Effects of nutrients and light on periphytic biomass and nutrient stoichiometry in a tropical black-water aquatic ecosystem

This article aims to test the light-nutrient hypothesis (LNH) in a periphytic community in a tropical black-water lake. Individual and interactive effects of light and nutrient availability were assessed with periphyton biomass accrual, nutrient content, and nutrient stoichiometry. We performed a manipulative field experiment with a 4 × 2 factorial design. We used nutrient diffusing substrates to produce four different nutrients treatments: Control (no nutrient added), nitrogen amended (N), phosphorus amended (P) and combined N and P amendment (NP). Two light levels were also considered: high light (near surface water) and low light (near bottom water). Light and nutrients individually and interactively caused significant changes in aggregate periphyton community properties. Total and autotrophic biomasses were significantly higher in high light conditions and in nutrient enriched treatments. Autotrophic biomass was significantly higher in N enriched treatment whereas total biomass was mainly affected by the joint addition of N and P. At lower light availability periphyton growth was limited, even in enriched treatments. Light also strongly affected periphyton nutrient content. Periphyton C, N and P in general increased when subjected to high light conditions. As predicted by the LNH, light promoted an increase in periphyton C:P ratios in P deprived treatments, but an opposite effect was observed on C:N ratios, especially in N-enriched treatments. This experiment revealed that light availability strongly limits the propagation of nutrient effects on periphyton growth. Such complex interdependencies on basal resources affect the proportion of autotrophic to total periphytic biomass that can be an important mechanism to explain variation in the nutrient stoichiometry of periphyton in nature.     

Effects of food‐web structure on periphyton stoichiometry in eutrophic lakes: a mesocosm study

1. Aquatic herbivores typically have much higher concentrations of nutrients (e.g. N and P) in their tissues than there is in the food they eat. These stoichiometric differences can cause herbivores to be limited by the elemental quality of their food, which could affect, in turn, the structure of consumer communities and even alter key ecosystem processes. 2. In streams and in the littoral zone of shallow lakes, periphyton is an important food resource for benthic animals. Studying the elemental composition of periphyton may help us to understand food‐web structure, and any reciprocal effect of this structure on periphyton stoichiometry. 3. To understand how alterations in the food‐web structure affect the elemental composition of periphyton in a eutrophic lake, we carried out a long‐term experiment (14 months) in large‐scale mesocosms (40 m³), in which we manipulated food‐web structure, and which were dominated either by planktivorous fish (Rutilus rutilus) or herbivorous invertebrates (without fish). Periphyton was sampled monthly at three depths (0.5, 1.5 and 2.5 m) to determine its biomass and elemental composition (C/N/P ratio). Food‐web structure, physical and chemical parameters were monitored throughout the experiment. 4. Fish had indirect positive effect on periphyton biomass, leading to twofold higher levels than in herbivore‐dominated mesocosms. This result was probably due to control of benthic consumers by fish, suggesting a strong top–down control on periphyton by their consumers in fishless enclosures. 5. The elemental ratios C/P and C/N were lower in deep water in both treatments, mainly mediated by light availability, in accordance with the light/nutrient ratio hypothesis. These ratios were also lower in fishless treatments, probably due to increases in inorganic nutrient availability and grazing pressure in herbivore‐dominated systems. During winter, periphyton elemental composition was similar in both treatments, and was unrelated to inorganic nutrient availability. 6. These results indicate that any alteration of food‐web structure in lakes, such as in biomanipulation experiments, is likely to modify both the biomass and elemental quality of periphyton. Resultant effects on the consumers of periphyton and macrophytes could play a key role in the success of biomanipulations and should be taken into account in further studies.     

