The role of light availability and herbivory on algal responses to nutrient enrichment in a riparian wetland,Alaska

We investigated how the relative availability of solar radiation in the presence or absence of grazing alters the ability of benthic algae to respond to nutrient enrichment in an Alaskan marsh. We used a factorial mesocosm experiment that included nutrient enrichment (enriched or control), grazing (grazed or ungrazed), and light (unshaded or shaded) to simulate shading by macrophytes early and late in the growing season, respectively. We found stronger effects of grazers and nutrients compared to light on benthic algal biomass and taxonomic composition. Algal biomass increased in nutrient‐enriched treatments and was reduced by grazing. Shading did not have an effect on algal biomass or taxonomic composition, but the concentration of chl a per algal biovolume increased with shading, demonstrating the ability of algae to compensate for changes in light availability. Algal taxonomic composition was more affected by grazer presence than nutrients or light. Grazer‐resistant taxa (basal filaments of Stigeoclonium) were replaced by diatoms (Nitzschia) and filamentous green algae (Ulothrix) when herbivores were removed. The interacting and opposing influences of nutrients and grazing indicate that the algal community is under dual control from the bottom‐up (nutrient limitation) and from the top‐down (consumption by herbivores), although grazers had a stronger influence on algal biomass and taxonomic composition than nutrient enrichment. Our results suggest that low light availability will not inhibit the algal response to elevated nutrient concentrations expected with ongoing climate change, but grazers rapidly consume algae following enrichment, masking the effects of elevated nutrients on algal production.     

Promotion of harmful algal blooms by zooplankton predatory activity

The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients. Eutrophication raises nutrient levels, but does not simply increase normal predator-prey activity; rather, harmful algal bloom (HAB) events develop often with serious ecological and aesthetic implications. Generally, HAB species are outwardly poor competitors for nutrients, while their development of grazing deterrents during nutrient stress ostensibly occurs too late, after the nutrients have largely been consumed already by fast-growing non-HAB species. A new mechanism is presented to explain HAB dynamics under these circumstances. Using a multi-nutrient predator-prey model, it is demonstrated that these blooms can develop through the self-propagating failure of normal predator-prey activity, resulting in the transfer of nutrients into HAB growth at the expense of competing algal species. Rate limitation of this transfer provides a continual level of nutrient stress that results in HAB species exhibiting grazing deterrents protecting them from top-down control. This process is self-stabilizing as long as nutrient demand exceeds supply, maintaining the unpalatable status of HABs; such events are most likely under eutrophic conditions with skewed nutrient ratios.     

Responses to fish predation and nutrients by plankton at different levels of taxonomic resolution

1. To improve mechanistic understanding of plankton responses to eutrophication, a mesocosm experiment was performed in the shallow littoral zone of a south Swedish lake, in which nutrient and fish gradients were crossed in a fully factorial design. 2. Food chain theory accurately predicted total biomass development of both phyto‐ and zooplankton. However, separating zooplankton and algae into finer taxonomic groups revealed a variety of responses to both nutrient and fish gradients. 3. That both nutrients and fish are important for phytoplankton dynamics was seen more clearly when viewing each algal group separately, than drawing conclusions only from broad system variables such as chlorophyll a concentration or total phytoplankton biovolume. 4. In some taxa, physiological constraints (e.g. sensitivity to high pH and low concentrations of free CO₂) and differences in competitive ability may be more important for the biomass development than fish predation, grazing by herbivorous zooplankton, and nutrient availability. 5. We conclude that food chain theory accurately predicted responses in system variables, such as total zooplankton or algal biomass, which are shaped by the dynamics of certain strong interactors (‘keystone species’), such as large cladocerans, cyanobacteria and edible algae (<50 μm), whereas responses at finer taxonomic levels cannot be predicted from current theory.     

Predator diet breadth influences the relative importance of bottom-up and top-down control of prey biomass and diversity

We investigated the effects of predator diet breadth on the relative importance of bottom-up and top-down control of prey assemblages, using microbial food webs containing bacteria, bacterivorous protists and rotifers, and two different top predators. The experiment used a factorial design that independently manipulated productivity and the presence or absence of two top predators with different diet breadths. Predators included a "specialist" predatory ciliate Euplotes aediculatus, which was restricted to feeding on small prey, and a "generalist" predatory ciliate Stentor coeruleus, which could feed on the entire range of prey sizes. Both total prey biomass and prey diversity increased with productivity in the predator-free control and specialist predator treatments, a pattern consistent with bottom-up control, but both remained unchanged by productivity in the generalist predator treatment, a pattern consistent with top-down control. Linear food chain models adequately described responses in the generalist predator treatment, whereas food web models incorporating edible and inedible prey (which can coexist in the absence of predators) adequately described responses in the specialist predator treatment. These results suggest that predator diet breadth can play an important role in modulating the relative strength of bottom-up and top-down forces in ecological communities.     

