Trophic position, elemental ratios and nitrogen transfer in a planktonic host-parasite-consumer food chain including a fungal parasite.

Parasitism is arguably the most commonly occurring consumer strategy. However, only a few food web studies assess how well stable isotopes reflect the trophic position of parasitic consumers and results are variable. Even fewer studies have measured the nutrient transfer by parasitic consumers, hindering an assessment of their role in nutrient transfer through food webs. Here we used a food chain consisting of a diatom as host, a chytrid as its parasitic consumer and a rotifer as the predatory consumer of the chytrid, to assess the trophic position of all three food-chain components using their natural 13C and 15N isotope signatures, and to measure the nitrogen transfer from the host via the chytrid to the rotifer by tracing 15N of a labelled host up the food chain. Additionally, we measured the carbon to nitrogen (C:N) ratios of all food-chain components. Natural isotope abundance results showed no clear 15N enrichment in the chytrid or rotifer relative to the primary producer. However, estimates of nitrogen transfer indicated that about 14% of host nitrogen was transferred per day from host to chytrid during infection epidemics and that some of this nitrogen was also transferred onward to the rotifer. Moreover, C:N ratios decreased with trophic level, suggesting that the chytrid provided a high-quality food source to the rotifer. In conclusion, our results support the "mycoloop". The mycooloop proposes that chytrid infections allow the transfer of nutrients bound in large, inedible phytoplankton to zooplankton through the production of edible transmission spores, thereby rerouting nutrients back into the food web.

Evaluating early-warning indicators of critical transitions in natural aquatic ecosystems.

Ecosystems can show sudden and persistent changes in state despite only incremental changes in drivers. Such critical transitions are difficult to predict, because the state of the system often shows little change before the transition. Early-warning indicators (EWIs) are hypothesized to signal the loss of system resilience and have been shown to precede critical transitions in theoretical models, paleo-climate time series, and in laboratory as well as whole lake experiments. The generalizability of EWIs for detecting critical transitions in empirical time series of natural aquatic ecosystems remains largely untested, however. Here we assessed four commonly used EWIs on long-term datasets of five freshwater ecosystems that have experienced sudden, persistent transitions and for which the relevant ecological mechanisms and drivers are well understood. These case studies were categorized by three mechanisms that can generate critical transitions between alternative states: competition, trophic cascade, and intraguild predation. Although EWIs could be detected in most of the case studies, agreement among the four indicators was low. In some cases, EWIs were detected considerably ahead of the transition. Nonetheless, our results show that at present, EWIs do not provide reliable and consistent signals of impending critical transitions despite using some of the best routinely monitored freshwater ecosystems. Our analysis strongly suggests that a priori knowledge of the underlying mechanisms driving ecosystem transitions is necessary to identify relevant state variables for successfully monitoring EWIs.

Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial.

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year-round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n = 4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year-round warming nearly doubled the final carbon stock in plant biomass from 6.9 ± 1.1 g C in the control treatment to 12.8 ± 0.6 g C (mean ± SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96 ± 9.6 to 152 ± 16 g C m-2  yaer-1 (mean ± SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40 ± 5.7 g C m-2  year-1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte-dominated systems, while not necessarily affecting net carbon burial on a system scale.

Species sorting and stoichiometric plasticity control community C:P ratio of first-order aquatic consumers.

Ecological stoichiometry has proven to be invaluable for understanding consumer response to changes in resource quality. Although interactions between trophic levels occur at the community level, most studies focus on single consumer species. In contrast to individual species, communities may deal with trophic mismatch not only through elemental plasticity but also through changes in species composition. Here, we show that a community of first-order consumers (e.g. zooplankton) is able to adjust its stoichiometry (C:P) in response to experimentally induced changes in resource quality, but only to a limited extent. Furthermore, using the Price equation framework we show the importance of both elemental plasticity and species sorting. These results illustrate the need for a community perspective in ecological stoichiometry, requiring consideration of species-specific elemental composition, intraspecific elemental plasticity and species turnover.

Integrating chytrid fungal parasites into plankton ecology: research gaps and needs.

