Bacteria and algae in stream periphyton along a nutrient gradient

1. Stream riffles in southern Ontario and western Quèbec were sampled for biomass (58 stations from 51 streams) and production (22 stations from 21 streams) of algae and bacteria in periphyton to test the hypothesis that bacteria in benthic biofilms compete with algae for nutrients. 2. Algal and bacterial biomass were positively correlated, as were algal and bacterial production. Bacterial production was also positively correlated to algal and bacterial biomass, but the relationship was not significant. The ratio of algal to bacterial biomass did not vary with nutrients whereas algal production tended to increase with nutrients more rapidly than bacterial production. 3. Instream nitrogen concentrations explained 38–58% of the variability in algal biomass and production. Bacterial abundance explained an additional 9–29% of the residual variance in algal production and biomass. However, the relationship between bacterial abundance and algal production and biomass, once nutrients were taken into account, was positive, in contrast to the predicted effect of competition. 4. Hence, we reject our original hypothesis that bacteria in biofilms compete with algae for nutrients and instead suggest that bacteria and algae in biofilms coexist in an association that offers space and resources to sustain production of both groups of organisms.     

The effect of grazing and nutrient supply on periphyton associated bacteria

The effects of nutrient additions and grazing by macro-invertebrates on periphyton-associated algae and bacteria were studied by performing an enclosure experiment on three occasions from early spring to summer at mesotrophic Lake Erken and V?dd?, at the Swedish Baltic coast. There were significant interactions between nutrient additions and grazing on bacterial biomass and specific activity in Lake Erken. Thus, the importance of either bottom-up or top-down effects could not be singled out. Bacterial biomass increased with enrichment only in the absence of grazers. Grazer presence tended to increase bacterial biomass in ambient nutrient conditions, but to decrease bacterial biomass under enrichment. For specific activity the positive response to enrichment was restricted to grazer presence. Hence, grazing by macro-invertebrates may have an indirect positive effect on bacterial activity by enhancing nutrient conditions through their feeding activities and/or fecal pellets production. In addition, we found a significant relationship between bacterial production and chlorophyll a at both sites. This relationship weakened in the presence of macro-invertebrates. Thus, the importance of internal nutrient regeneration by bacteria and algae decreased, possibly due to increased nutrient availability, in the presence of macro-invertebrate grazers.     

Nutrient loading and grazing by the minnow Phoxinus erythrogaster shift periphyton abundance and stoichiometry in mesocosms

1. Anthropogenic activities in prairie streams are increasing nutrient inputs and altering stream communities. Understanding the role of large consumers such as fish in regulating periphyton structure and nutritional content is necessary to predict how changing diversity will interact with nutrient enrichment to regulate stream nutrient processing and retention. 2. We characterised the importance of grazing fish on stream nutrient storage and cycling following a simulated flood under different nutrient regimes by crossing six nutrient concentrations with six densities of a grazing minnow (southern redbelly dace, Phoxinus erythrogaster) in large outdoor mesocosms. We measured the biomass and stoichiometry of overstory and understory periphyton layers, the stoichiometry of fish tissue and excretion, and compared fish diet composition with available algal assemblages in pools and riffles to evaluate whether fish were selectively foraging within or among habitats. 3. Model selection indicated nutrient loading and fish density were important to algal composition and periphyton carbon (C): nitrogen (N). Nutrient loading increased algal biomass, favoured diatom growth over green algae and decreased periphyton C : N. Increasing grazer density did not affect biomass and reduced the C : N of overstory, but not understory periphyton. Algal composition of dace diet was correlated with available algae, but there were proportionately more diatoms present in dace guts. We found no correlation between fish egestion/excretion nutrient ratios and nutrient loading or fish density despite varying N content of periphyton. 4. Large grazers and nutrient availability can have a spatially distinct influence at a microhabitat scale on the nutrient status of primary producers in streams.     

