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Abstract

Ecological stoichiometry is considered a key concept in understanding constraints in energy transfer at the plant-herbivore interface. However, whether this concept is relevant for benthic freshwater ecosystems is not fully known. Therefore, a field survey was conducted in 2003 during the growing season in the littoral zone of Lake Constance, a large pre-alpine lake in central Europe. The aim was to assess temporal variation in the elemental stoichiometric composition in both herbivorous macro-invertebrates and their food resource, the periphyton in two different lakes. The periphyton showed large temporal and spatial variation in carbon, nitrogen, and phosphorus content, with particularly high molar C:P ratios of up to 1225:1. Periphyton C:P and C:N ratios were often high and constantly above the Redfield ratio that is considered optimal for autotrophic growth. In contrast to the pronounced fluctuations in the nutrient ratios of their food resource, the herbivorous macroinvertebrates showed only very little variation in their nutrient ratios, which indicated that they are homeostatic, i.e., physiologically restricted to a comparatively narrow range of C:P and C:N ratios. Distinct species-specific C:P and C:N ratios were found for different taxonomic groups of macroinvertebrates, which indicated different requirements of optimal dietary C: P and C: N ratios and which might influence the ability of the taxa to compete for limiting elemental nutrients. Considering the temporally very high C: P and C: N ratios of the periphytic resource and the very low ratios of the consumer body tissue, this stoichiometric mismatch is likely to constrain growth and reproduction of these littoral invertebrates. Therefore, the concept of stoichiometric food quality limitation might also be applicable to the littoral food web in lakes.