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.     

Contrasting trophic‐cascade effects driven by variation in morphology of the perches used by a larval damselfly

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Seasonal shifts in the importance of bottom–up and top–down factors on stream periphyton community structure

We examined the importance of temporal variability in top–down and bottom–up effects on the accumulation of stream periphyton, which are complex associations of autotrophic and heterotrophic microorganisms. Periphyton contributes to primary production and nutrient cycling and serves as a food resource for herbivores (grazers). Periphyton growth is often limited by the availability of nitrogen and phosphorus, and biomass can be controlled by grazers. In this study we experimentally manipulated nutrients and grazers simultaneously to determine the relative contribution of bottom–up and top–down controls on periphyton over time. We used nutrient diffusing substrates to regulate nutrient concentrations and an underwater electric field to exclude grazing insects in three sequential 16–17 day experiments from August to October in montane Colorado, USA. We measured algal biomass, periphyton organic mass, and algal community composition in each experiment and determined densities of streambed insect species, including grazers. Phosphorus was the primary limiting nutrient for algal biomass, but it did not influence periphyton organic mass across all experiments. Effects of nutrient additions on algal biomass and community composition decreased between August and October. Grazed substrates supported reduced periphyton biomass only in the first experiment, corresponding to high benthic abundances of a dominant mayfly grazer (Rhithrogena spp.). Grazed substrates in the first experiment also showed altered algal community composition with reduced diatom relative abundances, presumably in response to selective grazing. We showed that top–down grazing effects were strongest in late summer when grazers were abundant. The effects of phosphorus additions on algal biomass likely decreased over time because temperature became more limiting to growth than nutrients, and because reduced current velocity decreased nutrient uptake rates. These results suggest that investigators should proceed with caution when extending findings based on short‐term experiments. Furthermore, these results support the need for additional seasonal‐scale field research in stream ecology.     

Long-term warming affects ecosystem functioning through species turnover and intraspecific trait variation

Effects of climate change on natural ecosystems can be mediated by ecological processes, but also by rapid evolutionary adaptations and/or non-heritable trait changes in organisms. So far, most studies testing the importance of inter- versus intraspecific changes for how communities and their functioning responds to climate change are either short-term laboratory experiments in highly controlled (artificial) environments, or long-term field surveys suffering from lack of experimental manipulation. Here, we quantified how community composition and functioning has changed in response to long-term warming, including the potential direct and indirect effects via immediate and delayed physiological, non-heritable plastic, ecological, evolutionary and eco-evolutionary responses. We used a site-for-time approach, sampling sites in an artificially heated basin and a nearby area to quantify how >30 years of experimental warming in situ affects benthic grazer communities and traits of grazer taxa, as well as their contribution to a key ecosystem function: grazing on filamentous algae. The community composition shifted with warming, because a non-native species was highly common, and taxa with higher mobility, became more common in the heated areas compared to the control sites. Warming altered community functioning but the underlying mechanisms varied between traits: increased metabolism was caused by intraspecific trait change, while increased grazing rate was mainly driven by species turnover. Our results suggest that both population- and community-level processes mediate the responses of natural communities to long-term environmental change, and that the ongoing warming of coastal waters is likely to alter the functioning of key marine ecosystems.     

Biodiversity mediates top–down control in eelgrass ecosystems: a global comparative‐experimental approach

Nutrient pollution and reduced grazing each can stimulate algal blooms as shown by numerous experiments. But because experiments rarely incorporate natural variation in environmental factors and biodiversity, conditions determining the relative strength of bottom–up and top–down forcing remain unresolved. We factorially added nutrients and reduced grazing at 15 sites across the range of the marine foundation species eelgrass (Zostera marina) to quantify how top–down and bottom–up control interact with natural gradients in biodiversity and environmental forcing. Experiments confirmed modest top–down control of algae, whereas fertilisation had no general effect. Unexpectedly, grazer and algal biomass were better predicted by cross‐site variation in grazer and eelgrass diversity than by global environmental gradients. Moreover, these large‐scale patterns corresponded strikingly with prior small‐scale experiments. Our results link global and local evidence that biodiversity and top–down control strongly influence functioning of threatened seagrass ecosystems, and suggest that biodiversity is comparably important to global change stressors.     