A comparison of algal,macroinvertebrate, and fish assemblage indices for assessing low-level nutrient enrichment in wadeable Ozark streams

Biotic indices for algae, macroinvertebrates, and fish assemblages can be effective for monitoring stream enrichment, but little is known regarding the value of the three assemblages for detecting perturbance as a consequence of low-level nutrient enrichment. In the summer of 2006, we collected nutrient and biotic samples from 30 wadeable Ozark streams that spanned a nutrient-concentration gradient from reference to moderately enriched conditions. Seventy-three algal metrics, 62 macroinvertebrate metrics, and 60 fish metrics were evaluated for each of the three biotic indices. After a group of candidate metrics had been identified with multivariate analysis, correlation procedures and scatter plots were used to identify the four metrics having strongest relations to a nutrient index calculated from log transformed and normalized total nitrogen and total phosphorus concentrations. The four metrics selected for each of the three biotic indices were: algae—the relative abundance of most tolerant diatoms, the combined relative abundance of three species of Cymbella, mesosaprobic algae percent taxa richness, and the relative abundance of diatoms that are obligate nitrogen heterotrophs; macroinvertebrate—the relative abundance of intolerant organisms, Baetidae relative abundance, moderately tolerant taxa richness, and insect biomass; fish—herbivore and detritivore taxa richness, pool species relative abundance, fish catch per unit effort, and black bass (Micropterus spp.) relative abundance.All three biotic indices were negatively correlated to nutrient concentrations but the algal index had a higher correlation (rho = ?0.89) than did the macroinvertebrate and fish indices (rho = ?0.63 and ?0.58, respectively). Biotic index scores were lowest and nutrient concentrations were highest for streams with basins having the highest poultry and cattle production. Because of the availability of litter for fertilizer and associated increases in grass and hay production, cattle feeding capacity increases with poultry production. Studies are needed that address the synergistic effect of poultry and cattle production on Ozark streams in high production areas before ecological risks can be adequately addressed.     

Experimental demonstration of the roles of bacteria and bacterivorous protozoa in plankton nutrient cycles

We have used a model food chain composed of a natural bacterial assemblage, a pennate diatom and a bacterivorous microflagellate to investigate the factors controlling the relative importance of bacteria and protozoa as sources for regenerated nitrogen in plankton communities. In bacterized diatom cultures in which diatom growth was nitrogen-limited, the carbon:nitrogen (C:N) ratio of the bacterial substrate greatly affected which population was responsible for the uptake of nitrogen. When nitrogen was added as NH ₄ ⁺ and the cultures were supplemented with glucose, the bacteria competed successfully with the algae for NH ₄ ⁺ and prevented the growth of algae by rapidly assimilating all NH ₄ ⁺ in the cultures. Bacterivorous protozoa inoculated into these cultures grazed the bacterial population and remineralized NH ₄ ⁺ , thus relieving the nitrogen limitation of algal growth and allowing an increase in algal biomass. In contrast, bacteria in cultures supplemented with the amino acid glycine (C:N = 2) were major remineralizers of nitrogen, and the influence of protozoan grazing was minimal. We conclude that the relative importance of bacteria and protozoa as nutrient regenerators in the detrital food loop is dependent largely on the overall carbon:nutrient ratio of the bacterial substrate. The role of bacterivorous protozoa as remineralizers of a growth-limiting nutrient is maximal in situations where the carbon:nutrient ratio of the bacterial substrate is high.     

Changes in phosphorus and nitrogen cycling following food web manipulations in a shallow Dutch lake

1. The flow of phosphorus and nitrogen through the food web of the shallow, eutrophic lake Wolderwijd was analysed for 2 different years before and for 1 year after food web manipulation.
2. After fish removal the water became clear and the growth of macrophytes began. Fish removal resulted in a significant reduction of the total nutrient pool in the water, but differences between the nutrient cycles before and after the experiment were mainly caused by a gradual change driven by a reduced phosphorus input.
3. The zooplankton biomass before and after food web manipulation did not change significantly. Unfavourable food conditions and predation by young fish limited zooplankton biomass after the food web manipulation.
4. After fish removal benthic algae, fish, zoobenthos and macrophytes form the largest pools of nutrients apart from the sediment top layer. However, they contribute only little to nutrient cycles in the water column.     