How light and nutrients affect the relationship between autotrophic and heterotrophic biomass in a tropical black water periphyton community

This study examines how nutrients and light affect the relationship between autotrophic biomass and non-autotrophic periphyton organic matter in a tropical black water lake biofilm community. We hypothesized that there is no positive correlation between autotrophic and non-autotrophic organic matter in the periphytic community of a black water humic lake, where non-algal components of periphyton can rely on carbon sources external to the periphyton matrix and where nutrient availability is low. Second, we sought to test our hypothesis that non-autotrophic periphyton organic matter will benefit from nutrient enhancement in a lake where the availability of DOC is high. We performed a field experiment using in situ lake mesocosms to manipulate nutrient concentrations and light availability in a 2 × 2 factorial design. Control treatments (no nutrient added) and nutrient treatments (N + P) were compared in different light conditions: high light (near surface water) and low light (near bottom). No positive correlation was found between autotrophic biomass and non-autotrophic periphyton organic matter, but a negative correlation was observed in high nutrient and light conditions. The low C:P and N:P ratios revealed that the non-autotrophic organic matter mostly comprised a heterotrophic microbial biofilm. High levels of light and nutrients together caused significant changes in periphyton community properties. The non-autotrophic periphyton organic matter was negatively affected by nutrient addition, whereas autotrophic biomass was positively affected, especially in high light conditions. Our results strongly suggest that non-autotrophic periphyton organic matter in a humic lake is primarily comprised of a bacterial biofilm that directly competes for nutrients with autotrophs in the periphytic community. We also observed no effect of nutrient addition on periphyton growing in light-limited conditions. These results suggest that heterotrophic periphytic organisms might experience carbon limitation despite the high availability, but usually low quality, of dissolved carbon in the water column of humic lakes.     

Food web complexity affects stoichiometric and trophic interactions

The stoichiometry of trophic interactions has mainly been studied in simple consumer–prey systems, whereas natural systems often harbour complex food webs with abundant indirect effects. We manipulated the complexity of trophic interactions by using simple laboratory food webs and complex field food webs in enclosures in Lake Erken. In the simple food web, one producer assemblage (periphyton) and its consumers (benthic snails) were amended by perch, which was externally fed by fish food. In the complex food web, two producer assemblages (periphyton and phytoplankton), their consumers (benthic invertebrates and zooplankton) and perch feeding on zooplankton were included. In the simple food web perch affected the stoichiometry of periphyton and increased periphyton biomass and the concentration of dissolved inorganic nitrogen. Grazers reduced periphyton biomass but increased its nutrient content. In the complex food web, in contrast to the simple food web, perch affected periphyton biomass negatively but increased phytoplankton abundance. Perch had no influence on benthic invertebrate density, zooplankton biomass or periphyton stoichiometry. Benthic grazers reduced periphyton biomass and nutrient content. The difference between the simple and the complex food web was presumably due to the increase of pelagic cyanobacteria ( Gloeotrichia sp.) with fish presence in the complex food web, thus fish had indirect negative effects on periphyton biomass through nutrient competition and shading by cyanobacteria. We conclude that the higher food web complexity through the presence of pelagic primary producers (in this case Gloeotrichia sp.) influences the direction and strength of trophic and stoichiometric interactions.     

Ecological stoichiometry of indirect grazer effects on periphyton nutrient content

Ecological stoichiometry has been successful in enhancing our understanding of trophic interactions between consumer and prey species. Consumer and prey dynamics have been shown to depend on the nutrient composition of the prey relative to the nutrient demand of the consumer. Since most experiments on this topic used a single consumer species, little is known about the validity of stoichiometric constraints on trophic interactions across consumers and ecosystems. We conducted a quantitative meta-analysis on grazer–periphyton experiments to test (1) if benthic grazers have consistent effects on the nutrient composition of their prey, and (2) whether these effects can be aligned to the nutrient stoichiometry of grazer and periphyton, other environmental factors, or experimental constraints. Grazers significantly lowered periphyton C:N and C:P ratios, indicating higher N- and P-content of grazed periphyton across studies. Grazer presence on average increased periphyton N:P ratios, but across studies the effect size did not differ significantly from zero. The sign and strength of grazer effects on periphyton nutrient ratios was strongly dependent on the nutrient content of grazers and their food, but also on grazer biomass, the amount of biomass removal and water column nutrients. Grazer with low P-content tended to reduce periphyton P-content, whereas grazers with high P-content increased periphyton P-content. This result suggests that low grazer P-content can be an indication of physiological P-limitation rather than a result of having relatively low and fixed P-requirements. At the across-system scale of this meta-analysis, predictions from stoichiometric theory are corroborated, but the plasticity of the consumer nutrient composition has to be acknowledged. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Gastropod grazers and nutrients,but not light,interact in determining periphytic algal diversity