Indirect effects shape macroalgal epifaunal communities

We tested the response of algal epifauna to the direct effects of predation and the indirect consequences of habitat change due to grazing and nutrient supply through upwelling using an abundant intertidal rhodophyte, Gelidium pristoides. We ran a mid‐shore field experiment at four sites (two upwelling sites interspersed with two non‐upwelling sites) along 450 km of the south coast of South Africa. The experiment was started in June 2014 and ran until June 2015. Four treatments (predator exclusion, grazer exclusion, control, and procedural control) set out in a block design (n = 5) were monitored monthly for algal cover for the first 6 months and every 2 months for the last 6 months. Epifaunal abundance, species composition, algal cover, and algal architectural complexity (measured using fractal geometry) were assessed after 12 months. Predation had no significant effect on epifaunal abundances, while upwelling interacted with treatment. Grazing reduced the architectural complexity of algae, with increased fractal dimensions in the absence of grazers, and also reduced algal cover at all sites, though the latter effect was only significant for upwelling sites. Epifaunal community composition was not significantly affected by the presence of herbivores or predators but differed among sites independently of upwelling; sites were more similar to nearby sites than those farther away. In contrast, total epifaunal abundance was significantly affected by grazing, when normalized to algal cover. Grazing reduced the cover of algae; thus, epifaunal abundances were not affected by the direct top‐down effects of predation but did respond to the indirect effects of grazing on habitat availability and quality. Our results indicate that epifaunal communities can be strongly influenced by the indirect consequences of biotic interactions.     

Warming affects different components of plant–herbivore interaction in a simplified community but not net interaction strength

Global warming impacts natural communities through effects on performance of individual species and through changes in the strength of interactions between them. While there is a body of evidence of the former, we lack experimental evidence on potential changes in interaction strengths. Knowledge about multispecies interactions is fundamental to understand the regulation of biodiversity and the impact of climate change on communities. This study investigated the effect of warming on a simplified community consisting of three species: rosy apple aphid Dysaphis plantaginea feeding on plantain, Plantago lanceolata, and a heterospecific neighbouring plant species, perennial ryegrass, Lolium perenne. The aphid does not feed on L. perenne. The experimental design consisted of monocultures and mixtures of L. perenne and P. lanceolata at three temperature levels. We did not find indication for indirect temperature effects on D. plantaginea through changes in leaf nitrogen or relative water content. However, experimental warming affected the life history traits of the aphid directly, in a non‐linear manner. Aphids performed best at moderate warming, where they grew faster and had a shorter generation time. In spite of the increased population growth of the aphids under warming, the herbivory rates were not changed and consequently the plant–herbivore interaction was not altered under warming. This suggests reduced consumption rates at higher temperature. Also plant competition affected the aphids but through an interaction with temperature. We provide proof‐of‐concept that net interactions between plants and herbivores should not change under warming despite direct effects of warming on herbivores when plant–plant interaction are considered. Our study stresses the importance of indirect non–trophic interactions as an additional layer of complexity to improve our understanding of how trophic interactions will alter under climate change.     

Extreme heat changes post‐heat wave community reassembly

Climate forecasts project further increases in extremely high‐temperature events. These present threats to biodiversity, as they promote population declines and local species extinctions. This implies that ecological communities will need to rely more strongly on recovery processes, such as recolonization from a meta‐community context. It is poorly understood how differences in extreme event intensity change the outcome of subsequent community reassembly and if such extremes modify the biotic environment in ways that would prevent the successful re‐establishment of lost species. We studied replicated aquatic communities consisting of algae and herbivorous rotifers in a design that involved a control and two different heat wave intensity treatments (29°C and 39°C). Animal species that suffered heat‐induced extinction were subsequently re‐introduced at the same time and density, in each of the two treatments. The 39°C treatment led to community closure in all replicates, meaning that a previously successful herbivore species could not re‐establish itself in the postheat wave community. In contrast, such closure never occurred after a 29°C event. Heat wave intensity determined the number of herbivore extinctions and strongly affected algal relative abundances. Re‐introduced herbivore species were thus confronted with significantly different food environments. This ecological legacy generated by heat wave intensity led to differences in the failure or success of herbivore species re‐introductions. Reassembly was significantly more variable, and hence less predictable, after an extreme heat wave, and was more canalized after a moderate one. Our results pertain to relatively simple communities, but they suggest that ecological legacies introduced by extremely high‐temperature events may change subsequent ecological recovery and even prevent the successful re‐establishment of lost species. Knowing the processes promoting and preventing ecological recovery is crucial to the success of species re‐introduction programs and to our ability to restore ecosystems damaged by environmental extremes.     