Chytridiomycota, often referred to as chytrids, can be virulent parasites with the potential to inflict mass mortalities on hosts, causing e.g. changes in phytoplankton size distributions and succession, and the delay or suppression of bloom events. Molecular environmental surveys have revealed an unexpectedly large diversity of chytrids across a wide range of aquatic ecosystems worldwide. As a result, scientific interest towards fungal parasites of phytoplankton has been gaining momentum in the past few years. Yet, we still know little about the ecology of chytrids, their life cycles, phylogeny, host specificity and range. Information on the contribution of chytrids to trophic interactions, as well as co-evolutionary feedbacks of fungal parasitism on host populations is also limited. This paper synthesizes ideas stressing the multifaceted biological relevance of phytoplankton chytridiomycosis, resulting from discussions among an international team of chytrid researchers. It presents our view on the most pressing research needs for promoting the integration of chytrid fungi into aquatic ecology.

Warming accelerates termination of a phytoplankton spring bloom by fungal parasites.

Climate change is expected to favour infectious diseases across ecosystems worldwide. In freshwater and marine environments, parasites play a crucial role in controlling plankton population dynamics. Infection of phytoplankton populations will cause a transfer of carbon and nutrients into parasites, which may change the type of food available for higher trophic levels. Some phytoplankton species are inedible to zooplankton, and the termination of their population by parasites may liberate otherwise unavailable carbon and nutrients. Phytoplankton spring blooms often consist of large diatoms inedible for zooplankton, but the zoospores of their fungal parasites may serve as a food source for this higher trophic level. Here, we investigated the impact of warming on the fungal infection of a natural phytoplankton spring bloom and followed the response of a zooplankton community. Experiments were performed in ca. 1000 L indoor mesocosms exposed to a controlled seasonal temperature cycle and a warm (+4 °C) treatment in the period from March to June 2014. The spring bloom was dominated by the diatom Synedra. At the peak of infection over 40% of the Synedra population was infected by a fungal parasite (i.e. a chytrid) in both treatments. Warming did not affect the onset of the Synedra bloom, but accelerated its termination. Peak population density of Synedra tended to be lower in the warm treatments. Furthermore, Synedra carbon: phosphorus stoichiometry increased during the bloom, particularly in the control treatments. This indicates enhanced phosphorus limitation in the control treatments, which may have constrained chytrid development. Timing of the rotifer Keratella advanced in the warm treatments and closely followed chytrid infections. The chytrids' zoospores may thus have served as an alternative food source to Keratella. Our study thus emphasizes the importance of incorporating not only nutrient limitation and grazing, but also parasitism in understanding the response of plankton communities towards global warming.

Pharmaceuticals May Disrupt Natural Chemical Information Flows and Species Interactions in Aquatic Systems: Ideas and Perspectives on a Hidden Global Change.

Pharmaceuticals consumption by humans and animals is increasing substantially, leading to unprecedented levels of these compounds in aquatic environments worldwide. Recent findings that concentrations reach levels that can directly have negative effects on organisms are important per se, but also sound an alarm for other potentially more pervasive effects that arise from the interconnected nature of ecological communities. Aquatic organisms use chemical cues to navigate numerous challenges, including the location of mates and food, and the avoidance of natural enemies. Low concentrations of pharmaceuticals can disrupt this "smellscape" of information leading to maladaptive responses. Furthermore, direct effects of pharmaceuticals on the traits and abundance of one species can cascade through a community, indirectly affecting other species. We review mechanisms by which pharmaceuticals in surface waters can disrupt natural chemical information flows and species interactions. Pharmaceuticals form a new class of chemical threats, which could have far-reaching implications for ecosystem functioning and conservation management.

Community stoichiometry in a changing world: combined effects of warming and eutrophication on phytoplankton dynamics.

The current changes in our climate will likely have far-reaching consequences for aquatic ecosystems. These changes in the climate, however, do not act alone, and are often accompanied by additional stressors such as eutrophication. Both global warming and eutrophication have been shown to affect the timing and magnitude of phytoplankton blooms. Little is known about the combined effects of rising temperatures and eutrophication on the stoichiometry of entire phytoplankton communities. We exposed a natural phytoplankton spring community to different warming and phosphorus-loading scenarios using a full-factorial design. Our results demonstrate that rising temperatures promote the growth rate of an entire phytoplankton community. Furthermore, both rising temperatures and phosphorus loading stimulated the maximum biomass built up by the phytoplankton community. Rising temperatures led to higher carbon: nutrient stoichiometry of the phytoplankton community under phosphorus-limited conditions. Such a shift towards higher carbon: nutrient ratios, in combination with a higher biomass buildup, suggests a temperature-driven increase in nutrient use efficiency, the phytoplankton community. Importantly, with higher carbon: nutrient stoichiometry, phytoplankton is generally of poorer nutritional value for zooplankton. Thus, although warming may result in higher phytoplankton biomass, this may be accompanied by a stoichiometric mismatch between phytoplankton and their grazers, with possible consequences for the entire aquatic food web.

