laboratory experiments confirmed that the difference in the carbon : nutrient ratio across the gradient is sufficient to impair Daphnia growth. The overall patterns are consistent with our prediction that shifts in the environmental light : nutrient ratio as a result of glacial melting would contribute to shifts in the dominance of stoichiometrically contrasting taxa in consumer guilds.     

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high natural solar radiation

Impacts of rising atmospheric CO₂ concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii, the effects of moderate and high natural solar radiation combined with either ambient or future pCO₂ on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO₂ concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO₂ in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (F_v/F_m) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom‐forming Antarctic coastal diatoms might increase carbon contents under future pCO₂ conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO₂ sequestration potential of diatom communities in the future coastal Southern Ocean.     

Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO₂ and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO₂ related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO₂. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO₂. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO₂ levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high‐nitrogen batch cultures and lower‐nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO₂ independent of the culture regime. We also find that, despite species‐specific variation in growth response to high CO₂, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO₂. The CO₂‐induced growth response is a good predictor of CO₂‐induced changes in competition (R² > .93) and community composition (R² > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO₂ levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.     

ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY EMILIANIA HUXLEYI (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS1

Phytoplankton play a key role in determining the partitioning of CO₂ between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw‐down of atmospheric CO₂ and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon‐to‐nitrogen (C:N) and carbon‐to‐phosphorus (C:P) assimilation ratios modify the oceanic CO₂ storage capacity. Here we show that increased atmospheric CO₂ concentration enhances CO₂ fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO₂ led to further increases in the particulate C:N and C:P ratios. Enhanced CO₂ uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO₂, could increase carbon storage in the nitrogen‐limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO₂ levels.     

Stoichiometry of regenerated nutrients differs between native and invasive freshwater mussels with implications for algal growth

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Reversal in competitive dominance of a toxic versus non-toxic cyanobacterium in response to rising CO2

Climate change scenarios predict a doubling of the atmospheric CO₂ concentration by the end of this century. Yet, how rising CO₂ will affect the species composition of aquatic microbial communities is still largely an open question. In this study, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and non-toxic strain of the freshwater cyanobacterium Microcystis aeruginosa. The toxic strain was able to reduce dissolved CO₂ to lower concentrations than the non-toxic strain, and became dominant in competition at low CO₂ levels. Conversely, the non-toxic strain could grow at lower light levels, and became dominant in competition at high CO₂ levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether microcystins might have a role in this reversal of competitive dominance, we performed further competition experiments with the wild-type strain M. aeruginosa PCC 7806 and its mcyB mutant impaired in microcystin production. The microcystin-producing wild type had a strong selective advantage at low CO₂ levels but not at high CO₂ levels. Our results thus demonstrate both in theory and experiment that rising CO₂ levels can alter the community composition and toxicity of harmful algal blooms.     

The response of a natural phytoplankton community from the Godavari River Estuary to increasing CO2 concentration during the pre-monsoon period

This paper reports for the first time upon the effects of increasing CO₂ concentrations on a natural phytoplankton assemblage in a tropical estuary (the Godavari River Estuary in India). Two short-term (5-day) bottle experiments were conducted (with and without nutrient addition) during the pre-monsoon season when the partial pressure of CO₂ in the surface water is quite low. The results reveal that the concentrations of total chlorophyll, the phytoplankton growth rate, the concentrations of particulate organic matter, the photosynthetic oxygen evolution rates, and the total bacterial count were higher under elevated CO₂ treatments, as compared to ambient conditions (control). δ¹³C of particulate organic matter (POM) varied inversely with respect to CO₂, indicating a clear signature of higher CO₂ influx under the elevated CO₂ levels. Whereas, δ¹³C_POM in the controls indicated the existence of an active bicarbonate transport system under limited CO₂ supply. A considerable change in phytoplankton community structure was noticed, with marker pigment analysis by HPLC revealing that cyanobacteria were dominant over diatoms as CO₂ concentrations increased. A mass balance calculation indicated that insufficient nutrients (N, P and Si) might have inhibited diatom growth compared to cyanobacteria, regardless of increased CO₂ supply. The present study suggests that CO₂ concentration and nutrient supply could have significant effects on phytoplankton physiology and community composition for natural phytoplankton communities in this region. However, this work was conducted during a non-discharge period (nutrient-limited conditions) and the responses of phytoplankton to increasing CO₂ might not necessarily be the same during other seasons with high physicochemical variability. Further investigation is therefore needed.     

