ECOLOGICAL STOICHIOMETRY IN RUNNING WATERS: PERIPHYTON CHEMICAL COMPOSITION AND SNAIL GROWTH

Abstract

Mismatches between the elemental composition of primary consumers and their food suggest that the production of primary consumers may be limited by food mineral content. Although the consequences of these mismatches for consumers have been studied extensively for zooplankton and terrestrial herbivores, the relationship between periphyton elemental composition and primary consumer performance has been rarely explored. We manipulated periphyton elemental composition by growing stream periphyton under all combinations (2 × 2 factorial) of low and high inorganic N and P and offered low and high quantities to Elimia livescens snails. Periphyton quantity, as measured by ash-free dry mass and total carbon, was kept the same among nutrient treatments within a level of food quantity. We measured Elimia growth as change in blotted wet mass and shell-free ash-free dry mass. Periphyton chemical composition was described by measurements of percentages of C, N, P, ash and chlorophyll a. At low periphyton quantity, Elimia growth on periphyton grown under high P was 40–66% higher than on periphyton grown under low P. Higher P content of the periphyton grown under high P is the most likely explanation for the increased growth of Elimia. At high food quantity, periphyton chemical composition had no effect on Elimia growth. Snails had increased growth when offered more periphyton of lower quality, but generally not when offered an increased quantity of high-quality periphyton. Although snails removed more periphyton when offered a higher quantity of food, snails did not actively compensate for the lower quality food by increasing removal rates when the same amount of each food type was offered. The elemental composition of Elimia was not affected by food quality or quantity, consistent with ecological stoichiometry theory. We suggest that dissolved P was the ultimate cause of differences in Elimia growth at low food quantity, mediated through changes in periphyton chemical composition. Limitation of primary consumer growth by periphyton chemical composition may be widespread in oligotrophic streams, because periphyton production and food quality are typically low.