Size‐balanced community reorganization in response to nutrients and warming

It is widely accepted that global warming will adversely affect ecological communities. As ecosystems are simultaneously exposed to other anthropogenic influences, it is important to address the effects of climate change in the context of many stressors. Nutrient enrichment might offset some of the energy demands that warming can exert on organisms by stimulating growth at the base of the food web. It is important to know whether indirect effects of warming will be as ecologically significant as direct physiological effects. Declining body size is increasingly viewed as a universal response to warming, with the potential to alter trophic interactions. To address these issues, we used an outdoor array of marine mesocosms to examine the impacts of warming, nutrient enrichment and altered top‐predator body size on a community comprised of the predator (shore crab Carcinus maenas), various grazing detritivores (amphipods) and algal resources. Warming increased mortality rates of crabs, but had no effect on their moulting rates. Nutrient enrichment and warming had near diametrically opposed effects on the assemblage, confirming that the ecological effects of these two stressors can cancel each other out. This suggests that nutrient‐enriched systems might act as an energy refuge to populations of species under metabolic constraints due to warming. While there was a strong difference in assemblages between mesocosms containing crabs compared to mesocosms without crabs, decreasing crab size had no detectable effect on the amphipod or algal assemblages. This suggests that in allometrically balanced communities, the expected long‐term effect of warming (declining body size) is not of similar ecological consequence to the direct physiological effects of warming, at least not over the six week duration of the experiment described here. More research is needed to determine the long‐term effects of declining body size on the bioenergetic balance of natural communities.     

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.     

Moderate weather extremes alter phytoplankton diversity—A microcosm study

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Effects of Chiton granosus (Frembly, 1827) and other molluscan grazers on algal succession in wave exposed mid-intertidal rocky shores of central Chile

Molluscan grazers can have important effects on the abundance, colonization rates, and successional pathways of algal assemblages and the entire intertidal community. In general, early successional algae are more readily consumed than corticated algae and kelps, which usually get established later in the community succession. To generalize, however, the effect of different grazers on algal assemblages must be examined on different coasts and under different scenarios. This information could help us understand the mechanisms of ecosystem processes and situations in which general models do not apply. Along the coast of Chile, humans harvest large keyhole limpets, which seem to be the only invertebrate grazers capable of controlling the dominant corticated alga Mazzaella laminarioides, a canopy-forming species that can cover extensive areas of the mid intertidal zone. In this scenario, where large limpets are harvested, the overall effects of the diverse molluscan assemblage of limpets, chitons and snails on algal succession and on corticated algae in particular are not clear. We conducted a 26-month-long experiment to evaluate the effects of molluscan grazers on mid-intertidal algal succession and to isolate the effects of Chiton granosus, the most conspicuous member of the assemblage at these tidal elevations. At sites heavily impacted by humans the molluscan grazer assemblage had strong negative effects on colonization and abundance of green algae such as ulvoids and Blidingia minima. In doing so, the grazer assemblage had a strong negative indirect effect on the establishments of chironomid fly larvae, which were only observed on green algal mats and rarely on bare rock. No significant effects were detected on epilithic microalgae, and effects on sessile invertebrates were highly variable over space and time. C. granosus also had significant negative effects on green algae but did not account for the total grazing pressure exerted by the guild. Limited foraging excursions (ca. 35 cm) from refuges and moderate site (crevice) fidelity in this species may contribute to the patchiness in green algal distribution observed in the field. Nearly 13 months after rock surface were experimentally cleared, M. laminarioides appeared in all experimental plots, but increased over three times faster in enclosures containing C. granosus than in exclosures plots or controls, suggesting that moderate levels of herbivory could actually facilitate the establishment of this alga in the succession and that the green algal cover found in the absence of grazers may delay its establishment.     

