Effects of periphyton stoichiometry on mayfly excretion rates and nutrient ratios

Abstract

We evaluated the effects of periphyton elemental composition on the rate and ratios of N and P excretions by heptageniid mayfly larvae (Ephemeroptera). We predicted that excretion N:P ratios would relate primarily to mismatches between periphyton N:P and mayfly N:P. We immersed periphyton grown in a stream above (inlet: high N) and below (outlet: low N) a lake in P-amended or control (no P) water for 12 h to increase periphyton P content and reduce periphyton N:P. We then measured release rates and ratios of N and P from Cinygmula sp. larvae that had consumed periphyton from 1 of the 4 treatments. Reducing periphyton N:P increased consumer P excretion rates (range for P-amended periphyton: 0.055 ± 0.010 to 0.076 ± 0.022 μg P g−1 dry mass [DM] h−1; range for control periphyton: 0.027 ± 0.008 to 0.031 ± 0.006 μg P g−1 DM h−1). Mayflies that consumed P-amended periphyton from the inlet excreted N at a higher rate than did mayflies that consumed P-amended periphyton from the outlet (inlet P-amended: 0.661 ± 0.244 μg N g−1 DM h−1; outlet P-ammended: 0.245 ± 0.020 μg N g−1 DM h−1). We found some evidence for N:P imbalance between consumer and resource as a predictor of excretion N:P, but other factors also appear to influence nutrient release rates. Mayflies retained significantly more P than predicted by a stoichiometric mass-balance model, a result that suggests that mayflies might be able to store P when it available in their food and use it for growth when needed. Our model also suggests that assimilation efficiencies of N and P are more important determinants of excretion N:P than elemental imbalances between mayflies and their food. We conclude that elemental imbalances between lotic macroinvertebrates and their food resources play an important role in governing nutrient release rates and ratios, but that resource characteristics (e.g., periphyton community composition) and organismal physiology (e.g., nutrient assimilation efficiency) also must be considered. The possibly substantial role of these additional factors in determining nutrient excretion bears further investigation.