The potential interactions of grazing, nutrients and light in influencing autotroph species diversity have not previously been considered. Earlier studies have shown that grazing and nutrients interact in determining autotroph species diversity, since grazing decreases species diversity when nutrients (i.e. N or P) limit autotroph growth, but increases it when nutrients are replete. We hypothesized that increased light intensities would intensify the interactions between grazing and nutrients on algal species diversity, resulting in even stronger reductions in algal species diversity through grazing under nutrient–poor conditions, and to even stronger increases of algal species diversity through grazing under nutrient-rich conditions. We studied the effects of grazing (absent, present), nutrients (ambient, N + P enriched) and light (low light, high light) on benthic algal diversity and periphyton C:nutrient ratios (which can indicate algal nutrient limitation) in a factorial laboratory experiment, using the gastropod grazer Viviparus viviparus. Grazing decreased algal biomass and algal diversity, but increased C:P and N:P ratios of periphyton. Grazing also affected periphyton species composition, by decreasing the proportion of Spirogyra sp. and increasing the proportion of species in the Chaetophorales. Grazing effects on diversity as well as on periphyton N:P ratios were weakened when nutrients were added (interaction between grazing and nutrients). Chlorophyll a (Chl a) per area increased with nutrient addition and decreased with high light intensities. Light did not increase the strength of the interaction between grazing and nutrients on periphytic algal diversity. This study shows that nutrient addition substantially reduced the negative effects of grazing on periphytic algal diversity, whereas light did not interact with grazing or nutrient enrichment in determining periphytic algal diversity.     

Effects of macrograzers and light on periphyton stoichiometry

Ecological stoichiometry describes the biochemical constraints of trophic interactions emerging from the different nutrient content and nutrient demand of producers and consumers, respectively. Most research on this topic originates from well-mixed pelagic food webs, whereas the idea has received far less attention in spatially structured habitats. Here, we test how light as well as grazing and nutrient regeneration by consumers affects growth and biomass of benthic primary producers. In the first laboratory experiment, we manipulated grazer presence (two different snail species plus ungrazed control), in the second experiment we factorially combined manipulation of grazer presence and light intensity. We monitored snail and periphyton biomass as well as dissolved and particulate nutrients (nitrogen and phosphorus) over time. Grazers significantly reduced algal biomass in both experiments. Grazers affected periphyton nutrient content depending on the prevailing nutrient limitation and their own body stoichiometry. In the nitrogen (N-) limited first experiment, grazers increased N both in the periphyton and in the water column. The effect was stronger for grazers with lower N-content. In the phosphorus (P-) limited second experiment, grazers increased the P-content of the periphyton, but the grazer with lower N-content had additionally positive effects on algal N. Light reduction did not affect periphyton biomass, but increased chlorophyll-, N- and P-content of the periphyton. These experiments revealed that the indirect effects of grazers on periphyton were bound by stoichiometric constraints of nutrient incorporation and excretion.     

Effects of nutrients and light on periphyton biomass and nitrogen uptake in Mediterranean streams with contrasting land uses

1. Nutrient diffusing substrata (NDS) were used to determine the relative importance of nutrients and light as potential limiting factors of periphyton biomass and nitrogen (N) uptake in Mediterranean streams subjected to different human impacts. The nutrients examined were phosphorus (P) and N, and we also further differentiated between the response of periphyton communities to N species (i.e. NO₃‐N and NH₄‐N). To examine the effect of light and nutrients on periphyton biomass, chlorophyll a accrual rates on NDS located at open and closed canopy sites were compared. The effect of nutrient availability on periphyton uptake was measured by ¹⁵N changes on the NDS after NO₃‐¹⁵N short‐term nutrient additions. 2. Results show that light was the main factor affecting algal biomass in the study streams. Algal biomass was in general higher at open than at closed canopy sites. Nutrient availability, as simulated with the NDS experiments, did not enhance algal biomass accrual in either of the 2 light conditions. 3. In the control treatments (i.e. ambient concentrations), periphyton NO₃‐N uptake rates increased and C : N molar ratios decreased consistently with increases in N availability across streams. NO₃‐N uptake rates were altered when ambient N concentrations were increased artificially in the N amended NDS. Periphyton assemblages growing on N enriched substrata seemed to preferentially take up N diffusing from the substratum rather than N from the water column. This response differed among streams, and depended on ambient N availability. 4. Periphyton biomass was not significantly different between substrata exposed to the two forms of available N sources. Nonetheless, we found differences in the effects of both N sources on the uptake of N from the water column. NH₄‐N seemed to be the preferred source of N for periphyton growing on NDS. 5. Results suggest that the effect of riparian zones on light availability, although seldom considered by water managers, may be more important than nutrients in controlling eutrophication effects derived from human activities. Finally, our results confirm that not only increases in concentration, but also stoichiometric imbalances should be considered when examining N retention in human altered streams.     