Cascading effects of predatory fish on the composition of benthic algae in high‐altitude streams

Cascading effects of predators can affect ecosystem properties by changing plant biomass, distribution and assemblage composition. Using data from field surveys and whole‐stream experiments we tested the hypothesis that predatory trout change assemblage composition of benthic algae in high‐elevation streams mediated by grazer behavior. Field surveys revealed that the taxonomic composition of algal assemblages differed significantly between streams that contained trout and those that were fishless; but comparisons of palatable versus unpalatable algal taxa between fish and fishless streams were equivocal because of high natural variability. Therefore, we tested for a behavioral (non‐consumptive) trophic cascade experimentally by adding brook trout chemical cues to six naturally fishless streams for 25 days and compared responses of grazers and algae to six reference streams without fish cues added. Algal response variables included rates of change in the abundance of three physiognomic categories, from most palatable (attached erect and prostrate diatoms) to least palatable (non‐diatoms), as determined from food selectivity analyses of the most common grazers (mayflies and caddisflies). Fish cues did not affect the mean densities or changes in densities of total grazers or any individual grazer species. However, in streams where fish cues were added, rates of accrual of attached erect diatoms, which was the preferred algal type for the grazer most vulnerable to trout predation (Baetis), were higher and their densities increased significantly faster with increasing densities of this grazer species than in reference streams. Results of his experiment support the hypothesis that predator induced suppression of grazer foraging behavior, rather than cascading effects of top predators on grazer density, may contribute to variation in the composition of algal assemblages among streams by allowing proliferation of most palatable algal species.     

Predaceous insects may limit algal grazers in high-nutrient environments: implications for wastewater remediation in open bioreactors

Algal biofuel has potential as a source of renewable fuel and a tool for wastewater remediation. Open algal bioreactors fertilized with wastewater can have net energy gain but are vulnerable to colonization by algal grazers. However, colonizing predaceous insects may limit grazer impacts on algae. Here, we investigate the effects of grazers, predators, and invading algae species on algal production and community structure in high-nutrient environments. First, we grew diverse algal assemblages in treated municipal wastewater in a greenhouse with Daphnia grazers and different insect predators that were added experimentally. When Daphnia were present without predators, they eliminated suspended algae. But, dragonfly larvae [Odonata: Libellulidae] and backswimmers [Hemiptera: Notonectidae], but not larval diving beetles [Coloeoptera: Dytiscidae], suppressed Daphnia allowing suspended algae to persist. Second, we grew Chlorella algae in field tanks that were open or protected from natural invertebrate colonization and half the tanks received wild-collected plankton in a factorial design. Mosquito larvae [Culex sp.] readily colonized open tanks and reduced algal mass and dissolved phosphorus concentrations. Colonist addition to open tanks shifted algal functional and taxonomic composition but did not impact suspended algal production. Our study indicates that large numbers of grazer individuals can rapidly colonize open bioreactors. Experimentally added and naturally colonizing grazers altered algal community structure and reduced algal standing crops but may also aid in nutrient removal from wastewater-fed bioreactors. Effective operation of open algal bioreactors must consider cultivated algae species’ vulnerability to competition and local grazers as well as the ability of potential predators to both naturally disperse into bioreactors and to control grazers.     

典型红树林生态系统藻类多样性及其在生态过程中的作用

藻类是红树林生态系统重要的生物类群, 根据生态习性可分为浮游植物、底栖微藻和大型藻类三个生态类群, 它们在红树林生态系统生物多样性、初级生产、元素循环等方面起着重要作用。但在红树林生态系统中, 关注重点多集中在红树植物和动物, 对其中的藻类重视不够, 且多数研究集中在近20年以及亚洲的红树林区。事实上, 红树林生态系统藻类非常丰富, 其多样性研究有助于深入揭示红树林生态系统的结构与功能。本文介绍了红树林生态系统藻类的组成类群及其重要性, 重点对红树林区浮游植物、底栖硅藻和大型海藻的种类组成、地理分布及其与初级生产力、水质污染、元素循环、碳库形成等生态过程中的作用的研究动态和进展等进行了总结。根据已有研究, 红树林区浮游植物和底栖硅藻的种类数一般为几十到上百种, 其中硅藻在种类和数量上都占绝对优势, 它们是重要的初级生产者、饵料生物和水质污染指示生物; 红树林区底栖大型藻类主要由红藻、绿藻、褐藻、蓝藻组成, 绿藻的种类较多, 红藻在数量上占优势; 藻类是红树林湿地碳库的重要贡献者, 在红树林湿地生态系统碳汇和碳循环中起重要作用。红树林生态系统是个高度动态和异质的系统, 今后应加强红树林藻类多样性的长周期、大尺度变化及不同生境藻类的综合研究, 关注大陆径流和潮汐对藻类多样性和蓝碳的影响, 借助沉积物藻类记录, 探明红树林区藻类的长周期变化, 反演气候变化和人类活动对红树林生态系统的影响过程和机制。     