Linking herbivore-induced defences to population dynamics

1. Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi‐ and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator–prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2. We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3. A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4. We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities.     

Environmental context determines multi‐trophic effects of consumer species loss

Loss of biodiversity and nutrient enrichment are two of the main human impacts on ecosystems globally, yet we understand very little about the interactive effects of multiple stressors on natural communities and how this relates to biodiversity and ecosystem functioning. Advancing our understanding requires the following: (1) incorporation of processes occurring within and among trophic levels in natural ecosystems and (2) tests of context‐dependency of species loss effects. We examined the effects of loss of a key predator and two groups of its prey on algal assemblages at both ambient and enriched nutrient conditions in a marine benthic system and tested for interactions between the loss of functional diversity and nutrient enrichment on ecosystem functioning. We found that enrichment interacted with food web structure to alter the effects of species loss in natural communities. At ambient conditions, the loss of primary consumers led to an increase in biomass of algae, whereas predator loss caused a reduction in algal biomass (i.e. a trophic cascade). However, contrary to expectations, we found that nutrient enrichment negated the cascading effect of predators on algae. Moreover, algal assemblage structure varied in distinct ways in response to mussel loss, grazer loss, predator loss and with nutrient enrichment, with compensatory shifts in algal abundance driven by variation in responses of different algal species to different environmental conditions and the presence of different consumers. We identified and characterized several context‐dependent mechanisms driving direct and indirect effects of consumers. Our findings highlight the need to consider environmental context when examining potential species redundancies in particular with regard to changing environmental conditions. Furthermore, non‐trophic interactions based on empirical evidence must be incorporated into food web‐based ecological models to improve understanding of community responses to global change.     

Algal defenses,population stability,and the risk of herbivore extinctions: a chemostat model and experiment

The effects of inducible defenses and constitutive defenses on population dynamics were investigated in a freshwater plankton system with rotifers as predators and different algal strains as prey. We made predictions for these systems using a chemostat predator–prey model and focused on population stability and predator persistence as a function of flow-through rate. The model exhibits three major types of behavior at a high nutrient concentration: (1) at high dilution rates, only algae exist; (2) at intermediate dilution rates, algae and rotifers show stable coexistence; (3) at low dilution rates, large population fluctuations occur, with low minimum densities entailing a risk of stochastic rotifer extinctions. The size and location of the corresponding areas in parameter space critically depend on the type of algal defense strategy. In an 83-day high-nutrient chemostat experiment we changed the dilution rate every 3 weeks, from 0.7 to 0.5 to 0.3 to 0.1 per day. Within this range of dilution rates, rotifers and algae coexisted, and population fluctuations of algae clearly increased as dilution rates decreased. The CV of herbivore densities was highest at the end of the experiment, when the dilution rate was low. On day 80, herbivorous rotifers had become undetectable in all three chemostats with permanently defended algae (where rotifer densities had already been low) and in two out of three chemostats where rotifers had been feeding on algae with inducible defenses (that represented more edible food). We interpret our results in relation to the paradox of enrichment.     

Quantification of invertebrate predation and herbivory in food chains of low complexity