Feedbacks of consumer nutrient recycling on producer biomass and stoichiometry: separating direct and indirect effects

Herbivores can have both direct (consumptive) and indirect (nutrient‐mediated) effects on primary producer biomass and nutrient stoichiometry. Ecological stoichiometry theory predicts that herbivores of contrasting body stoichiometry will differentially remineralize nutrients, resulting in feedbacks on producer stoichiometry. We experimentally separated direct and indirect effects of aquatic vertebrate grazers on periphyton by manipulating grazer abundance and identity in mesocosms, and using grazer exclusion cages to expose periphyton to recycled nutrients in the absence of direct grazing. In experiment 1, we used a catfish with high body phosphorus (low body N:P), Ancistrus triradiatus, to assess consumptive versus nutrient‐mediated effects of grazer density on periphyton. In experiment 2, we compared the nutrient‐mediated effects of grazing by Ancistrus triradiatus and Rana palmipes, a tadpole with low body phosphorus and high body N:P. In experiment 1, we found that increasing catfish density led to lower biomass and particulate nutrients in periphyton through direct consumptive effects, but that nutrient‐mediated indirect effects enhanced periphyton biomass when grazers were experimentally separated from direct contact with periphyton. As predicted by stoichiometry theory, nutrient recycling by this P‐rich grazer tended to increase algal C:P and N:P (although effects were not statistically significant), while their consumptive effects reduced algal C:P and N:P. In experiment 2, grazer identity had strong effects on dissolved water nutrient concentrations, N recycling (measured with a ¹⁵N tracer), and periphyton stoichiometry. In accordance with stoichiometry theory, catfish increased N concentrations and recycling rates leading to higher periphyton N:P, while tadpoles had greater effects on P availability leading to lower periphyton N:P. Our experiments elucidate the importance of both the density and identity of grazers in controlling periphyton biomass and stoichiometry through consumptive and nutrient‐mediated effects, and support the power of ecological stoichiometry theory to predict feedbacks on producer stroichiometry arising from consumer stoichiometry through nutrient recycling.     

Interactive effects of light and nutrients on phytoplankton stoichiometry

The stoichiometric composition of autotrophs can vary greatly in response to variation in light and nutrient availability, and can mediate ecological processes such as C sequestration, growth of herbivores, and nutrient cycling. We investigated light and nutrient effects on phytoplankton stoichiometry, employing five experiments on intact phytoplankton assemblages from three lakes varying in productivity and species composition. Each experiment employed two nutrient and eight irradiance levels in a fully factorial design. Light and nutrients interactively affected phytoplankton stoichiometry. Thus, phytoplankton C:N, C:P, and N:P ratios increased with irradiance, and slopes of the stoichiometric ratio versus irradiance relationships were steeper with ambient nutrients than with nutrients added. Our results support the light–nutrient hypothesis, which predicts that phytoplankton C:nutrient ratios are functions of the ratio of available light and nutrients; however, we observed considerable variation among lakes in the expression of this relationship. Phytoplankton species diversity was positively correlated with the slopes of the C:N and C:P versus irradiance relationships, suggesting that diverse assemblages may exhibit greater flexibility in the response of phytoplankton nutrient stoichiometry to light and nutrients. The interactive nature of light and nutrient effects may render it difficult to generate predictive models of stoichiometric responses to these two factors. Our results point to the need for future studies that examine stoichiometric responses across a wide range of phytoplankton communities.     

