Empirical models for predicting the excretion of nutrients (N and P) by aquatic metazoans: taxonomic differences in rates and element ratios

1. We developed empirical models for predicting the release of nutrients [nitrogen (N) and phosphorus (P)] by aquatic metazoans (zooplankton, mussels, benthic macroinvertebrates and fish). 2. The number of species represented in each model ranged from 9 to 74 (n = 40 – 1122), organism dry mass from 1 × 10^?5 to 8 × 10⁴ mg and water temperature from ?1.8 to 32 °C for all models. Organisms were from marine and freshwater (both lotic and lentic) environments. 3. Rates and ratios of nutrient excretion were modelled and intra‐ and intertaxon differences in excretion were examined. Rates of N and P excretion were not significantly different between marine and freshwater species within the same taxon (e.g. zooplankton). However, rates of excretion (as a function of organism dry mass and water temperature) were significantly different among different orders of zooplankton, mussels and fish. However, excretion of N was similar among different orders of benthic macroinvertebrates. 4. Detritivorous fish excreted both N and P at rates greater than all other taxa; whereas mussels excreted N and P generally at rates less than other taxa. There were no significant differences in the rate of N and P excretion between zooplankton and fish (i.e. the allometry of N and P excretion was similar between zooplankton and fish). 5. Molar N : P ratios of nutrients excreted increased with increasing organism dry mass for each group of metazoans, except for zooplankton and detritivorous fish (where N : P ratios declined with increasing organism dry mass). Molar N : P ratios in the excretions of aquatic metazoans were generally below the Redfield ratio of 16:1. 6. We examined the influence of variable abundance of zooplankton, benthic macroinvertebrates and fish on assemblage excretion rates. Rates of N and P excretion were calculated by applying our models to metazoan biomass and abundance data over seven consecutive years in two oligotrophic lakes. Rates of N and P excretion (g ha^?1 day^?1) increased linearly with increasing assemblage biomass (kg ha^?1). However, rates of N and P excretion were significantly and negatively correlated with the relative abundance of fish and positively correlated with the relative abundance of zooplankton.     

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.     

Food web architecture and basal resources interact to determine biomass and stoichiometric cascades along a benthic food web

Understanding the effects of predators and resources on primary producers has been a major focus of interest in ecology. Within this context, the trophic cascade concept especially concerning the pelagic zone of lakes has been the focus of the majority of these studies. However, littoral food webs could be especially interesting because base trophic levels may be strongly regulated by consumers and prone to be light limited. In this study, the availability of nutrients and light and the presence of an omnivorous fish (Hyphessobrycon bifasciatus) were manipulated in enclosures placed in a humic coastal lagoon (Cabiúnas Lagoon, Macaé - RJ) to evaluate the individual and interactive effects of resource availability (nutrients and light) and food web configuration on the biomass and stoichiometry of periphyton and benthic grazers. Our findings suggest that light and nutrients interact to determine periphyton biomass and stoichiometry, which propagates to the consumer level. We observed a positive effect of the availability of nutrients on periphytic biomass and grazers' biomass, as well as a reduction of periphytic C∶N∶P ratios and an increase of grazers' N and P content. Low light availability constrained the propagation of nutrient effects on periphyton biomass and induced higher periphytic C∶N∶P ratios. The effects of fish presence strongly interacted with resource availability. In general, a positive effect of fish presence was observed for the total biomass of periphyton and grazer's biomass, especially with high resource availability, but the opposite was found for periphytic autotrophic biomass. Fish also had a significant effect on periphyton stoichiometry, but no effect was observed on grazers' stoichiometric ratios. In summary, we observed that the indirect effect of fish predation on periphyton biomass might be dependent on multiple resources and periphyton nutrient stoichiometric variation can affect consumers' stoichiometry.     