Changes in phosphorus and nitrogen cycling following food web manipulations in a shallow Dutch lake

1. The flow of phosphorus and nitrogen through the food web of the shallow, eutrophic lake Wolderwijd was analysed for 2 different years before and for 1 year after food web manipulation.
2. After fish removal the water became clear and the growth of macrophytes began. Fish removal resulted in a significant reduction of the total nutrient pool in the water, but differences between the nutrient cycles before and after the experiment were mainly caused by a gradual change driven by a reduced phosphorus input.
3. The zooplankton biomass before and after food web manipulation did not change significantly. Unfavourable food conditions and predation by young fish limited zooplankton biomass after the food web manipulation.
4. After fish removal benthic algae, fish, zoobenthos and macrophytes form the largest pools of nutrients apart from the sediment top layer. However, they contribute only little to nutrient cycles in the water column.     

Experimental impacts of fish on small and large cladocerans under eutrophic conditions

The impacts of fish and nutrient enrichment on the biomass of small and large cladoceran species were studied in mesocosms under eutrophic conditions. Fish shifted the ratio between small and large species to favour small cladocerans compared to control mesocosms, while nutrient enrichment favoured large cladocerans. Both fish vital activity and nutrient enrichment increased total and blue-green algae concentrations. While chlorophyll concentrations increased in response to fish and nutrient enrichment, there were no significant effects of fish on total cladoceran biomass and the relative proportion of large species due to high variability among replicates within treatments. However, fish and nutrient enrichment affected average total cladoceran biomass and the biomass of large species compared to control conditions.     

Factors regulating autotrophy and heterotrophy in the main channel and an embayment of a large river impoundment

We characterized seasonal patterns of phytoplankton and bacterial biomass, production and nutrient limitation along a lateral transect within a large river impoundment. We hypothesized that the balance between autotrophy and heterotrophy was related to depth gradients and differences in water residence time (WRT) between the main channel and an embayment. Heterotrophy predominated in the main channel with bacterial production exceeding phytoplankton production by a factor of 3.3. In the embayment, autotrophy and heterotrophy were more closely balanced (ratios of bacterial to phytoplankton production ca. 0.8). Phytoplankton and bacterial biomass were positively correlated with WRT. However, WRT accounted for less than 50% of variation and its predictive power was comparable to models based on nutrient or DOC concentrations. Bacterial production was correlated with phytoplankton biomass and production suggesting that algal-derived C may be an important substrate for bacterial growth even in systems dominated by allochthonous inputs. Our experimental data suggest that nutrient limitation may be important particularly in embayments where biomass was somewhat higher and substrate concentrations were lower. Nutrient limitation in the main channel was rare whereas N and P amendments consistently stimulated phytoplankton growth rates in the embayment. Bacterial cell densities did not respond to nitrogen or phosphorus additions in either the main channel or embayment.     

Effects of omnivorous filter‐feeding fish and nutrient enrichment on the plankton community and water transparency of a tropical reservoir