Zooplankton grazing impact on algae, heterotrophic flagellates and bacteria, as well as invertebrate predation on herbivorous zooplankton, were investigated in two sub-Antarctic lakes with extremely simple food chains. The two species of herbivorous zooplankton present in the lakes (the copepods boeckella michaelseni and Pseudoboeckella poppei) exerted substantial grazing pressure on algae. However, the dominant algal species exhibited properties that enabled them to avoid (large size or extruding spines, e.g. Staurastrum sp., Tribonema sp.) or compensate (recruitment from the sediment, Mallomonas sp.) grazing. There are only two potential invertebrate predators on the herbivorous copepods in the two lakes: the copepod Parabroteas sarsi and the diving beetle Lancetes claussi. Vertebrate predators are entirely abscent from sub-Antarctic lakes. Based on our experiments, we estimated that the predators would remove at most about 0.4% of the herbivorous copepods per day, whereas planktivorous fish, if present in the lakes, would have removed 5–17% of the zooplankton each day. Consequently, the invertebrate predators in these high-latitude lakes had only a marginal predation impact compared to the predation pressure on zooplankton in the presence of vertebrate predators in temperate lakes. The study of these simple systems with only two quantitatively functionally important trophic links, suggests that high grazing pressure foreces the algal community towards forms with grazer resistant adaptations such as large size, recruitment from another habitat, and grazer avoidance spines. We propose that due to such adaptations, predictions from food web theory are only partly corroborated, i.e. algal biomass actually increases with increasing productivity, although the grazer community is released from predation. In more species-rich and complex systems, e.g temperate lakes with three functionally important links, such adaptations are likely to be even more important, and, consequently, the observable effects of trophic interactions from top predators on lower trophic levels even more obscured.     

Differential responses of size‐based functional groups to bottom–up and top–down perturbations in pelagic food webs: a meta‐analysis

We performed a meta‐analysis of 31 lake mesocosm experiments to investigate differences in the responses of pelagic food chains and food webs to nutrient enrichment and fish presence. Trophic levels were divided into size‐based functional groups (phytoplankton into highly edible and poorly edible algae, and zooplankton into small herbivores, large herbivores and omnivorous zooplankton) in the food webs. Our meta‐analysis shows that 1) nutrient enrichment has a positive effect on phytoplankton and zooplankton, while fish presence has a positive effect on phytoplankton and a negative effect on zooplankton in the food chains; 2) nutrient enrichment has a positive effect on highly edible algae and small herbivores, but no effect on poorly edible algae, large herbivores and omnivorous zooplankton in the food webs. Planktivorous fish have a positive effect on highly edible algae and small herbivores, a negative effect on large herbivores and omnivorous zooplankton, and no effect on poorly edible algae. Our meta‐analysis confirms that nutrient enrichment and planktivorous fish affect functional groups differentially within trophic levels, revealing important changes in the functioning of food webs. The analysis of fish effects shows the well‐described trophic cascade in the food chain and reveals two trophic cascades in the food web: one transmitted by large herbivores that benefit highly edible phytoplankton, and one transmitted by omnivorous zooplankton that benefit small herbivores. Comparison between the responses of food webs and simple food chains also shows consistent biomass compensation between functional groups within trophic levels.     

Can a community of small-bodied grazers control phytoplankton in rivers?

1. Phytoplankton, zooplankton and grazing were monitored throughout the growing season for three years (1994–96) in the Belgian section of the River Meuse.
2. A size structure analysis of the algal community shows that there was a summer shift toward larger algal units, following a decline in phytoplankton biomass. These changes occurred after an increase in zooplankton biomass and diversity.
3. Daily filtration rates of grazers ranged from 1 to 113% day^–1 and maxima were observed during the summer period. Higher rates tended to correspond with peaks of rotifer biomass. A decline in total phytoplankton biomass within two weeks followed the increase in zooplankton biomass and filtration rate. A rapid biomass recovery was then observed, along with a shift of the algal community toward larger units. When grazing activity was not sustained, due to zooplankton fluctuations, the change in phytoplankton size structure was less marked.
4. We suggest that the composition of the phytoplankton community of large rivers may at times be controlled by grazers. However, such biotic interactions can take place only when physical constraints are reduced, i.e. when discharge is low, and when increased transfer time, high temperature and availability of grazeable algae allow high zooplankton biomass.     

Growth,Grazing, and Starvation Survival in Three Heterotrophic Dinoflagellate Species

To assess the effects of fluctuating prey availability on predator population dynamics and grazing impact on phytoplankton, we measured growth and grazing rates of three heterotrophic dinoflagellate species—Oxyrrhis marina, Gyrodinium dominans and Gyrodinium spirale—before and after depriving them of phytoplankton prey. All three dinoflagellate species survived long periods (> 10 d) without algal prey, coincident with decreases in predator abundance and cell size. After 1–3 wks, starvation led to a 17–57% decrease in predator cell volume and some cells became deformed and transparent. When re‐exposed to phytoplankton prey, heterotrophs ingested prey within minutes and increased cell volumes by 4–17%. At an equivalent prey concentration, continuously fed predators had ~2‐fold higher specific growth rates (0.18 to 0.55 d^?1) than after starvation (?0.16 to 0.25 d^?1). Maximum specific predator growth rates would be achievable only after a time lag of at least 3 d. A delay in predator growth poststarvation delays predator‐induced phytoplankton mortality when prey re‐emerges at the onset of a bloom event or in patchy prey distributions. These altered predator‐prey population dynamics have implications for the formation of phytoplankton blooms, trophic transfer rates, and potential export of carbon.     