Elemental stoichiometry of the key calcifying marine phytoplankton Emiliania huxleyi under ocean climate change: A meta-analysis

The elemental composition of marine microorganisms (their C:N:P ratio, or stoichiometry) is central to understanding the biotic and biogeochemical processes underlying key marine ecosystem functions. Phytoplankton C:N:P is species specific and flexible to changing environmental conditions. However, bulk or fixed phytoplankton stoichiometry is usually assumed in biogeochemical and ecological models because more realistic, environmentally responsive C:N:P ratios have yet to be defined for key functional groups. Here, a comprehensive meta-analysis of experimental laboratory data reveals the variable C:N:P stoichiometry of Emiliania huxleyi, a globally significant calcifying phytoplankton species. Mean C:N:P of E. huxleyi is 124C:16N:1P under control conditions (i.e. growth not limited by one or more environmental stressors) and shows a range of responses to changes in nutrient and light availability, temperature and pCO₂. Macronutrient limitation caused strong shifts in stoichiometry, increasing N:P and C:P under P deficiency (by 305% and 493% respectively) and doubling C:N under N deficiency. Responses to light, temperature and pCO₂ were mixed but typically shifted cellular elemental content and C:N:P stoichiometry by ca. 30% or less. Besides these independent effects, the interactive effects of multiple environmental changes on E. huxleyi stoichiometry under future ocean conditions could be additive, synergistic or antagonistic. To synthesise our meta-analysis results, we explored how the cellular elemental content and C:N:P stoichiometry of E. huxleyi may respond to two hypothetical future ocean scenarios (increased temperature, irradiance and pCO₂ combined with either N deficiency or P deficiency) if an additive effect is assumed. Both future scenarios indicate decreased calcification (which is predominantly sensitive to elevated pCO₂), increased C:N, and up to fourfold shifts in C:P and N:P. Our results strongly suggest that climate change will significantly alter the role of E. huxleyi (and potentially other calcifying phytoplankton species) in marine biogeochemical processes.     

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO₂, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO₂ as drivers of air–sea CO₂ exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO₂ fluxes of the coastal ecosystem. For this application, the MIRO‐CO₂ model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO₂ and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO₂ before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO₂ sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO₂ to the atmosphere. R‐MIRO‐CO₂ scenarios testing the relative impact of temperature, wind speed, atmospheric CO₂ and river loads variability on the simulated air–sea CO₂ fluxes suggest that the trend in air–sea CO₂ fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO₂.     

Elevated CO2 enhances resprouting of a tropical savanna tree

The savannas (cerrado) of south-central Brazil are currently subjected to frequent anthropogenic burning, causing widespread reduction in tree density. Increasing concentrations of atmospheric CO₂ could reduce the impact of such frequent burning by increasing the availability of nonstructural carbohydrate, which is necessary for resprouting. We tested the hypotheses that elevated CO₂ stimulates resprouting and accelerates replenishment of carbohydrate reserves. Using a factorial experiment, seedlings of a common Brazilian savanna tree, Keilmeyera coriacea, were grown at 350 ppm and 700 ppm CO₂ and at two nutrient levels. To simulate burning, the plants were either clipped at 15 weeks or were left unclipped. Among unclipped plants, CO₂ and nutrients both stimulated growth, with no significant interaction between nutrient and CO₂ effects. Among clipped plants, both CO₂ and nutrients stimulated resprouting. However, there was a strong interaction between CO₂ and nutrient effects, with CO₂ having a significant effect only in the presence of high nutrient availability. Under elevated CO₂, carbohydrate reserves remained at higher levels following clipping. Root total nonstructural carbohydrate remained above 36% in all treatments, so carbohydrate reserves did not limit regrowth. These results indicate that under elevated CO₂ this species may be better able to endure the high frequency of anthropogenic burning in the Brazilian savannas. Received: 19 May 1999 / Accepted: 8 November 1999     

Evolution of nutrient uptake reveals a trade-off in the ecological stoichiometry of plant-herbivore interactions

Nutrient limitation determines the primary production and species composition of many ecosystems. Here we apply an adaptive dynamics approach to investigate evolution of the ecological stoichiometry of primary producers and its implications for plant-herbivore interactions. The model predicts a trade-off between the competitive ability and grazing susceptibility of primary producers, driven by changes in their nutrient uptake rates. High nutrient uptake rates enhance the competitiveness of primary producers but also increase their nutritional quality for herbivores. This trade-off enables coexistence of nutrient exploiters and grazing avoiders. If herbivores are not selective, evolution favors runaway selection toward high nutrient uptake rates of the primary producers. However, if herbivores select nutritious food, the model predicts an evolutionarily stable strategy with lower nutrient uptake rates. When the model is parameterized for phytoplankton and zooplankton, the evolutionary dynamics result in plant-herbivore oscillations at ecological timescales, especially in environments with high nutrient availability and low selectivity of the herbivores. High herbivore selectivity stabilizes the community dynamics. These model predictions show that evolution permits nonequilibrium dynamics in plant-herbivore communities and shed new light on the evolutionary forces that shape the ecological stoichiometry of primary producers.     