1. The major aim of this study was to test the hypothesis that nutrient enrichment and the introduction of the Nile tilapia (Oreochromis niloticus), an exotic omnivorous filter‐feeding fish, operate interdependently to regulate plankton communities and water transparency of a tropical reservoir in the semi‐arid northeastern Brazil. 2. A field experiment was performed for 5 weeks in 20 enclosures (9.8 m³) to which four treatments were randomly allocated: tilapia addition (F), nutrient addition (N), tilapia and nutrient addition (F + N) and a control treatment with no tilapia or nutrient addition (C). A two‐way repeated measures anova was undertaken to test for time, tilapia and nutrient effects and their interactions on water transparency, total phosphorus and total nitrogen concentrations, phytoplankton biovolume and zooplankton biomass. 3. Nutrient addition had no effect except on rotifer biomass, but there were significant fish effects on the biomass of total zooplankton, copepod nauplii, rotifers, cladocerans and calanoid copepods and on the biovolume of total phytoplankton, large algae (GALD ≥ 50 μm), Bacillariophyta and Zygnemaphyceae and on Secchi depth. In addition, we found significant interaction effects between tilapia and nutrients on Secchi depth and rotifers. Overall, tilapia decreased the biomass of most zooplankton taxa and large algae (diatoms) and decreased water transparency, while nutrient enrichment increased the biomass of rotifers, but only in the absence of tilapia. 4. In conclusion, the influence of fish on the reservoir plankton community and water transparency was significant and even greater than that of nutrient loading. This suggests that biomanipulation of filter‐feeding tilapias may be of importance for water quality management of eutrophic reservoirs in tropical semi‐arid regions.     

Responses of phytoplankton to fish predation and nutrient loading in shallow lakes: a pan-European mesocosm experiment

1. The impacts of nutrients (phosphorus and nitrogen) and planktivorous fish on phytoplankton composition and biomass were studied in six shallow, macrophyte‐dominated lakes across Europe using mesocosm experiments. 2. Phytoplankton biomass was more influenced by nutrients than by densities of planktivorous fish. Nutrient addition resulted in increased algal biomass at all locations. In some experiments, a decrease was noted at the highest nutrient loadings, corresponding to added concentrations of 1 mg L^?1 P and 10 mg L^?1 N. 3. Chlorophyll a was a more precise parameter to quantify phytoplankton biomass than algal biovolume, with lower within‐treatment variability. 4. Higher densities of planktivorous fish shifted phytoplankton composition toward smaller algae (GALD < 50 μm). High nutrient loadings selected in favour of chlorophytes and cyanobacteria, while biovolumes of diatoms and dinophytes decreased. High temperatures also may increase the contribution of cyanobacteria to total phytoplankton biovolume in shallow lakes.     

Empirical and dynamical models of production and biomass of benthic algae in lakes

This work presents new empirical and dynamical models for benthic algae in lakes. The models were developed within the framework of a more comprehensive lake ecosystem model, LakeWeb, which also accounts for phytoplankton, bacterioplankton, two types of zooplankton (herbivorous and predatory), macrophytes, prey fish and predatory fish. The new dynamic model provides seasonal variations (the calculation time is 1 week). It is meant to account for all factors regulating the production and biomass of benthic algae for lakes in general. This work also presents and uses a new data-base established by us from published sources. Many of the lakes included in this study are situated in the former Soviet Union. They were investigated during the Soviet period and the data and results have up until now been largely unknown in the West. We present empirical models for benthic algae, and show that the biomass of benthic algae in whole lakes can be estimated from the ratio between the lake area above the Secchi depth to the total lake area and the primary production of phytoplankton. We also present several critical tests of the dynamical model. The dynamical and empirical models give corresponding results over a wide limnological domain. We provide algorithms for (1) the production rate of benthic algae (2) the elimination rate (related to the turnover time of benthic algae), (3) the rate of benthic algae consumption by zoobenthos, and (4) the rate of physical erosion of benthic algae. Our results indicate that the production of benthic algae is highly dependent on lake morphometry and sediment character, as well as water clarity, and less dependent on nutrient (phosphorus) concentrations in water and sediments. This work provides new quantitative support to such conclusions and also a useful model for predictions of production and biomass of benthic algae.     