Top-down control of natural phyto- and bacterioplankton prey communities by Daphnia magna and by the natural zooplankton community of the hypertrophic Lake Blankaart

Biomanipulation, through the reduction of fish abundance resulting in an increase of large filter feeders and a stronger top-down control on algae, is commonly used as a lake restoration tool in eutrophic lakes. However, cyanobacteria, often found in eutrophic ponds, can influence the grazing capacity of filter feeding zooplankton. We performed grazing experiments in hypertrophic Lake Blankaart during two consecutive summers (1998, with and 1999, without cyanobacteria) to elucidate the influence of cyanobacteria on the grazing pressure of zooplankton communities. We compared the grazing pressure of the natural macrozooplankton community (mainly small to medium-sized cladocerans and copepods) with that of large Daphnia magna on the natural bacterioplankton and phytoplankton prey communities. Our results showed that in the absence of cyanobacteria, Daphnia magna grazing pressure on bacteria was higher compared to the grazing pressure of the natural zooplankton community. However, Daphnia grazing rates on phytoplankton were not significantly different compared to the grazing rates of the natural zooplankton community. When cyanobacteria were abundant, grazing pressure of Daphnia magnaseemed to be inhibited, and the grazing pressure on bacteria and phytoplankton was similar to that of the natural macrozooplankton community. Our results suggest that biomanipulation may not always result in a more effective top-down control of the algal biomass.     

Nonconsumptive effects in a multiple predator system reduce the foraging efficiency of a keystone predator

Many studies have demonstrated that the nonconsumptive effect (NCE) of predators on prey traits can alter prey demographics in ways that are just as strong as the consumptive effect (CE) of predators. Less well studied, however, is how the CE and NCE of multiple predator species can interact to influence the combined effect of multiple predators on prey mortality. We examined the extent to which the NCE of one predator altered the CE of another predator on a shared prey and evaluated whether we can better predict the combined impact of multiple predators on prey when accounting for this influence. We conducted a set of experiments with larval dragonflies, adult newts (a known keystone predator), and their tadpole prey. We quantified the CE and NCE of each predator, the extent to which NCEs from one predator alters the CE of the second predator, and the combined effect of both predators on prey mortality. We then compared the combined effect of both predators on prey mortality to four predictive models. Dragonflies caused more tadpoles to hide under leaf litter (a NCE), where newts spend less time foraging, which reduced the foraging success (CE) of newts. Newts altered tadpole behavior but not in a way that altered the foraging success of dragonflies. Our study suggests that we can better predict the combined effect of multiple predators on prey when we incorporate the influence of interactions between the CE and NCE of multiple predators into a predictive model. In our case, the threat of predation to prey by one predator reduced the foraging efficiency of a keystone predator. Consequently, the ability of a predator to fill a keystone role could be compromised by the presence of other predators.     

Resuspension of algal cells by benthivorous fish boosts phytoplankton biomass and alters community structure in shallow lakes

1. Positive effects of fish on algal biomass have variously been attributed to cascading top‐down effects and to nutrient enrichment by fish excretion. 2. Here, we used a combination of field and laboratory approaches to test an additional hypothesis, namely that the physical resuspension of settled algal cells by fish enhances algal biomass and alters community composition. 3. A multi‐lake survey showed that phytoplankton biomass and the fraction of motile algae increased with the concentration of inorganic suspended solids. This correlation could not be explained by wind‐induced resuspension because of the small size of the lakes. 4. In an enclosure experiment, chlorophyll‐a concentration, phytoplankton abundance and inorganic suspended solids increased significantly in the presence of Cyprinus carpio (common carp), but only if the fish had access to the sediment. No such effects were seen when a net prevented carp reaching the sediment. 5. The effects of enhanced nutrients and reduced zooplankton grazing as a result of fish feeding could not (fully) explain these observations, suggesting that the resuspension by carp of settled algae from a surface film on the sediment was the major factor in the outcome of the experiment. 6. An increase in diatoms and green algae (organisms with a relatively large sedimentation velocity) only in enclosures where carp could reach the sediment supported this view. 7. Several lines of evidence indicate that fish‐induced resuspension of algal cells from the sediment is an important mechanism that affects phytoplankton biomass and community composition in shallow lakes.     