Biological control of toxic cyanobacteria by mixotrophic predators: an experimental test of intraguild predation theory.

Intraguild predators both feed on and compete with their intraguild prey. In theory, intraguild predators can therefore be very effective as biological control agents of intraguild prey species, especially in productive environments. We investigated this hypothesis using the mixotrophic chrysophyte Ochromonas as intraguild predator and the harmful cyanobacterium Microcystis aeruginosa as its prey. Ochromonas can grow photoautotrophically, but can also graze efficiently on Microcystis. Hence, it competes with its prey for inorganic resources. We developed a mathematical model and parameterized it for our experimental food web. The model predicts dominance of Microcystis at low nutrient loads, coexistence of both species at intermediate nutrient loads, and dominance of Ochromonas but a strong decrease of Microcystis at high nutrient loads. We tested these theoretical predictions in chemostat experiments supplied with three different nitrogen concentrations. Ochromonas initially suppressed the Microcystis abundance by > 97% compared to the Microcystis monocultures. Thereafter, however, Microcystis gradually recovered to -20% of its monoculture abundance at low nitrogen loads, but to 50-60% at high nitrogen loads. Hence, Ochromonas largely lost control over the Microcystis population at high nitrogen loads. We explored several mechanisms that might explain this deviation from theoretical predictions, and found that intraspecific interference at high Ochromonas densities reduced their grazing rates on Microcystis. These results illustrate the potential of intraguild predation to control pest species, but also show that the effectiveness of their biological control can be reduced in productive environments.

Understanding the differential impacts of two antidepressants on locomotion of freshwater snails (Lymnaea stagnalis).

There is growing evidence of negative impacts of antidepressants on behavior of aquatic non-target organisms. Accurate environmental risk assessment requires an understanding of whether antidepressants with similar modes of action have consistent negative impacts. Here, we tested the effect of acute exposure to two antidepressants, fluoxetine and venlafaxine (0-50 µg/L), on the behavior of non-target organism, i.e., freshwater pond snail, Lymnaea stagnalis. As compounds interact with chemical cues in the aquatic ecosystems, we also tested whether the effects altered in the presence of bile extract containing 5α-cyprinol sulfate (5α-CPS), a characterized kairomone of a natural predator, common carp (Cyprinus carpio). Behavior was studied using automated tracking and analysis of various locomotion parameters of L. stagnalis. Our results suggest that there are differences in the effects on locomotion upon exposure to venlafaxine and fluoxetine. We found strong evidence for a non-monotonic dose response on venlafaxine exposure, whereas fluoxetine only showed weak evidence of altered locomotion for a specific concentration. Combined exposure to compounds and 5α-CPS reduced the intensity of effects observed in the absence of 5α-CPS, possibly due to reduced bioavailability of the compounds. The results highlight the need for acknowledging different mechanisms of action among antidepressants while investigating their environmental risks. In addition, our results underline the importance of reporting non-significant effects and acknowledging individual variation in behavior for environmental risk assessment.

Effect of an antidepressant on aquatic ecosystems in the presence of microplastics: A mesocosm study.

Emerging pollutants, such as pharmaceuticals and microplastics have become a pressing concern due to their widespread presence and potential impacts on ecological systems. To assess the ecosystem-level effects of these pollutants within a multi-stressor context, we simulated real-world conditions by exposing a near-natural multi-trophic aquatic food web to a gradient of environmentally relevant concentrations of fluoxetine and microplastics in large mesocosms over a period of more than three months. We measured the biomass and abundance of different trophic groups, as well as ecological functions such as nutrient availability and decomposition rate. To explore the mechanisms underlying potential community and ecosystem-level effects, we also performed behavioral assays focusing on locomotion parameters as a response variable in three species: Daphnia magna (zooplankton prey), Chaoborus flavicans larvae (invertebrate pelagic predator of zooplankton) and Asellus aquaticus (benthic macroinvertebrate), using water from the mesocosms. Our mesocosm results demonstrate that presence of microplastics governs the response in phytoplankton biomass, with a weak non-monotonic dose-response relationship due to the interaction between microplastics and fluoxetine. However, exposure to fluoxetine evoked a strong non-monotonic dose-response in zooplankton abundance and microbial decomposition rate of plant material. In the behavioral assays, the locomotion of zooplankton prey D. magna showed a similar non-monotonic response primarily induced by fluoxetine. Its predator C. flavicans, however, showed a significant non-monotonic response governed by both microplastics and fluoxetine. The behavior of the decomposer A. aquaticus significantly decreased at higher fluoxetine concentrations, potentially leading to reduced decomposition rates near the sediment. Our study demonstrates that effects observed upon short-term exposure result in more pronounced ecosystem-level effects following chronic exposure.

