Food webs in nature have multiple, reticulate connections between a diversity of consumers and resources. Such complexity affects web dynamics: it first spreads the direct effects of consumption and productivity throughout the web rather than focusing them at particular "trophic levels." Second, consumer densities are often donor controlled with food from across the trophic spectrum, the herbivore and detrital channels, other habitats, life-history omnivory, and even trophic mutualism. Although consumers usually do not affect these resources, increased numbers often allow consumers to depress other resources to levels lower than if donor-controlled resources were absent. We propose that such donor-controlled and "multichannel" omnivory is a general feature of consumer control and central to food web dynamics. This observation is contrary to the normal practice of inferring dynamics by simplifying webs into a few linear "trophic levels," as per "green world" theories. Such theories do not accommodate commo...

Food Web Complexity and Community Dynamics

▪ Abstract Animals are important in nutrient cycling in freshwater ecosystems. Via excretory processes, animals can supply nutrients (nitrogen and phosphorus) at rates comparable to major nutrient sources, and nutrient cycling by animals can support a substantial proportion of the nutrient demands of primary producers. In addition, animals may exert strong impacts on the species composition of primary producers via effects on nutrient supply rates and ratios. Animals can either recycle nutrients within a habitat, or translocate nutrients across habitats or ecosystems. Nutrient translocation by relatively large animals may be particularly important for stimulating new primary production and for increasing nutrient standing stocks in recipient habitats. Animals also have numerous indirect effects on nutrient fluxes via effects on their prey or by modification of the physical environment. Future studies must quantify how the importance of animal-mediated nutrient cycling varies among taxa and along environme...

Nutrient Cycling by Animals in Freshwater Ecosystems

http://www3.carleton.ca/fecpl/courses/ESS.pdf

Ecosystem services generated by fish populations

Biologists concerned with the evolutionary consequences of competition seem unaware of a large literature on freshwater fishes that bears directly on the debates over ecological character displacem...

Character release and displacement in fishes: a neglected literature

The recent invasion of San Francisco Bay by the suspension-feeding clam Putamocorbula amurensis has provided an opportunity to document the ecological consequences of a major biological disturbance. Previous work over the last two decades has shown that phytoplankton biomass in the upper estuary is low (2-3 mg Chl a m-3) during seasonal periods of high river flow and short residence time, and it is usually high (peak > 30 mg Chl a m-3) during the summer-autumn seasons of low river flow and long residence time. However since P. amurensis became widespread and abundant in 1987, the summer phytoplankton biomass maximum has disappeared, presumably because of increased grazing pressure by this newly introduced species. For the period 1977-l 990, mean estimated primary production was only 39 g C mm2 yr-’ during years when bivalve suspension feeders were abundant (>2,000 mm2), compared to 106 g C m-2 yr-’ when bivalves were absent or present in low numbers. These observations support the hypothesis that seasonal and interannual fluctuations in estuarine phytoplankton biomass and primary production can be regulated jointly by direct physical effects (e.g. river-driven transport) and trophic interactions (episodes of enhanced grazing pressure by immigrant populations of benthic suspension feeders).

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1992.37.5.0946

Trophic interactions and direct physical effects control phytoplankton biomass and production in an estuary

Observations and experiments on zooplankton grazing and nutrient (N and P) limitation of phytoplankton of Lake Mendota, Wisconsin, revealed that the relative strength of the two factors varied markedly during the seasonal succession of phytoplankton. Furthermore, seasonal changes in grazing and nutrient limitation were caused not only by seasonal changes in zooplankton community structure and nutrient availability but also by changes in phytoplankton community structure, which led to changes in edibility to zooplankton. The spring phytoplankton bloom of diatoms and small flagellates was highly vulnerable to grazing by both cyclopoid copepods and Duphnia, although Daphnia had a much stronger effect. Increased Duphnia grazing in late spring resulted in low phytoplankton biomass (the clear-water period) and a phytoplankton community dominated by colonial green algae- taxa vulnerable to grazing by Daphnia but not other zooplankton. Nutrients (N and P) did not limit phytoplankton growth during the spring bloom or the clear-water period. After the clear-water period, the summer phytoplankton community was dominated by blue-greens and Ceratium. Grazing effects by both Daphnia and copepods were low in summer, while nutrient limitation (both N and P) became severe. Reduced grazing impacts in summer may have resulted from prior intense grazing impacts, which led to dominance of grazing-resistant taxa. These results suggest that seasonal variation in the strength of grazing and nutrient controls in eutrophic lakes results from changes in zooplankton biomass and community structure, nutrient availability, and phytoplankton community structure interacting to determine phytoplankton seasonal succession.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1990.35.3.0697