Prey subsidy or predator cue? Direct and indirect effects of caged predators on aquatic consumers and resources

The non-consumptive effects of predators on prey can affect prey phenotypes, potentially having important consequences for communities due to trait-mediated indirect interactions. Predicting non-consumptive effects and their impacts on communities can be difficult because predators can affect resources directly through nutrient cycling and indirectly by altering prey resource use, which can lead to complex interactions among resources and consumers. In this study we examined the effects of caged dragonfly predators on aquatic resources in the presence and absence of two focal herbivores, the tadpoles of Neotropical tree frogs Agalychnis callidryas and Dendropsophus ebraccatus. We crossed the presence/absence of caged dragonflies with four tadpole treatments: no tadpoles, each tadpole species alone, and both species together to examine interactions among tadpole composition, predator presence, and time on tadpole growth, resources, and zooplankton abundances. Predator effects on growth changed through ontogeny and was species-dependent. Predators initially reduced then dramatically increased A. callidryas growth, but had no effect on D. ebraccatus. Predators also increased the abundances of both periphyton and phytoplankton. However, there was no evidence of a trait-mediated trophic cascade (i.e., tadpole by predator interaction). Instead, nutrients from prey carcass subsidies likely played an increasingly important role in facilitating resources, and shaping tadpole growth, competitive interactions, and zooplankton abundances through time. In nutrient-poor aquatic systems the release of nutrients via the consumption of terrestrially derived prey items by aquatic predators may have important impacts on food webs by facilitating resources independent of the role of trait-mediated trophic cascades.     

Intraspecific variation in a predator drives cascading variation in primary producer community composition

Predation has important cascading impacts on primary producer biomass and community composition in many ecosystems. While most studies have focused on the consequences of interspecific or density differences in predators, it is recognized that phenotypic variation within species can have strong and cascading community and ecosystem consequences at lower trophic levels. In coastal New England lakes, both the presence and life history form of the zooplanktivorous fish alewife, Alosa pseudoharengus, have strong influence on the biomass, size structure and community composition of crustacean zooplankton communities. Here we test the hypothesis that alewife presence and life history will have cascading impacts on phytoplankton biomass and community composition in a mesocosm experiment that previously reported strong biomass and compositional differences of crustacean zooplankton communities among alewife treatments. We show that alewife life history led to small but statistically significant differences in phytoplankton community composition among treatments. This compositional difference was driven primarily by an increase in the density of two edible phytoplankton genera associated with lower zooplankton biomass in the anadromous alewife treatment. Our results show that intraspecific variation in a predator can have cascading effects on primary producer communities. However we did not observe significant differences in total algal biomass.     

Trophic cascades in a temperate seagrass community

We assessed the relative importance of bottom–up and top–down processes in structuring an eelgrass community in Sweden, a system impacted both by eutrophication and overfishing. Using artificial seagrass as substrate, we manipulated nutrient levels and predator abundance in a full‐factorial cage‐experiment. The results revealed a seagrass community dominated by strong top–down processes controlling the aggregate biomass of mesograzers and macroalgae. In the absence of predators the large amphipod Gammarus locusta became very abundant resulting in a leaf community with low biomass of algae and smaller mobile fauna. One enclosed gobid fish predator reduced the abundance of adult G. locusta by >90%, causing a three to six times increase in the biomass of algae, smaller mesograzers and meiofauna. Numerous small predators in uncaged habitats reduced the biomass of G. locusta and other mesograzers by >95% in comparison to the fish treatment, further increasing the biomass of epiphytic algae and meiofauna. Although water column nutrient enrichment caused a temporal bloom of the filamentous macroalgae Ulva spp., no significant nutrient‐effects were found on the algal community at the end of the experiment. The only lasting nutrient‐effect was a significant increase in the biomass of G. locusta, but only in the absence of ambient predators. These results demonstrate that mesograzers can respond to enhanced food supply, increase their biomass and control the algal growth when predation rates are low. However, in the assessed system, high predation rates appear to make mesograzers functionally extinct, causing a community‐wide trophic cascade that promotes the growth of ephemeral algae. This top–down effect could penetrate down, despite a complex food‐web because the interaction strength in the community was strongly skewed towards two functionally dominant algal and grazer species that were vulnerable to consumption. These results indicate that overexploitation of gadoid fish may be linked to increased macroalgal blooms and loss of eelgrass in the area through a trophic cascade affecting the abundance of mesograzers.     