Light Increases Energy Transfer Efficiency in a Boreal Stream

Periphyton communities of a boreal stream were exposed to different light and nutrient levels to estimate energy transfer efficiency from primary to secondary producers using labeling with inorganic ¹³C. In a one-day field experiment, periphyton grown in fast-flow conditions and dominated by opportunistic green algae were exposed to light levels corresponding to sub-saturating (forest shade) and saturating (open stream section) irradiances, and to N and P nutrient additions. In a two-week laboratory experiment, periphyton grown in low-flow conditions and dominated by slowly growing diatoms were incubated under two sub-saturating light and nutrient enrichment levels as well as grazed and non-grazed conditions. Light had significant positive effect on ¹³C uptake by periphyton. In the field experiment, P addition had a positive effect on ¹³C uptake but only at sub-saturating light levels, whereas in the laboratory experiment nutrient additions had no effect on the periphyton biomass, ¹³C uptake, biovolume and community composition. In the laboratory experiment, the grazer (caddisfly) effect on periphyton biomass specific ¹³C uptake and nutrient content was much stronger than the effects of light and nutrients. In particular, grazers significantly reduced periphyton biomass and increased biomass specific ¹³C uptake and C:nutrient ratios. The energy transfer efficiency, estimated as a ratio between ¹³C uptake by caddisfly and periphyton, was positively affected by light conditions, whereas the nutrient effect was not significant. We suggest that the observed effects on energy transfer were related to the increased diet contribution of highly palatable green algae, stimulated by higher light levels. Also, high heterotrophic microbial activity under low light levels would facilitate energy loss through respiration and decrease overall trophic transfer efficiency. These findings suggest that even a small increase in light intensity could result in community-wide effects on periphyton in boreal streams, with a subsequent increase in energy transfer and system productivity.     

Food web pathways for fish communities in small tropical streams

1. It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species.
2. We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow.
3. We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects.
4. Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass.
5. This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high.

     

Arbuscular mycorrhizal fungi affect plant community structure under various nutrient conditions and stabilize the community productivity

Soil biota could have a significant impact on plant productivity and diversity through benefiting plants and mediating plant–plant interaction. However, it is poorly understood how soil biotic factors interaction with abiotic environments affect plant community diversity and composition. Here, we investigate the community‐level consequences of arbuscular mycorrhizal fungi (AMF) interactions with multiple nutrients and their ecological stoichiometry. We conducted a greenhouse experiment manipulating nitrogen (N) and phosphorus (P) to create soil nutrient availability and N:P gradients for microcosm communities with and without AMF. We found that AMF suppressed plant diversity at low P levels, whereas it did not alter the diversity at high P levels because of trade‐offs in the abundance of the dominant and subordinate species. AMF reduced plant diversity at the intermediate N:P ratios, while AMF did not affect the diversity at low and high N:P ratios. P addition decreased the mycorrhizal contribution to community productivity, whereas N addition reduced the negative effects of AMF on productivity at high P levels. AMF decreased community productivity at low N:P ratios but increased it at high N:P ratios. AMF increased the stoichiometric homoeostasis of plant communities, which was positively correlated with the stability of productivity under variations in soil N:P ratios. Our study demonstrates that both resource availability and stoichiometry influence the effect of AMF on plant community productivity and diversity and suggests that AMF may increase the stability of plant communities under variations in the soil nutrients by increasing the stoichiometric homoeostasis of the plant community.     

Weedbeds and big bugs: the importance of scale in detecting the influence of nutrients and predation on macroinvertebrates in plant‐dominated shallow lakes