The importance of nutrient supply by fish excretion and watershed streams to a eutrophic lake varies with temporal scale over 19 years

Animals transform and translocate nutrients at ecologically relevant rates, contributing to eutrophication in aquatic ecosystems by mobilizing otherwise unavailable nutrients. Yet we know little about how animal-mediated nutrient cycling compares with external abiotic nutrient sources over long periods (years–decades) and at multiple timescales. To address this, we conducted a 19-year study in a eutrophic reservoir examining nitrogen (N) and phosphorus (P) inputs from watershed streams versus excretion by an abundant fish (gizzard shad, Dorosoma cepedianum) at weekly, monthly and seasonal timescales. Over the entire time period, watershed N and P loading was 33- and 3-fold greater than fish N and P excretion, respectively. However, fish N excretion exceeded watershed nutrient loading in 36% of weeks and 43% of months, and fish P excretion in 68% of weeks and 58% of months during the growing season. Fish excretion had lower temporal variability in both supply rate and N:P ratio than watershed loading. Fish excretion also supplied nutrients at a much lower molar N:P ratio than the watershed (mean of daily N:P supply ratios were 15 and 723, respectively). In eutrophic lakes with high fish biomass, fish excretion can strongly influence algal biomass and community composition. Eutrophication management efforts should consider removal of benthivorous fish, like gizzard shad, in addition to other watershed management practices to improve water quality. Future climate change will modulate the interplay between fish- and watershed-mediated nutrient dynamics by altering the geographic distribution of detritivorous fish and the frequency and severity of storm and drought events.     

Stoichiometry of nutrient excretion by fish: interspecific variation in a hypereutrophic lake

This study investigates how nutrient cycling rates and ratios vary among fish species, with a particular focus on comparing an ecologically dominant detritivore (gizzard shad) to other fishes in a productive lake. We also examined how nutrient cycling rates are mediated by body size (as predicted by allometry theory), and how variation in nutrient cycling is related to body and food nutrient contents (according to predictions of ecological stoichiometry). As predicted by allometry, per capita nitrogen and phosphorus excretion rates increased and mass-specific excretion rates decreased, with increasing mass. Body phosphorus content was correlated with body mass only in one species, bluegill. Contrary to stoichiometric predictions, there was no relationship between body P and mass-normalized P excretion rate, or between body N:P and excreted N:P, when all individuals of all species were considered.
However, at the species level, we observed some support for a body nutrient content effect on excretion as predicted by stoichiometry theory. For example, gizzard shad had lower body P (high body N:P) and also excreted P at higher rates (lower N:P) than bluegill, which had high body P (lower body N:P). We applied the Sterner (1990) homeostatic stoichiometry model to the two most common species in the study – gizzard shad and bluegill and found that food N:P had a greater effect than consumer body N:P on excreted N:P. This indicates that, in terms of variation among these species, nutrient excretion may be more of a function of food nutrient content than the nutrient content of the consumer. These results suggest that stoichiometry can provide a framework for variation among species in nutrient cycling and for evaluating the ecosystem consequences of biodiversity loss.     

Stoichiometry of nutrient recycling by vertebrates in a tropical stream: linking species identity and ecosystem processes

Ecological stoichiometry offers a framework for predicting how animal species vary in recycling nutrients, thus providing a mechanism for how animal species identity mediates ecosystem processes. Here we show that variation in the rates and ratios at which 28 vertebrate species (fish, amphibians) recycled nitrogen (N) and phosphorus (P) in a tropical stream supports stoichiometry theory. Mass-specific P excretion rate varied 10-fold among taxa and was negatively related to animal body P content. In addition, the N : P ratio excreted was negatively related to body N : P. Body mass (negatively related to excretion rates) explained additional variance in these excretion parameters. Body P content and P excretion varied much more among taxonomic families than among species within families, suggesting that familial composition may strongly influence ecosystem-wide nutrient cycling. Interspecific variation in nutrient recycling, mediated by phylogenetic constraints on stoichiometry and allometry, illustrates a strong linkage between species identity and ecosystem function.     

Light and nutrients regulate energy transfer through benthic and pelagic food chains