Shifting nutrient-mediated interactions between algae and bacteria in a microcosm: evidence from alkaline phosphatase assay

The impacts of different nutrient additions (N + P, N + P + C, 4N + P, 4N + P + C, N + 2P) on the growth of algae and bacteria were studied in a microcosm experiment. Since alkaline phosphatase activity (APA) provides an indication of phosphorus deficiency, the higher value for algal APA in the treatments with excess nitrogen and for bacterial APA in the treatments with excess carbon suggested that, algal and bacterial phosphorus-limited status were induced by abundant nitrogen and carbon input, respectively. Bacterial phosphorus-limited status was weakened due to higher bacterial competition for phosphorus, compared to algae. In comparison with the bacterial and specific bacterial APA, higher values of algal and specific algal APA were found, which showed a gradual increase that coincided with the increase of chlorophyll a concentration. This fact indicated not only a stronger phosphorus demand by algae than by bacteria, but also a complementary relationship for phosphorus demand between algae and bacteria. However, this commensalism could be interfered by glucose input resulting in the decline of chlorophyll a concentration. Furthermore, the correlation between bacterial numbers and chlorophyll a concentration was positive in treatments without carbon and blurry in treatments with carbon. These observations validate a hypothesis that carbon addition can stimulate bacterial growth justifying bacterial nutrient demand, which decreases the availability of nutrients to algae and affects nutrient relationship between algae and bacteria. However, this interference would terminate after algal and bacterial adaption to carbon input.     

Effects of light, dissolved organic carbon, and inorganic nutrients [2pt] on the relationship between algae and heterotrophic bacteria in stream periphyton

Depending on the chemical and physical environment, algae and heterotrophic bacteria in stream periphyton communities likely engage in both positive and negative interactions. We tested the hypothesis that bacteria are more closely associated with algae when allochthonous sources of labile DOC are low and algae are not light limited. Secondly, we tested the hypothesis that, under extremely oligotrophic conditions, bacteria will out-compete algae for inorganic nutrients if their carbon requirements are met by allochthonous sources. Experiments were carried out using in situ light manipulations and nutrient diffusing substrates (releasing inorganic nutrients and /or glucose) in Harts Run, an oligotrophic stream located in north central Kentucky. Although we found that both algal and bacterial biomass were higher under ambient light, bacteria did not respond to glucose in the dark. This may indicate that bacteria were associated with algae not as a carbon source, but as a substrate for colonization. In the nutrient × glucose manipulation, we found that bacteria were co-limited by inorganic nutrients. There was no evidence of algae being negatively affected by competition with bacteria for nitrogen and phosphorus. Although low temperatures might have played a role in preventing inorganic nutrient competition between these two groups of organisms, the results of both experiments may indicate that the quantitative link between periphytic bacteria and algae is stronger under oligotrophic conditions.     

Toward enhanced performance of integrated photo-bioelectrochemical systems: Taxa and functions in bacteria–algae communities

An integrated photo-bioelectrochemical (IPB) system uses microalgae in the cathode of a microbial fuel cell to achieve higher electricity generation and nutrient removal from wastewater. Using multivariate analysis and surveys of IPB studies, this paper identifies key algal and bacterial taxa and discusses their functions critical for IPB performance. Unicellular algae with high photosynthetic oxygen production and biofilm formation can enhance IPB energy production. Diverse bacterial taxa achieve nitrogen transformations and can improve total nitrogen removal. Understanding bacteria–algae interactions via quorum sensing in the IPB cathode may potentially aid in boosting system performance. Future advances in development of IPBs for wastewater treatment will benefit from interdisciplinary collaboration in analysis of microbial community functions.     

Comparing Effects of Nutrients on Algal Biomass in Streams in Two Regions with Different Disturbance Regimes and with Applications for Developing Nutrient Criteria

