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

Lakes were surveyed to assess the potential patterns of latitudinal variati on in carbon:nitrogen:phosphorus (C:N:P) stoichiometry of lower food web co mponents. Thirty-four lakes were surveyed at an arctic latitude (68 degrees 38'N, 149 degrees 38'W) and 10 lakes at a temperate latitude (46 degrees 1 3'N, 89 degrees 32'W) during 1997. The temperate data set was augmented wit h earlier survey results employing similar methods. It was hypothesized tha t differences in environmental variables across latitude would cause differ ences in community C:N:P ratios, leading to differences in trophic interact ions. Physical measurements (light, temperature), seston and zooplankton we re collected from each lake. Seston and zooplankton were analyzed for C, N and P content, and zooplankton were counted and measured for biomass estima tes. The degree of trophic interaction between seston and zooplankton was a ssessed by (i) measuring elemental imbalances between seston and zooplankto n and (ii) calculating the potential recycling ratio by the zooplankton com munity available for seston. Seston C:nutrient, but not N:P, ratios were hi gher in temperate than arctic lakes. Conversely, arctic zooplankton had hig her C:nutrient, but not N:P, ratios than zooplankton in temperate lakes. El emental imbalances were greater in temperate than in arctic lakes, but N:P stoichiometry of potential zooplankton recycling was nearly identical betwe en the two latitudes. Zooplankton community C:N:P ratios were not related t o either latitude or seston C:N:P. In accordance with stoichiometric theory , relative abundances of calanoid copepods were positively correlated with seston C:N in temperate lakes. Additionally, 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.