The amount of energy flowing to top trophic levels depends on primary production and the efficiency at which it is converted to production at each trophic level. In aquatic systems, algal production is often limited by light and nutrients, and the nutritional quality of algae depends on the relative balance of these two resources. In this study, we used a mesocosm experiment to examine how light and nutrient variation affected food chain efficiency (FCE, defined as the proportion of primary production converted to top trophic level production), using a food web with benthic and pelagic food chains. We also related variation in benthic and pelagic efficiencies to the nutritional quality of primary producers, i.e. carbon:nitrogen:phosphorus stoichiometry. As predicted, pelagic and benthic FCEs were highest under low light/high nutrient conditions, the treatment with the best algal food quality, i.e. the lowest C:nutrient ratios. Pelagic FCE and pelagic herbivore efficiency (HE_P) were more responsive than benthic FCE to variation in light and nutrients. Furthermore, pelagic FCE and HE_P were highly correlated with algal C:P, suggesting ‘carryover effects’ of algal food quality on carnivores (larval fish) via effects on herbivore (zooplankton) quality. Benthic (tadpole) production was primarily explained by primary production rate, suggesting food quantity rather than quality drives their production. However, benthic FCE was also highest at low light/high nutrients and was significantly correlated with food quality. The stronger effect of food quality in mediating pelagic compared to benthic efficiencies, is consistent with differences in the stoichiometric mismatches between algae and consumers. Pelagic FCE and HE_P were more likely to be P‐limited, whereas benthic FCE was more likely N‐limited. This study is the first to examine both pelagic and benthic FCE within the same system, and highlights the importance of differential consumer needs in determining how food quality affects energy transfer efficiency.     

Factors influencing nitrogen and phosphorus excretion rates of fish in a shallow lake

Summary 1. Fish excretion can be an important source of nutrients in aquatic ecosystems. Nitrogen (N) and phosphorus (P) excretion rates are influenced by many factors, including fish diet, fish growth rate and fish size. However, the relative influence of these and other factors on community‐level excretion rates of fish is unknown. 2. We used bioenergetics modeling to estimate excretion rates of eight fish species in a shallow, Minnesota (U.S.A.) lake over four months in both 2004 and 2005. Excretion rates of each species were summed for community‐level N and P excretion rates, as well as the N : P ratio of excretion. We then used a model‐selection approach to assess ability of independent variables to predict excretion rates, and to identify the most parsimonious model for predicting N : P excretion ratios and P and N excretion rates at the community scale. Predictive models were comprised of the independent variables water temperature and average fish density, fish size, fish growth rate, nutrient content of fish and nutrient content of fish diets at the community scale. 3. Fish density and nutrient content of fish diets (either N or P) were the most parsimonious models for predicting both N and P excretion rates, and explained 96% and 92% of the variance in N and P excretion, respectively. Moreover, fish density and nutrient models had 1400‐fold more support for predicting N and 21‐fold more support for predicting P excretion relative to models based on fish density only. Water temperature, fish size, fish growth rates and nutrient content of fish showed little influence on excretion rates, and none of our independent variables showed a strong relationship with N : P ratios of excretion. 4. Past work has focused on the importance of fish density as a driver of fish excretion rates on a volumetric basis. However, our results indicate that volumetric excretion rates at the community scale will also change substantially in response to changes in relative abundance of fish prey or shifts in relative dominance of planktivores, benthivores, or piscivores. Changes in community‐scale excretion rates will have subsequent influences on algal abundance, water clarity, and other ecosystem characteristics.     

Spatial and seasonal variation in nutrient excretion by benthic invertebrates in a eutrophic reservoir

1. Nitrogen (N) and phosphorus (P) fluxes via excretion by benthic invertebrates were quantified in a eutrophic reservoir (Acton Lake, Ohio, U.S.A.). We quantified variation in nutrient fluxes seasonally (June until November 1997), spatially (three sites) and among taxa (chironomids, tubificid oligochaetes and Chaoborus). 2. The three taxa differed in spatial distribution and contribution to nutrient fluxes. Tubificids were the most abundant taxon at two oxic sites (1.5 and 4 m depth), and were exceedingly rare at an anoxic, hypolimnetic site (8 m). Chironomids were abundant only at the shallowest oxic site. Chaoborus was the only abundant taxon at the anoxic site. Total benthic invertebrate biomass was greatest at the shallowest site and lowest at the anoxic, hypolimnetic site. 3. Mass‐specific excretion rate [μmol NH₄–N or soluble reactive P (SRP) excreted mg dry mass^–1 h^–1] varied among experiments and was influenced by temperature. Differences among taxa were not significant. Thus, nutrient flux through benthic invertebrates was affected more by total invertebrate biomass and temperature than by species composition. 4. Fluxes of N and P via benthic invertebrate excretion (μmol NH₄–N or SRP m^–2 day^–1) were greatest at the oxic sites, where fluxes were dominated by the excretion of tubificids and chironomids. The N and P fluxes at the anoxic site were much lower, and were dominated by excretion by Chaoborus. The ratio of N and P excreted by the benthic invertebrate assemblage varied seasonally and was lowest at the anoxic site. 5. Comparison with other measured inputs shows that excretion by benthic invertebrates could be an important source of nutrients, especially of P. However, the relative importance of nutrient excretion by the benthos varies greatly spatially and temporally.     