Responses of stream algal biomass to nutrient enrichment were studied in two regions where differences in hydrologic variability cause great differences in herbivory. Around northwestern Kentucky (KY) hydrologic variability constrains invertebrate biomass and their effects on algae, but hydrologic stability in Michigan (MI) streams permits accrual of high herbivore densities and herbivory of benthic algae. Multiple indicators of algal biomass and nutrient availability were measured in 104 streams with repeated sampling at each site over a 2−month period. Many measures of algal biomass and nutrient availability were positively correlated in both regions, however the amount of variation explained varied with measures of biomass and nutrient concentration and with region. Indicators of diatom biomass were higher in KY than MI, but were not related to nutrient concentrations in either region. Chl a and % area of substratum covered by Cladophora were positively correlated to nutrient concentrations in both regions. Cladophora responded significantly more to nutrients in MI than KY. Total phosphorus (TP) and total nitrogen (TN) explained similar amounts of variation in algal biomass, and not significantly more variation in biomass than dissolved nutrient concentrations. Low N:P ratios in the benthic algae indicated N as well as P may be limiting their accrual. Most observed responses in benthic algal biomass occurred in nutrient concentrations between 10 and 30 μg TP l⁻¹ and between 400 and 1000 μg TN l⁻¹.     

Community resistance and change to nutrient enrichment and fish manipulation in a vegetated lake littoral

1. High biomass of macrophytes is considered important in the maintenance of a clear‐water state in shallow eutrophic lakes. Therefore, rehabilitation and protection of aquatic vegetation is crucial to the management of shallow lakes. 2. We conducted field mesocosm experiments in 1998 and 1999 to study community responses in the plant‐dominated littoral zone of a lake to nutrient enrichment at different fish densities. We aimed to find the threshold fish biomass for the different nutrient enrichment levels below which large herbivorous zooplankton escapes control by fish. The experiments took place in the littoral of Lake Vesijärvi in southern Finland and were part of a series of parallel studies carried out jointly at six sites across Europe. 3. In 1998, when macrophyte growth was poor, a clear‐water state with low phytoplankton biomass occurred only in unenriched mesocosms without fish or with low fish biomass (4 g fresh mass m^?2). Both nutrient enrichment and high fish biomass (20 g fresh mass m^?2) provoked a turbid water state with high planktonic and periphytic algal biomass. The zooplankton community was dominated by rotifers and failed to control the biomass of algae in nutrient enriched mesocosms. The littoral community thus had low buffer capacity against nutrient enrichment. 4. In 1999, macrophytes, especially free‐floating Lemna trisulca L., grew well and the zooplankton community was dominated by filter‐feeding cladocerans. The buffer capacity of the littoral community against nutrient enrichment was high; a clear‐water state with low phytoplankton biomass prevailed even under the highest nutrient enrichment. High grazing rates by cladocerans, together with reduced light penetration into the water caused by L. trisulca, were apparently the main mechanisms behind the low algal biomass. 5. Effects of fish manipulations were less pronounced than effects of nutrient enrichment. In 1999, clearance rates of cladocerans were similar in fish‐free and low‐fish treatments but decreased in the high‐fish treatment. This suggests that the threshold fish biomass was between the low‐ and high‐fish treatments. In 1998, such a threshold was found only between fish‐free and low‐fish treatments. 6. The pronounced difference in the observed responses to nutrient enrichment and fish additions in two successive years suggests that under similar nutrient conditions and fish feeding pressure either clear or turbid water may result depending on the initial community structure and on weather.     

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.     

Cultivation of Ulva lactuca with manure for simultaneous bioremediation and biomass production

The potential of liquid manure as sole nutrient source for cultivation of Ulva lactuca was investigated with the perspective of utilizing the produced biomass for feed and/or energy. Algae grown with manure demonstrated equal growth rates to algae grown with standard f/2-medium. The optimum manure concentration, expressed as ammonium concentration, was 25?μM. At these conditions, the biomass produced was potentially suitable for anaerobic digestion, due to a relative high carbon/nitrogen ratio (approximately 19). At higher manure concentrations, tissue concentrations of nitrogen, phosphorus, proteins, and amino acids increased, making the biomass less suitable for anaerobic digestion but potentially interesting as a feed supplement. Cultivating U. lactuca with manure as nutrient source has potential in terms of bioremediation as well as production of bioenergy and protein-feed.     

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.