The relative role of dispersal and local interactions for alpine plant community diversity under simulated climate warming

Most studies on factors determining diversity are conducted in temperate or warm regions, whereas studies in climatically harsh and low productivity areas, such as alpine regions, are rare. We examined the relative roles of seed availability and different biotic and abiotic factors for the diversity of an alpine plant community in southern Norway. Furthermore, because climate warming is predicted to be an important driver of alpine species diversity, we assessed how the relative impacts of dispersal and local interactions on diversity might change under experimental warming (open top chambers, OTCs).
Addition of seeds from 27 regional species increased community diversity. The establishment of the species was negatively related both to the diversity of the existing system and the cover of the abundant dwarf shrub Dryas octopetala . These results show that both species dispersal limitation and local biotic interactions are important factors for alpine plant community diversity. Despite relatively harsh environmental conditions and low productivity, competition from the resident vegetation appeared to have a greater role for species establishment and diversity than facilitation and experimental warming. Higher temperature appeared to increase the negative relationship between resident species diversity and species establishment. This may suggest that climate warming can increase the role of interspecific competition for alpine plant community structure, and thus alter the long-term effects of biotic interactions on diversity.     

Exploring the compass of potential changes induced by climate warming in plant communities

New models are required to predict the impacts of future climate change on biodiversity. A move must be made away from individual models of single species toward approaches with synergistically interacting species. The focus should be on indirect effects due to biotic interactions. Here we propose a new parsimonious approach to simulate direct and indirect effects of global warming on plant communities. The methodology consists of five steps: a) field survey of species abundances, b) quantitative assessment of species co-occurrences, c) assignment of a theorised effect of increased temperature on each species, d) creation of a community model to project community dynamics, and e) exploration of the potential range of temperature change effects on plant communities.We explored the possible climate-driven dynamics in an alpine vegetation community and gained insights into the role of biotic interactions as determinants of plant species response to climate change at local scale. The study area was the uppermost portion of Alpe delle Tre Potenze (Northern Apennines, Italy) from 1500 m up to the summit at 1940 m.Our work shows that: 1) unexpected climate-driven dynamics can emerge, 2) interactive communities with indirect effects among species can overcome direct effects induced by global warming; 3) if just one or few species react to global warming the new community configuration could be unexpected and counter-intuitive; 4) timing of species reactions to global warming is an important driver of community dynamics; 5) using simulation models with a limited amount of data in input, it is possible to explore the full range of potential changes in plant communities induced by climate warming.     

Epiphyte grazing enhances productivity of remnant seagrass patches

Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P. australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C. fragum density, while P. australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above‐ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting.     

Erosion of community diversity and stability by herbivore removal under warming

Climate change has the potential to influence the persistence of ecological communities by altering their stability properties. One of the major drivers of community stability is species diversity, which is itself expected to be altered by climate change in many systems. The extent to which climatic effects on community stability may be buffered by the influence of species interactions on diversity is, however, poorly understood because of a paucity of studies incorporating interactions between abiotic and biotic factors. Here, I report results of a 10-year field experiment, the past 7 years of which have focused on effects of ongoing warming and herbivore removal on diversity and stability within the plant community, where competitive species interactions are mediated by exploitation through herbivory. Across the entire plant community, stability increased with diversity, but both stability and diversity were reduced by herbivore removal, warming and their interaction. Within the most species-rich functional group in the community, forbs, warming reduced species diversity, and both warming and herbivore removal reduced the strength of the relationship between diversity and stability. Species interactions, such as exploitation, may thus buffer communities against destabilizing influences of climate change, and intact populations of large herbivores, in particular, may prove important in maintaining and promoting plant community diversity and stability in a changing climate.     

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.