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.     

Using size distributions to detect nutrient and sediment effects within and between habitats

We studied the use of size distributions as a response variable in limnological experiments. Previous quantifications of size distributions were incomplete or difficult to use in experimental settings, and we developed a multivariate approach that more fully describes the shape and biomass of planktonic and benthic size distributions. We re-evaluate the hypothesis that fish affect the shape and nutrients affect the biomass of size distributions, and show that the multivariate approach is more responsive to detecting treatment effects. In a mesocosm experiment, we use this new quantification and analysis of size distributions to detect the main and interactive effects of nutrient addition and sediment type on both benthic and pelagic size distributions. Size distributions in both habitats responded to the nutrient and sediment treatments, indicating linkage since a treatment applied in one habitat affected the size distribution in the opposite habitat. Since size distributions reduce each habitat into a common currency, we were able to examine the nature of the linkage. The relative response of each habitat to the nutrient treatment was different with regard to the shape of the distributions, while the relative response to the sediment treatment was different with regard to the biomass in the distributions.     

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.     

Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

Productivity and trophic structure of aquatic ecosystems result from a complex interplay of bottom‐up and top‐down forces that operate across benthic and pelagic food web compartments. Projected global changes urge the question how this interplay will be affected by browning (increasing input of terrestrial dissolved organic matter), nutrient enrichment and warming. We explored this with a process‐based model of a shallow lake food web consisting of benthic and pelagic components (abiotic resources, primary producers, grazers, carnivores), and compared model expectations with the results of a browning and warming experiment in nutrient‐poor ponds harboring a boreal lake community. Under low nutrient conditions, the model makes three major predictions. (a) Browning reduces light and increases nutrient supply; this decreases benthic and increases pelagic production, gradually shifting productivity from the benthic to the pelagic habitat. (b) Because of active habitat choice, fish exert top‐down control on grazers and benefit primary producers primarily in the more productive of the two habitats. (c) Warming relaxes top‐down control of grazers by fish and decreases primary producer biomass, but effects of warming are generally small compared to effects of browning and nutrient supply. Experimental results were consistent with most model predictions for browning: light penetration, benthic algal production, and zoobenthos biomass decreased, and pelagic nutrients and pelagic algal production increased with browning. Also consistent with expectations, warming had negative effects on benthic and pelagic algal biomass and weak effects on algal production and zoobenthos and zooplankton biomass. Inconsistent with expectations, browning had no effect on zooplankton and warming effects on fish depended on browning. The model is applicable also to nutrient‐rich systems, and we propose that it is a useful tool for the exploration of the consequences of different climate change scenarios for productivity and food web dynamics in shallow lakes, the worldwide most common lake type.     

Nutrient recycling by two phosphorus-rich grazing catfish: the potential for phosphorus-limitation of fish growth

In ecosystems where excretion by fish is a major flux of nutrients, the nitrogen (N) to phosphorus (P) ratio released by fish can be important in shaping patterns of algal biomass, community composition, primary production, and nutrient limitation. Demand for N and P as well as energy influences N/P excretion ratios and has broad implications in ecosystems where nutrient recycling by fishes is substantial. Bioenergetics and stoichiometric models predict that natural fish populations are generally energy-limited and therefore N/P recycling by fishes is relatively invariant. Yet, the potential for P limitation of growth has not been examined in herbivorous fishes, which are common in many aquatic habitats. We examined N/P excretion ratios and P demand in two P-rich herbivorous catfishes of the family Loricariidae, Ancistrus triradiatus (hereafter Ancistrus) and Chaetostoma milesi (hereafter Chaetostoma). Both fishes are common grazers in the Andean piedmont region of Venezuela where we conducted this study. Mass balance (MB) models indicate that these fishes have a high P demand. In fact, our Ancistrus’ P MB model predicted negative P excretion rates, indicating that Ancistrus did not consume enough P to meet its P demand for growth. Direct measurement of excretion rates showed positive, but very low P excretion rates and high N/P excretion ratios for both taxa. To obtain measured P excretion rates of Ancistrus from the MB model, gross growth efficiency must be reduced by 90%. Our results suggest that growth rates of both of these herbivorous and P-rich fish are likely P-limited. If P limitation of growth is common among herbivorous fish populations, herbivorous fishes recycle likely at high N/P ratios and act to diminish the quality of their food.     

