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The effects of global and local environmental changes are transmitted through networks of interacting organisms to shape the structure of communities and the dynamics of ecosystems. We tested the impact of elevated temperature on the top-down and bottom-up forces structuring experimental freshwater pond food webs in western Canada over 16 months. Experimental warming was crossed with treatments manipulating the presence of planktivorous fish and eutrophication through enhanced nutrient supply. We found that higher temperatures produced top-heavy food webs with lower biomass of benthic and pelagic producers, equivalent biomass of zooplankton, zoobenthos and pelagic bacteria, and more pelagic viruses. Eutrophication increased the biomass of all organisms studied, while fish had cascading positive effects on periphyton, phytoplankton and bacteria, and reduced biomass of invertebrates. Surprisingly, virus biomass was reduced in the presence of fish, suggesting the possibility for complex mechanisms of top-down control of the lytic cycle. Warming reduced the effects of eutrophication on periphyton, and magnified the already strong effects of fish on phytoplankton and bacteria. Warming, fish and nutrients all increased whole-system rates of net production despite their distinct impacts on the distribution of biomass between producers and consumers, plankton and benthos, and microbes and macrobes. Our results indicate that warming exerts a host of indirect effects on aquatic food webs mediated through shifts in the magnitudes of top-down and bottom-up forcing.

The possibility of P limitation for zooplankton growth has many implications for understanding changes in production efficiency and feedback dynamics between consumers and resoutxes. However, there have been no direct tests to determine whether the putative P limitation is real. To answer this question, we directly supplied inorganic P to Daphnia magna apart from food algae, Scenedesmus acutus, and then evamined changes in body mass. During the period from birth to age 6 d, D. magna were fed on live algae For 19 h and placed in water of high inorganic P (4 mM: P treatment) for 5 h each day. We used P-free water as a control treatment. Growth rate estimated from initial and final body mass during the 6-d incubation was significantly larger in the P treatment than in the control treatment when Daphnia fed on P-deficient algae, whereas a significant dij'ference was not detected between the treatments for Daphnia fed on P-sufficient algae (C : P atomic < 300). The results clearly demonstrate that Daphnia growth is in fact limited by P itself when they feed on P-deficient algae.Recently, controversy has sparked over the possibility of direct P limitation for zooplankton production. Several studies have demonstrated lower individual and population growth rates of Daphnia when they feed on algae with low P content relative to carbon, i.e. high C : P ratios (Sommer 1992;Sterner 1993;Sterner et al. 1993). Using mass-balance models, Olsen et al. (1986) and Urabe and Watanabe (1992) estimated the threshold food C : P ratio, above which net production of cladocerans is limited by P content rather than C in the food. In both these studies, the threshold was calculated to be -300 (atomic ratio) for Daphnia, although the ratio changes according to food concentration and digestibility of carbon in the food (Hessen 1992; Urabe and Watanabe 1992) and because of changes in the proportion of metabolic cost (respiration) within assimilated carbon (Sterner and Robinson 1994;Sterner 1997). Seston has C : P ratios >300 in many north temperate lakes (Hecky et al. 1993; Elser and Hassett 1994;Sterner et al. 1997), implying that P limitation of Daphnia growth may be common.Direct P limitation of zooplankton growth, however, has been called into question (Brett 1993; Mtiller-Navarra 1995a, b) in part because of the lack of direct evidence showing whether P is in fact the actual substance limiting the growth rate of Daphnia. Although direct demonstrations of the precise limiting factors for zooplankton feeding on low-quality foods have been lacking, several studies suggest that the growth rate of Daphnia is affected by the availability of individual long-chain unsaturated fatty acids such as EPA (eicosapentaenoic acid, 20 : 503) and DHA (docosahexaenoic acid, 22 : 603) (Ahlgren et al. 1989; Miiller-Navarra 1995b). Fatty acids such as these are essential substances for

Incident light was manipulated in large plankton towers containing algae, microbes, and herbivores. Paradoxically, food chain production was lower with greater light energy input. This apparent paradox is resolved by recognizing stoichiometric constraints to food chain production. At high light, elevated algal biomass was achieved mainly by increases in cellular carbon. Consumers have a high phosphorus demand for growth, and thus a large excess of carbon inhibited, rather than stimulated, their growth. These experiments may help us predict the consequences of anthropogenic perturbations in nutrients, carbon, and solar energy. They also may help us to understand the wide range of consumer biomass and production at a given level of primary productivity in ecosystems.

1. Samples from 16 lakes in central (n = 145) and western (n = 12) North America, the coastal northeast Pacific (n = 302) and the western Canadian Arctic Oceans (n = 142) were collected and analysed for viral, bacterial and cyanobacterial abundances and chlorophyll-a concentration.2. Viral abundance was significantly different among the environments. It was highest in the coastal Pacific Ocean and lowest in the coastal Arctic Ocean. The abundances of bacteria and cyanobacteria as well as chlorophyll-a concentrations also differed significantly among the environments, with both bacterial abundance and chlorophyll-a concentration highest in lakes. As a consequence, the association of these variables with viral abundance varied among the environments. 3. Discriminant analyses with the abundance data indicated that the marine and freshwater environments were predictably different from each other. Multiple-regression analysis included bacterial and cyanobacterial abundances, and chlorophyll-a concentration as significant variables in explaining viral abundance in lakes. In regression models for the coastal Pacific Ocean, bacterial and cyanobacterial abundances were significant variables, and for the coastal Arctic Ocean viral abundance was predicted by bacterial abundance and chlorophyll-a concentration. 4. The relationship of viral and bacterial abundance differed between the investigated freshwater and marine environments, probably because of differences in viral production and loss rates. However, freshwaters had fewer viruses compared to bacteria, despite previously documented higher burst sizes and frequencies of infected cells, suggesting that loss rates may be more important in lakes. 5. Together, these findings suggest that there are different drivers of viral abundance in different aquatic environments, including lakes and oceans.