Mixotrophic organisms become more heterotrophic with rising temperature.

The metabolic theory of ecology predicts that temperature affects heterotrophic processes more strongly than autotrophic processes. We hypothesized that this differential temperature response may shift mixotrophic organisms towards more heterotrophic nutrition with rising temperature. The hypothesis was tested in experiments with the mixotrophic chrysophyte Ochromonas sp., grown under autotrophic, mixotrophic and heterotrophic conditions. Our results show that (1) grazing rates on bacterial prey increased more strongly with temperature than photosynthetic electron transport rates, (2) heterotrophic growth rates increased exponentially with temperature over the entire range from 13 to 33 °C, while autotrophic growth rates reached a maximum at intermediate temperatures and (3) chlorophyll contents during mixotrophic growth decreased at high temperature. Hence, the contribution of photosynthesis to mixotrophic growth strongly decreased with temperature. These findings support the hypothesis that mixotrophs become more heterotrophic with rising temperature, which alters their functional role in food webs and the carbon cycle.

Strong interactions between stoichiometric constraints and algal defenses: evidence from population dynamics of Daphnia and algae in phosphorus-limited microcosms.

The dynamic interactions among nutrients, algae and grazers were tested in a 2 × 3 factorial microcosm experiment that manipulated grazers (Daphnia present or absent) and algal composition (single species cultures and mixtures of an undefended and a digestion-resistant green alga). The experiment was run for 25 days in 10-L carboys under mesotrophic conditions that quickly led to strong phosphorus limitation of algal growth (TP is approximately equal to 0.5 µM, N:P 40:1). Four-day Daphnia juvenile growth assays tested for Daphnia P-limitation and nutrient-dependent or grazer-induced algal defenses. The maximal algal growth rate of undefended Ankistrodesmus (mean ± SE for three replicate microcosms; 0.92 ± 0.02 day(-1)) was higher than for defended Oocystis (0.62 ± 0.03 day(-1)), but by day 6, algal growth was strongly P-limited in all six treatments (molar C:P ratio >900). The P-deficient algae were poor quality resources in all three algal treatments. However, Daphnia population growth, reproduction, and survival were much lower in the digestion-resistant treatment even though growth assays provided evidence for Daphnia P-limitation in only the undefended and mixed treatments. Growth assays provided little or no support for simple threshold element ratio (TER) models that fail to consider algae defenses that result in viable gut passage. Our results show that strong P-limitation of algal growth enhances the defenses of a digestion-resistant alga, favoring high abundance of well-defended algae and energy limitation of zooplankton growth.

Environmental refuges from disease in host-parasite interactions under global change.

The physiological performance of organisms depends on their environmental context, resulting in performance-response curves along environmental gradients. Parasite performance-response curves are generally expected to be broader than those of their hosts due to shorter generation times and hence faster adaptation. However, certain environmental conditions may limit parasite performance more than that of the host, thereby providing an environmental refuge from disease. Thermal disease refuges have been extensively studied in response to climate warming, but other environmental factors may also provide environmental disease refuges which, in turn, respond to global change. Here, we (1) showcase laboratory and natural examples of refuges from parasites along various environmental gradients, and (2) provide hypotheses on how global environmental change may affect these refuges. We strive to synthesize knowledge on potential environmental disease refuges along different environmental gradients including salinity and nutrients, in both natural and food-production systems. Although scaling up from single host-parasite relationships along one environmental gradient to their interaction outcome in the full complexity of natural environments remains difficult, integrating host and parasite performance-response can serve to formulate testable hypotheses about the variability in parasitism outcomes and the occurrence of environmental disease refuges under current and future environmental conditions.

Making waves: Lessons learned from the COVID-19 anthropause in the Netherlands on urban aquatic ecosystem services provisioning and management.

