EFFECTS OF NUTRIENTS AND DISSOLVED ORGANIC CARBON ON MICROBIAL AND MACROINVERTEBRATE CONTRIBUTIONS TO LEAF LITTER BREAKDOWN.

N.A. Baer and W.O. Lamp. Marine-Estuarine-Environmental-Science Program, Department of Entomology, University of Maryland, College Park, MD 20742

Our sampling of two Coastal Plain watersheds in Maryland has demonstrated nitrogen-limited conditions in the streams with phosphorus levels ranging from 0.20 mg L^-1 to greater than 2.75 mg L^-1. These streams are also characterized by black or brown water with high dissolved organic carbon (DOC) levels. Other researchers have shown changes in leaf decomposition rates due to increase in nitrogen with or without an increase in phosphorus. Researchers have also suggested that elevated DOC levels may cause a shift in the bacterial community and therefore changes in the detrital foodweb. Thus we performed a laboratory experiment examining both microbial and macroinvertebrate response to changes in nitrogen, phosphorus and DOC. We hypothesized that increased DOC reduces leaf decomposition by the microbial community but not the macroinvertebrate, Gammarus sp. We also predicted an increase in detrital processing as we elevated nitrogen concentrations. Artificial stream environments were constructed in a temperature-controlled chamber. Microbes and Gammarus sp. were reared in stream water samples varying in nitrate, phosphorus and DOC concentrations. Dissolved oxygen and ash-free dry mass were measured to compare treatments. The results illustrate the relationship between nutrients, DOC, and leaf decomposition.