Elemental stoichiometry of lower food web components in arctic and temperate lakes

Lakes were surveyed to assess the potential patterns of latitudinal variation in carbon:nitrogen:phosphorus (C:N:P) stoichiometry of lower food web components. Thirty-four lakes were surveyed at an arctic latitude (68°38� N, 149°38� W) and 10 lakes at a temperate latitude (46°13� N, 89°32� W) during 1997. The temperate data set was augmented with earlier survey results employing similar methods. It was hypothesized that differences in environmental variables across latitude would cause differences in community C:N:P ratios, leading to differences in trophic interactions. Physical measurements (light, temperature), seston and zooplankton were collected from each lake. Seston and zooplankton were analyzed for C, N and P content, and zooplankton were counted and measured for biomass estimates. The degree of trophic interaction between seston and zooplankton was assessed by (i) measuring elemental imbalances between seston and zooplankton and (ii) calculating the potential recycling ratio by the zooplankton community available for seston. Seston C:nutrient, but not N:P, ratios were higher in temperate than arctic lakes. Conversely, arctic zooplankton had higher C:nutrient, but not N:P, ratios than zooplankton in temperate lakes. Elemental imbalances were greater in temperate than in arctic lakes, but N:P stoichiometry of potential zooplankton recycling was nearly identical between the two latitudes. Zooplankton community C:N:P ratios were not related to either latitude or seston C:N:P. In accordance with stoichiometric theory, relative abun- dances of calanoid copepods were positively correlated with seston C:N in temperate lakes. Addition- ally, relative abundances of Daphnia were negatively correlated with seston C:N ratios in temperate and arctic lakes, and positively correlated with N:P ratios in the arctic. In general, these results suggest that seston and zooplankton community stoichiometry differ across latitude, and these differences have the potential to affect trophic interactions.

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