Ontogeny, diet shifts, and nutrient stoichiometry in fish

Most stoichiometric models do not consider the importance of ontogenetic changes in body nutrient composition and excretion rates. We quantified ontogenetic variation in stoichiometry and diet in gizzard shad, Dorosoma cepedianum , an omnivorous fish with a pronounced ontogenetic diet shift; and zebrafish, Danio rerio, grown in the lab with a constant diet. In both species, body stoichiometry varied considerably along the life cycle. Larval gizzard shad and zebrafish had higher molar C:P and N:P ratios than larger fish. Variation in body nutrient ratios was driven mainly by body P, which increased with size. Gizzard shad body calcium content was highly correlated with P content, indicating that ontogenetic P variation is associated with bone formation. Similar trends in body stoichiometry of zebrafish, grown under constant diet in the laboratory, suggest that ontogeny (e.g. bone formation) and not diet shift is the main factor affecting fish body stoichiometry in larval and juvenile stages. The N:P ratio of nutrient excretion also varied ontogenetically in gizzard shad, but the decline from larvae to juveniles appears to be largely associated with variation in the N:P of alternative food resources (zooplankton vs detritus) rather than by fish body N:P. Furthermore, the N:P ratio of larval gizzard shad excretion appears to be driven more by the N:P ratio at which individuals allocate nutrients to growth, more so than static body N:P, further illustrating the need to consider ontogenetic variation. Our results thus show that fish exhibit considerable ontogenetic variation in body stoichiometry, driven by an inherent increase in the relative allocation of P to bones, whereas ontogenetic variation in excretion N:P ratio of gizzard shad is driven more by variation in food N:P than by body N:P.     

Stoichiometry of benthic invertebrate nutrient recycling: interspecific variation and the role of body mass

Ecological stoichiometry (ES) and allometry offer frameworks for predicting how nutrient recycling varies within and among animal species. Despite the importance of benthic-derived nutrients in most aquatic systems, predictions based on ES and allometry have been poorly tested among benthic invertebrate consumers. Here, we show that the rates and ratios at which three freshwater benthic invertebrate species (a crustacean, an insect, and a polychaeta) recycled nitrogen (N) and phosphorus (P) can be partially predicted by ES and allometry depending on whether data are analyzed intra- or interspecifically. Mass-specific N and P excretion rates were negatively correlated with invertebrate body size both among and within taxa, supporting allometric predictions. However, mass-specific N and P excretion rates were positively and negatively correlated to invertebrate body N and P, respectively, but only when data were analyzed intraspecifically. As a corollary, the mass-specific N:P excretion ratio was positively related to body N:P ratio. Such a contrasting pattern on excretion-mediated N and P recycling suggests that stoichiometric constraints regarding consumer-resource imbalances for the three species utilized in this study may be stronger for P than for N. Our results indicate that the variation in nutrient recycling, which is mediated by taxonomic constraints on stoichiometry and allometry, may substantially help us to understand the importance of benthic detritivorous species to the functioning of aquatic ecosystems.     

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.     

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.     

Effects of Nutrients,Fish, Charophytes and Algal Sediment Recruitment on the Phytoplankton Ecology of a Shallow Lake

The influence of nutrient levels, fish density and charophytes on the phytoplankton ecology of a shallow Mediterranean lake was studied by means of an in situ mesocosm experiment. Different levels of nutrients and fish were added over the course of an eight‐week experiment, during which charophytes were removed towards the end. After submerged plants were removed, phytoplankton biomass increased significantly in all the mesocosms, with a reduction of algal diversity and species richness and dominance of cyanobacteria. Cyanobacteria recruited from the sediment played an important role in sustaining planktonic populations of the dominant species. Oscillatorial species (Pseudanabaena galeata, Planktolyngbya limnetica) dominated at higher nutrient levels (0.5–1 mg L^–1 P and 5–10 mg L^–1 N) and chroococcal cyanobacteria (Merismopedia tenuissima) at lower nutrient levels. Density of planktivorous fish had little effect on the algal recruitment from the sediment and phytoplankton biomass and diversity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)