The anomalous past two years of the COVID-19 pandemic have been a test of human response to global crisis management as typical human activities were significantly altered. The COVID-instigated anthropause has illustrated the influence that humans and the biosphere have on each other, especially given the variety of national mobility interventions that have been implemented globally. These local COVID-19-era restrictions influenced human-ecosystem interactions through changes in accessibility of water systems and changes in ecosystem service demand. Four urban aquatic case studies in the Netherlands demonstrated shifts in human demand during the anthropause. For instance, reduced boat traffic in Amsterdam canals led to improved water clarity. In comparison, ongoing service exploitation from increased recreational fishing, use of bathing waters and national parks visitation are heightening concerns about potential ecosystem degradation. We distilled management lessons from both the case studies as well as from recent literature pertaining to ecological intactness and social relevance. Equally important to the lessons themselves, however, is the pace at which informed management practices are established after the pandemic ends, particularly as many communities currently recognize the importance of aquatic ecosystems and are amenable to their protection.

The value of novel ecosystems: Disclosing the ecological quality of quarry lakes.

Intense sand and gravel mining has created numerous man-made lakes around the world in the past century. These small quarry lakes (1-50 ha) are usually hydrologically isolated, often deep (6-40 m) and stratify during summer and in cold winters. Due to their small size, these deep man-made lakes are usually not included in the regular monitoring campaigns, e.g. as required for the European Water Framework Directive (WFD). Therefore, not much is known about the ecological functioning of these novel ecosystems. During two summers, we determined the macrophyte diversity and measured a range of physico-chemical and biological parameters in 51 quarry lakes in the catchment area of the rivers Meuse and Rhine. We compared the results of this campaign to the chemical and macrophyte sampling as performed for the WFD in the immediate surrounding shallow standing waters. Alpha (local) and beta diversity (regional), and local contribution to beta diversity were calculated for the whole region of which beta diversity was further partitioned into a true species replacement and richness difference component. Quarry lakes contain higher water quality reflected by lower nutrient and chlorophyll-a concentration compared with shallow water bodies. Additionally, quarry lakes contribute significantly to the regional macrophyte diversity pool by harboring distinctly different macrophyte communities (beta diversity - replacement). Specifically quarry lakes with a total phosphorus concentration in the water column below 35 µg P/l contribute most to beta diversity among quarry lakes. Novel ecosystems such as deep quarry lakes are often perceived as less valuable ecosystems, with strong implications regarding their management. Our results show that quarry lakes are in general of better chemical and biological quality compared with shallow standing waters. We therefore call for a more integrated assessment of the quality of quarry lakes and corresponding management strategy of these waters by water managers.

Distribution patterns of epiphytic reed-associated macroinvertebrate communities across European shallow lakes.

So far, research on plant-associated macroinvertebrates, even if conducted on a large number of water bodies, has mostly focused on a relatively small area, permitting limited conclusions to be drawn regarding potentially broader geographic effects, including climate. Some recent studies have shown that the composition of epiphytic communities may differ considerably among climatic zones. To assess this phenomenon, we studied macroinvertebrates associated with the common reed Phragmites australis (Cav.) Trin. ex Steud in 46 shallow lakes using a common protocol. The lakes, located in nine countries, covered almost the entire European latitudinal range (from <48°N to 61°N) and captured much of the variability in lake size and nutrient content in the region. A Poisson Generalized Linear Mixed Model (GLMM) showed the number of macroinvertebrate epiphytic taxa to be negatively associated with water conductivity and positively associated with medium ice cover duration (approximately 1 month). A Gamma GLMM showed a positive effect of chlorophyll a on the density of macroinvertebrates, and a significantly greater density in lakes located at the lowest and highest latitudes. Individual taxa responded differently to lake environmental conditions across climate zones. Chironomidae dominated in all climate zones, but their contribution to total density decreased with increasing latitude, with progressively greater proportions of Naidinae, Asellidae, Ephemeroptera and Trichoptera. Our study demonstrates that epiphytic macroinvertebrate fauna, even when analyzed at low taxonomic resolution, exhibits clear differences in diversity, relative abundance of individual taxa and total density, shaped both by geographic and anthropogenic variables. The results were discussed in the context of climate change. To our best knowledge this is the first study to examine epiphytic fauna carried out on a European scale.

The potential of zooplankton in constraining chytrid epidemics in phytoplankton hosts.