Isolation by Time During an Arctic Phytoplankton Spring Bloom

The arctic phytoplankton spring bloom, which is often diatom‐dominated, is a key event that provides the high latitude communities with a fundamental flux of organic carbon. During a bloom, phytoplankton may increase its biomass by orders of magnitude within days. Yet, very little is known about phytoplankton bloom dynamics, including for example how blooming affects genetic composition and diversity of a population. Here, we quantified the genetic composition and temporal changes of the diatom Fragilariopsis cylindrus, which is one of the most important primary producers in the Arctic, during the spring bloom in western Greenland, using 13 novel microsatellite markers developed for this study. We found that genetic differentiation (quantified using sample‐specific F_ST) decreased between time points as the bloom progressed, with the most drastic changes in F_ST occurring at the start of the bloom; thus the genetic structure of the bloom is characterized by isolation by time. There was little temporal variation in genetic diversity throughout the bloom (mean H_E = 0.57), despite marked fluctuations in F. cylindrus cell concentrations and the temporal change in sample‐specific F_ST. On the basis of this novel pattern of genetic differentiation, we suggest that blooming behavior may promote genetic diversity of a phytoplankton population.     

Does the growth response of woody plants to elevated CO2 increase with temperature? A model‐oriented meta‐analysis

The temperature dependence of the reaction kinetics of the Rubisco enzyme implies that, at the level of a chloroplast, the response of photosynthesis to rising atmospheric CO₂ concentration (C_a) will increase with increasing air temperature. Vegetation models incorporating this interaction predict that the response of net primary productivity (NPP) to elevated CO₂ (eC_a) will increase with rising temperature and will be substantially larger in warm tropical forests than in cold boreal forests. We tested these model predictions against evidence from eC_a experiments by carrying out two meta‐analyses. Firstly, we tested for an interaction effect on growth responses in factorial eC_a × temperature experiments. This analysis showed a positive, but nonsignificant interaction effect (95% CI for above‐ground biomass response = ?0.8, 18.0%) between eC_a and temperature. Secondly, we tested field‐based eC_a experiments on woody plants across the globe for a relationship between the eC_a effect on plant biomass and mean annual temperature (MAT). This second analysis showed a positive but nonsignificant correlation between the eC_a response and MAT. The magnitude of the interactions between CO₂ and temperature found in both meta‐analyses were consistent with model predictions, even though both analyses gave nonsignificant results. Thus, we conclude that it is not possible to distinguish between the competing hypotheses of no interaction vs. an interaction based on Rubisco kinetics from the available experimental database. Experiments in a wider range of temperature zones are required. Until such experimental data are available, model predictions should aim to incorporate uncertainty about this interaction.     

Inorganic carbon promotes photosynthesis,growth, and maximum biomass of phytoplankton in eutrophic water bodies

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Effect of Elevated Atmospheric CO2 and Temperature Increases on the Severity of Basil Downy Mildew Caused by Peronospora belbahrii Under Phytotron Conditions

Three experimental trials have been carried out on the basil (Ocimum basilicum)–downy mildew (Peronospora belbahrii) pathosystem, under phytotron conditions, to evaluate the effect of simulated elevated atmospheric CO₂ concentrations and temperatures as well as that of their interaction. Six CO₂ and temperature combinations were tested to establish their effect on disease development. The photosynthetic efficiency (PI) and chlorophyll content index (CCI) of the basil plants were monitored throughout the trials. Average disease incidence was 43.8% under standard conditions (18–22°C and 400–450 ppm of CO₂)_, while average disease severity was 22.1%. In the same temperature regime, a doubled level of CO₂ caused a significant increase in both disease incidence and severity. When temperatures ranged between 18 and 26°C, CO₂ at 800–850 ppm increased disease incidence. At the highest temperatures tested, that is at 26–30°C, which are not favourable for downy mildew development, the increase in CO₂ had no significant effect on disease incidence. A decreasing trend of PI was observed for the PI values of the inoculated plants. This trend was particularly pronounced for high CO₂ levels at the end of the experiment. In the same way as for disease development, lower values were recorded for the inoculated plants at the end of the experiment at 18–22°C for both CO₂ concentrations and at 22–26°C for 850 ppm of CO₂. The non‐inoculated plants showed higher photosynthetic efficiency than the inoculated plants. Similar trends were also observed for the CCI, thus confirming that downy mildew incidence and severity, which in particular caused foliar damage at high CO₂ concentrations, led to a decrease in the physiological performances.     

Coupling dissolved organic carbon,CO2 and productivity in boreal lakes

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