1. The scale of investigations influences the interpretation of results. Here, we investigate the influence of fish and nutrients on biotic communities in shallow lakes, using studies at two different scales: (i) within‐lake experimental manipulation and (ii) comparative, among‐lake relationships. 2. At both scales, fish predation had an overriding influence on macroinvertebrates; fish reduced macroinvertebrate biomass and altered community composition. Prey selection appeared to be size based. Fish influenced zooplankton abundance and light penetration through the water column also, but there was no indication that fish caused increased resuspension of sediment. 3. There were effects of nutrients at both scales, but these effects differed with the scale of the investigation. Nutrients increased phytoplankton and periphyton at the within‐lake scale, and were associated with increased periphyton at the among‐lake scale. No significant effect of nutrients on macroinvertebrates was observed at the within‐lake scale. However, at the among‐lake scale, nutrients positively influenced the biomass and density of macroinvertebrates, and ameliorated the effect of fish on macroinvertebrates. 4. Increased prey availability at higher nutrient concentrations would be expected to cause changes in the fish community. However, at the among‐lake scale, differences were not apparent in fish biomass among lakes with different nutrient conditions, suggesting that stochastic events influence the fish community in these small and relatively isolated shallow lakes. 5. The intensity of predation by fish significantly influences macroinvertebrate community structure of shallow lakes, but nutrients also play a role. The scale of investigation influences the ability to detect the influence of nutrients on the different components of shallow lake communities, particularly for longer lived organisms such as macroinvertebrates, where the response takes longer to manifest.     

Water temperature determines strength of top-down control in a stream food web

1. We examined effects of water temperature on the community structure of a three trophic level food chain (predatory fish, herbivorous caddisfly larvae and periphyton) in boreal streams. We used laboratory experiments to examine (i) the effects of water temperature on feeding activities of fish and caddisfly larvae and on periphyton productivity, to evaluate the thermal effects on each trophic level (species‐level experiment), and (ii) the effects of water temperature on predation pressure of fish on abundance of the lower trophic levels, to evaluate how temperature affects top‐down control by fish (community‐level experiment). 2. In the species‐level experiment, feeding activity of fish was high at 12 °C, which coincides with the mean summer temperature in forested streams of Hokkaido, Japan, but was depressed at 3 °C, which coincides with the mean winter temperature, and also above 18 °C, which coincides with the near maximum summer temperatures. Periphyton productivity increased over the range of water temperatures. 3. In the community‐level experiments, a top‐down effect of fish on the abundance of caddisfly larvae and periphyton was clear at 12 °C. This effect was not observed at 3 and 21 °C because of low predation pressure of fish at these temperatures. 4. These experiments revealed that trophic cascading effects may vary with temperature even in the presence of abundant predators. Physiological depression of predators because of thermal stress can alter top‐down control and lead to changes in community structure. 5. We suggest that thermal habitat alteration can change food web structure via combinations of direct and indirect trophic interactions.     

Ecological responses to simulated benthic-derived nutrient subsidies mediated by omnivorous fish

1. Fish can play an important role in coupling benthic and pelagic habitats by consuming benthic prey and providing essential nutrients to algae in dissolved form. However, little is known about the factors affecting the magnitude of this nutrient subsidy. 2. Using laboratory and mesocosm experiments we evaluated how varying ingestion rates of bluegill sunfish (Lepomis macrochirus) affects fish excretion rates of both nitrogen (N) and phosphorus (P). During the 10‐week mesocosm experiment, we also evaluated how varying ingestion rates may affect plankton community dynamics, and nutrient flux between pelagic and benthic habitats. Lastly, bioenergetic/mass balance models were used to examine the nutrient stoichiometry of fish body composition and excretion products. 3. Under laboratory conditions, both N and P excretion rates increased with increased ingestion of benthic prey surrogates (earthworms). This effect was more pronounced for N than P. Furthermore, under the more realistic conditions of the mesocosm experiment ingestion rate had no significant effect on P excretion rate. 4. Increased fish ingestion rate in the mesocosm experiment increased total algal biomass and the flux of nutrients from the water column to sediments. Effects of variable ingestion were much stronger on periphyton biomass and algal sedimentation rates than on phytoplankton or zooplankton biomass or composition. 5. Fish body nutrient composition was greatly affected by ingestion rate. N content increased and P content decreased with ingestion rate. As a result, the N : P ratio of fish bodies also increased with ingestion rate. The N : P ratio of nutrients excreted by fish also increased with ingestion rate, counter to predictions of stoichiometric theory, which predicts that excreted N : P ratio is negatively correlated to body N : P. However, this finding can be explained by relaxing the assumption of constant nutrient assimilation rates, and our mass balance data suggest that assimilation rates vary indeed with ingestion rate. 6. Our study provides experimental evidence that translocation of benthic‐derived nutrients by fish can affect the flux of nutrients among habitats, while also suggesting that stoichiometry models need to better incorporate how variable ingestion rates affect nutrient assimilation and excretion rates.     