Fungal diseases threaten natural and man-made ecosystems. Chytridiomycota (chytrids) infect a wide host range, including phytoplankton species that form the basis of aquatic food webs and produce roughly half of Earth's oxygen. However, blooms of large or toxic phytoplankton form trophic bottlenecks, as they are inedible to zooplankton. Chytrids infecting inedible phytoplankton provide a trophic link to zooplankton by producing edible zoospores of high nutritional quality. By grazing chytrid zoospores, zooplankton may induce a trophic cascade, as a decreased zoospore density will reduce new infections. Conversely, fewer infections will not produce enough zoospores to sustain long-term zooplankton growth and reproduction. This intricate balance between zoospore density necessary for zooplankton energetic demands (growth/survival), and the loss in new infections (and thus new zoospores) because of grazing was tested empirically. To this end, we exposed a cyanobacterial host (Planktothrix rubescens) infected by a chytrid (Rizophydium megarrhizum) to a grazer density gradient (the rotifer Keratella cf. cochlearis). Rotifers survived and reproduced on a zoospore diet, but the Keratella population growth was limited by the amount of zoospores provided by chytrid infections, resulting in a situation where zooplankton survived but were restricted in their ability to control disease in the cyanobacterial host. We subsequently developed and parameterized a dynamical food-chain model using an allometric relationship for clearance rate to assess theoretically the potential of different-sized zooplankton groups to restrict disease in phytoplankton hosts. Our model suggests that smaller-sized zooplankton may have a high potential to reduce chytrid infections on inedible phytoplankton. Together, our results point out the complexity of three-way interactions between hosts, parasites, and grazers and highlight that trophic cascades are not always sustainable and may depend on the grazer's energetic demand.

Interactive Effects of Rising Temperature and Nutrient Enrichment on Aquatic Plant Growth, Stoichiometry, and Palatability.

The abundance and stoichiometry of aquatic plants are crucial for nutrient cycling and energy transfer in aquatic ecosystems. However, the interactive effects of multiple global environmental changes, including temperature rise and eutrophication, on aquatic plant stoichiometry and palatability remain largely unknown. Here, we hypothesized that (1) plant growth rates increase faster with rising temperature in nutrient-rich than nutrient-poor sediments; (2) plant carbon (C): nutrient ratios [nitrogen (N) and phosphorus (P)] respond differently to rising temperatures at contrasting nutrient conditions of the sediment; (3) external nutrient loading to the water column limits the growth of plants and decreases plant C:nutrient ratios; and that (4) changes in plant stoichiometry affect plant palatability. We used the common rooted submerged plant Vallisneria spiralis as a model species to test the effects of temperature and nutrient availability in both the sediment and the water column on plant growth and stoichiometry in a full-factorial experiment. The results confirmed that plants grew faster in nutrient-rich than nutrient-poor sediments with rising temperature, whereas external nutrient loading decreased the growth of plants due to competition by algae. The plant C: N and C: P ratios responded differently at different nutrient conditions to rising temperature. Rising temperature increased the metabolic rates of organisms, increased the nutrient availability in the sediment and enhanced plant growth. Plant growth was limited by a shortage of N in the nutrient-poor sediment and in the treatment with external nutrient loading to the water column, as a consequence, the limited plant growth caused an accumulation of P in the plants. Therefore, the effects of temperature on aquatic plant C:nutrient ratios did not only depend on the availability of the specific nutrients in the environment, but also on plant growth, which could result in either increased, unaltered or decreased plant C:nutrient ratios in response to temperature rise. Plant feeding trial assays with the generalist consumer Lymnaea stagnalis (Gastropoda) did not show effects of temperature or nutrient treatments on plant consumption rates. Overall, our results implicate that warming and eutrophication might interactively affect plant abundance and plant stoichiometry, and therefore influence nutrient cycling in aquatic ecosystems.

The ecological stoichiometry of toxins produced by harmful cyanobacteria: an experimental test of the carbon-nutrient balance hypothesis.

The elemental composition of primary producers reflects the availability of light, carbon and nutrients in their environment. According to the carbon-nutrient balance hypothesis, this has implications for the production of secondary metabolites. To test this hypothesis, we investigated a family of toxins, known as microcystins, produced by harmful cyanobacteria. The strain Microcystis aeruginosa HUB 5-2-4, which produces several microcystin variants of different N:C stoichiometry, was cultured in chemostats supplied with various combinations of nitrate and CO(2). Excess supply of both nitrogen and carbon yielded high cellular N:C ratios accompanied by high cellular contents of total microcystin and the nitrogen-rich variant microcystin-RR. Comparable patterns were found in Microcystis-dominated lakes, where the relative microcystin-RR content increased with the seston N:C ratio. In total, our results are largely consistent with the carbon-nutrient balance hypothesis, and warn that a combination of rising CO(2) and nitrogen enrichment will affect the microcystin composition of harmful cyanobacteria.