Effects of consumers and enrichment on abundance and diversity of benthic algae in a rocky intertidal community

Human alteration of nutrient cycling and the densities of important consumers have intensified the importance of understanding how nutrients and consumers influence the structure of ecological systems. We examined the effects of both grazing and nutrient enrichment on algal abundance and diversity in a high-intertidal limpet-macroalgal community on the South Island of New Zealand, a relatively nutrient-poor environment. We used a fully factorial design with three levels each of grazing (manipulations of limpet and snail densities) and nutrients (nutrient-diffusers attached to the rock). Top-down control by grazers appears to be the driving organizing mechanism for algal communities in this system, with strong negative effects of grazing on algal diversity and abundance across all levels of nutrient enrichment. However, in contrast to the conclusions drawn from the analysis of the whole algal community, there was an interactive effect of grazing and enrichment on foliose algae, an important component of the algal system. When herbivory was reduced to very low levels, enrichment generated increases in the abundance and biomass of foliose algae. As expected, top-down control was the primary determinant of algal community structure in this system, controlling abundance and diversity of macrophytes on the upper shore. Contrary to expectations, however, increased nutrients had no community-wide effects, although foliose algal abundance increases were greatest with high nutrients and reduced grazing. It seems likely that most of the corticated algal species have limited capacity to respond to nutrient pulses in this nutrient-poor environment.     

Spatial patterns and relationships between phytoplankton, zooplankton and water quality in the Saimaa lake system, Finland

The interactions between phytoplankton and zooplankton were studied in two large lakes in the Saimaa lake system, Finland. Both are subjected to substantial waste water loading, and exhibit a clear trophic gradient between the loaded and unloaded areas. The phytoplankton and zooplankton were compared in terms of composition, abundance and biomass at 34–39 stations located in different parts of the lakes. At least four mechanisms were thought to affect the composition of plankton communities: (1) the amount of nutrients (trophic gradient), (2) grazing of algae by herbivores, (3) the effect of the algal species composition on feeding by zooplankters (large, colonial algae in the more loaded parts of the lakes) and (4) the regeneration and reorganization of nutrients.     

Independent and interactive effects of nutrients and grazers on benthic algal community structure

Algal responses to nutrients, grazing by Helicopsyche borealis, and concurrent grazing by Helicopsyche and Baetis tricaudatus were examined in recirculating stream chambers. Alagl communities, dominated by Achnanthes minutissima, Cocconeis placentula, and Synedra ulna, were primarily phosphorus-limited. Algal populations responded after only 6 days of nutrient enrichment. Initially, both the adnate diatom Cocconeis and erect diatom Synedra showed positive response to nutrient enrichment. Accumulation of algal biomass between day 3 and 6 in the P enriched treatment was resulted primarily from the growth of Synedra, an overstory rosette-like diatom colony. Such a shift in dominant growth from adnate to erect diatoms is a general phenomenon in periphyton succession in the absence of disturbance. Algal species showed differential responses to an increase of Helicopsyche densities. The accrual rate of Achnanthes continuously decreased with increasing grazer densities. The accrual rates of both Cocconeis and Synedra declined but reached plateaus between medium and high grazing densities. Baetis effectively and exclusively depressed Synedra and had no significant impact on Cocconeis. After concurrent grazing, algal communities were mainly dominated by Cocconeis (approximately 80% of total algal biovolume). The grazer' s mouth structures, grazing efficiencies, and mobility may account for the differential effects of concurrent grazing on algal communities. Significant interactive effects of P and grazing by Helicopsyche indicated that both nutrient addition and grazing may exert significant impact on algal communities. However, grazing may have a much stronger effect on algae than nutrients. Our results indicate that enhancement of algal biomass by P was dampened by grazing activities and that P had no effect on algal biomass in the presence of grazers.