Effects of a Low Head Dam on a Dominant Detritivore and Detrital Processing in a Headwater Stream

Thornwall, Brett

ABSTRACT: The caddisfly Pycnopsyche gentilis is a dominant detritivore in southern Appalachian streams. A dam on Sims Creek selectively removes P. gentilis from downstream reaches. I evaluated the breakdown of yellow birch leaves in the presence and absence of P. gentilis using a leaf pack breakdown experiment. Leaf packs were placed in reaches above the dam where P. gentilis is present and below the dam where it is essentially absent. I also conducted experiments to determine why P. gentilis was uncommon below the dam. Using enclosures, I evaluated whether there were differences in the survival and growth of P. gentilis in the two stream sections. I also evaluated the potential for intraguild predation by a larger caddisfly, Pycnopsyche luculenta, which is more abundant below the dam. Pycnopsyche gentilis larvae were placed in enclosures with or without P. luculenta above and below the dam; all enclosures included leaf packs. Leaf breakdown was significantly slower in the downstream section. Pycnopsyche gentilis was absent from downstream leaf packs. The stonefly Tallaperla was also less abundant below the dam. Detritivore diversity indices were similar for both stream sections. Calculations of P. gentilis per capita leaf ingestion in enclosures suggest that P. gentilis was primarily responsible for the higher breakdown rates in the upstream reach. Survival and growth of P. gentilis were similar in up and downstream sections and P. luculenta had no effect on P. gentilis growth or survival. My results are consistent with those of other studies that suggest that it is the presence of P. gentilis and not detritivore diversity that drives detrital breakdown in southern Appalachian streams. The absence of P. gentilis below the dam may be due to the dam and reservoir preventing downstream dispersal of larvae.