A test of stoichiometry across six Irish lakes of low-moderate nutrient status and contrasting hardness

The importance of stoichiometric food quality constraints in influencing zooplankton community structure and distribution in six lakes with categorical variations in alkalinity were investigated by assessing the relationship between seston nutrient content, bulk zooplankton nutrient content and the abundance of individual zooplankton taxa. Stoichiometric processes were viewed in the context of the overall biotic and abiotic environment. Distinct differences in the distribution of certain zooplankton taxa between high- and low-alkalinity lakes were observed and size-distribution analyses suggested that the size-selective foraging behaviour of fish may have been responsible for determining seasonal variation in community structure. Nevertheless, a positive correlation was observed between bulk zooplankton C:P content and bulk seston C:P content, and a negative relationship between Daphnia abundance and the seston C:P and N:P ratios, which suggests that with declining food P content, high-P taxa are replaced by low-P taxa. This would suggest that stoichiometric food quality constraints are in part involved in structuring the zooplankton community of the six lakes of this study. Although classic models of lake trophic structure based on carbon input (bottom-up) and predation (top-down) may provide explanations for broad community patterns, stoichiometry may provide an alternative or additional mechanism for understanding the details of zooplankton species composition and dynamics.