Seasonal patterns of grazing and nutrient limitation of phytoplankton in a eutrophic lake

PREDATION can be a potent force structuring ecological communities and affecting several trophic levels1–7. The cascading trophic interactions hypothesis predicts that lacustrine predators such as fish can have a strong effect on herbivorous zooplankton, which in turn can regulate phytoplankton8,9. Ascertaining the scale, scope and generality of this hypothesis is important for both development of ecological theory and aquatic ecosystem management10. Although small-scale tests of parts of this cascade are common11–15, whole-lake assessments are not, particularly in large, natural lakes9; also the influence of several trophic levels and nutrients, a requisite for unambiguous interpretation of predator effects, has not been considered. Here we present data from a natural experiment in Lake Mendota, Wisconsin, USA, which support the cascading trophic interactions hypothesis. Massive mortality of fish greatly reduced prEdation on zooplankton, resulting in an increase in the abundance of large Daphnia, and a dramatic decrease in phytoplankton biomass. Physical factors and concentrations of limiting nutrients were unchanged before and after fish mortality, indicating that these factors probably did not cause the observed decrease in phytoplankton. Our results demonstrate strong food web influences on phytoplankton, and support the idea that food web interactions can be managed to reduce phytoplankton abundance.

Effects on lower trophic levels of massive fish mortality

PREDATION can be a potent force structuring ecological communities and affecting several trophic levels1–7. The cascading trophic interactions hypothesis predicts that lacustrine predators such as fish can have a strong effect on herbivorous zooplankton, which in turn can regulate phytoplankton8,9. Ascertaining the scale, scope and generality of this hypothesis is important for both development of ecological theory and aquatic ecosystem management10. Although small-scale tests of parts of this cascade are common11–15, whole-lake assessments are not, particularly in large, natural lakes9; also the influence of several trophic levels and nutrients, a requisite for unambiguous interpretation of predator effects, has not been considered. Here we present data from a natural experiment in Lake Mendota, Wisconsin, USA, which support the cascading trophic interactions hypothesis. Massive mortality of fish greatly reduced prÉdation on zooplankton, resulting in an increase in the abundance of large Daphnia, and a dramatic decrease in phytoplankton biomass. Physical factors and concentrations of limiting nutrients were unchanged before and after fish mortality, indicating that these factors probably did not cause the observed decrease in phytoplankton. Our results demonstrate strong food web influences on phytoplankton, and support the idea that food web interactions can be managed to reduce phytoplankton abundance.

Effects on lower tropic levels of massive fish mortality

It is becoming increasingly clear that lake plankton communities are regulated by both predation and resources. Top predators, through effects on herbivores, can regulate phytoplankton community structure, biomass, and primary productivity (Carpenter et al. 1985; Carpenter and Kitchell 1988; Vanni and Findlay 1990). Increase in potential limiting nutrients, such as nitrogen and phosphorus, can stimulate phytoplankton production and biomass, and the ratio of limiting nutrients can influence community structure (Schindler 1977; Smith 1983). Recently, lake ecologists have realized that when attempting to explain phytoplankton dynamics, both nutrients and herbivory must be considered. Often the two interactions simultaneously influence phytoplankton, although their relative strengths may vary seasonally (e.g. Sommer et al. 1986; Vanni and Temte 1990). Furthermore, resource effects and herbivore effects may interact in complex ways to influence phytoplankton (Leibold 1980; Sterner 1990; Carpenter et al. 1991)—a case of the interactions themselves interacting (Roughgarden and Diamond 1986).

Herbivory, Nutrients, and Phytoplankton Dynamics in Lake Mendota, 1987–89

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Seasonal patterns of grazing and nutrient limitation of phytoplankton in a eutrophic lake

Effects on lower trophic levels of massive fish mortality

Effects on lower tropic levels of massive fish mortality

Herbivory, Nutrients, and Phytoplankton Dynamics in Lake Mendota, 1987–89