Snail grazers affect the fate of labile dissolved organic C in streams

The fate of organic C applied as radiolabeled dissolved organic C (DOC) was tracked in laboratory experimental chambers with and without snail grazers. Low concentrations of radiolabeled amino acids were incubated with microbial communities attached to pieces of rock collected from a central Alabama stream. The microbial communities assimilated a portion of the amino acids and were then placed in glass chambers with stream water only. Individuals of Elimia darn, a dominant snail grazer in small, carbonate streams, were then added to 5/2 of the chambers. C-14 was assayed in various components of the chamber systems, including overlying water and atmosphere, periphyton, snail tissue, and fecal/particulate material over a 24- and 48-h feeding cycle. The fate of the C-14 radiolabel was significantly changed by the presence of the snails during both sampling periods, compared to nongrazed systems. Snails consistently assimilated similar to 22% of the radiolabel. However, (DOC)-C-14 efflux from periphyton was significantly lower when snails were present (8% vs 34% in the no-snail treatment), and more radiolabel occurred in CO2 in the chambers containing snails (23% vs 10% without snails). The amount of C-14 radiolabel in particulate matter was also higher, in part because of fecal pellet formation (33% vs 27% in nongrazed chambers). This study showed that C-14 assimilated by periphyton communities as (DOC)-C-14 is transferred to snail grazers, which then modify the flow path of C.