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.     

Elevated light and nutrients alter the nutritional quality of stream periphyton

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Fish-mediated indirect effects in a littoral food web

The strength of the direct effect by scraping cladocerans and the indirect effect of nutrient regeneration by filtering herbivorous cladocerans on periphyton growth was investigated in a littoral food web. Ten enclosures were erected in a lake in an area with artificial vegetation. Five enclosures were stocked with juvenile perch ( Perca fluviatilis ) and five lacked fish. In addition, a reference area in the artificial vegetation was sampled. The mesh size of the net surrounding the cages was chosen to allow an inflow of phytoplankton into the cages from the surrounding water. The periphyton and filtering herbivorous cladoceran biomasses were highest in the fish-free treatment. There was no difference in phytoplankton biomass between treatments despite the large difference in filtering herbivorous cladoceran biomass, suggesting that the inflow of phytoplankton into enclosures completely compensated for the loss due to filtering. The reference area and the enclosure with fish showed the same patterns in developments with respect to filtering herbivorous cladocerans and periphyton. Scraping cladoceran biomass was higher in the fish-free treatment resulting in a positive correlation between scraping cladoceran and periphyton biomass. Our results suggest that the positive indirect effect of filtering herbivorous cladoceran nutrient regeneration on periphyton was stronger than the negative direct effect of grazing by scraping cladocerans on periphyton in this semi-open system, and that pelagic production by phytoplankton may foster periphyton growth in the littoral habitat via filtering herbivorous cladocerans. Furthermore, heterogeneity within trophic levels involving primary producers of different growth forms such as phytoplankton and periphyton may enhance the potential for compensatory responses via nutrient recycling.     

Periphyton responds differentially to nutrients recycled in dissolved or faecal pellet form by the snail grazer Theodoxus fluviatilis

1. We aimed to separate the effects of grazers on periphyton via grazing from that of nutrient recycling from their faecal pellets. 2. We set up three different experimental treatments (snails/no snails/faecal pellets) and sampled over 16 days. The ‘snail’ treatment contained a low density (snail biomass c. 14 g^?2) of the gastropod grazer Theodoxus fluviatilis and the ‘faecal pellet’ treatment received the same amount of faecal pellets as were produced in the ‘snail’ treatment. Whereas the ‘faecal pellet’ treatment provided extra nutrients to periphyton from the faeces, the ‘snail’ treatment provided nutrients in the form of both faeces and in excreta. There was also direct grazing on periphyton in the ‘snail’ treatment. The ‘no snail’ was not grazed and received no nutrients in faeces or excreta. 3. We measured periphyton C, N and P content, chlorophyll‐a (chl‐a), primary production, bacterial biomass, bacterial production and bacterial respiratory activity. In the water column we measured dissolved inorganic N and soluble reactive P. 4. Snails increased the amount of dissolved inorganic N in the water. On day 16, the periphyton N : P ratio in the ‘faecal pellet’ treatment was lower, and periphyton P content was higher, than in the other two treatments. N : P ratios decreased over time in the ‘faecal pellet’ treatment. Primary and bacterial production were positively correlated in all treatments. 5. Algal chl‐a and primary production of periphyton per unit area and periphyton chl‐a : C ratios increased over the 16 day in the ‘snail’ treatment, and thus excretion of dissolved N by snails had a stronger positive effect on the periphyton community than N and P in faecal pellets. 6. Our data show that excretion and egestion can have different effects on periphyton, probably because of the higher proportion of dissolved N in excreta and the higher proportion of P recycled in faecal pellets. The relative effect of nutrients recycled in egesta or in excretions, probably depends on the form of nutrient limitation of the periphyton. Further, the different components of the periphyton matrix could react differently to the different forms of nutrient recycling. 7. We conclude that direct grazing effects are less important than nutrient effects when nutrients are limiting and grazing pressure is low. Further, the spatial separation of different grazing effects can lead to differences in periphyton production and nutrient stoichiometry. This might be an explanation for the patchiness of periphyton in nature.