Seasonal Shifts in the Use of Green and Brown Food Resources by a Dominant Stream Grazer

Streams in deciduous forests experience seasonally distinct pulses of primary production and leaf litter input every year as a result of riparian leaf phenology. Many benthic invertebrates have evolved to harvest one or the other of these two very different organic resources, but few are effective in exploiting both. Multichannel feeding (using more than one food resource) should be advantageous for a long‐lived consumer, enabling it to attain larger populations than it could on a single resource. In this study, we examined the ability of a dominant primary consumer, the grazing snail Elimia clavaeformis , to assimilate both autochthonous (‘green’) and allochthonous (‘brown’) food resources in forest headwaters. The in situ growth of individually tagged snails and their accrual of nitrogen and phosphorus was measured over four separate years in Walker Branch (Tennessee, USA), a first‐order stream. The measurements were made in 2‐month increments chosen to include separate periods of maximum primary production and leaf litter abundance. Stable carbon isotope analysis of snails collected in spring and autumn was employed as a complementary means of assessing Elimia' s ability to switch between autochthony and allochthony. Elimia exhibited two distinct periods of positive growth each year: one associated with peak primary production in spring and the other with leaf fall in autumn. The snail accrued significant quantities of nitrogen and phosphorus while grazing on both epilithon and leaf litter, despite the latter's ostensibly low food quality. Stable carbon analysis confirmed the snails assimilated significant amounts of leaf carbon in autumn and integrated both green and brown food resources on an annual basis. Elimia lost substantial amounts of biomass and inorganic nutrients in summer when primary production was light‐limited and leaf litter was scarce. Gross primary production was not significantly correlated with snail growth; whereas ecosystem respiration, which incorporates respiration by both autotrophs and heterotrophs, was linearly related to growth. Our results indicate that leaf litter is an important resource subsidy for snails in Walker Branch, allowing them to attain high densities at which they overgraze epilithon. The rasping mode of feeding common to snails likely predisposes these grazers to effectively harvest nutrient‐rich microbes from leaf surfaces as well as from epilithon. The ability to change diets is very advantageous to long‐lived consumers exposed to seasonally variable supplies of autochthonous and allochthonous organic matter. Multichannel feeding by Elimia has important implications for multiple trophic levels in Walker Branch and helps explain why pleurocerid snails like Elimia dominate the benthos in many streams.