Stoichiometric mismatch between littoral invertebrates and their periphyton food

Abstract: Ecological stoichiometry is considered a key concept in understanding con-straints in energy transfer at the plant-herbivore interface. However, whether this con-cept is relevant for benthic freshwater ecosystems is not fully known. Therefore, afield survey was conducted in 2003 during the growing season in the littoral zone ofLake Constance, a large pre-alpine lake in central Europe. The aim was to assess tem-poral variation in the elemental stoichiometric composition in both herbivorous macro-invertebrates and their food resource, the periphyton in two different lakes. The peri-phyton showed large temporal and spatial variation in carbon, nitrogen, and phospho-rus content, with particularly high molar C:P ratios of up to 1225:1. Periphyton C:Pand C:N ratios were often high and constantly above the Redfield ratio that is consid-ered optimal for autotrophic growth. In contrast to the pronounced fluctuations in thenutrient ratios of their food resource, the herbivorous macroinvertebrates showed onlyvery 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 taxonomicgroups of macroinvertebrates, which indicated different requirements of optimal die-tary C:P and C:N ratios and which might influence the ability of the taxa to competefor limiting elemental nutrients. Considering the temporally very high C:P and C:Nratios 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 lit-toral invertebrates. Therefore, the concept of stoichiometric food quality limitationmight also be applicable to the littoral food web in lakes.Key words: benthos, C:N:P ratio, ecological stoichiometry, herbivory, homeostasis,lake littoral, macroinvertebrates, mismatch, nutrient ratios, phosphorus.