MACROINVERTEBRATE COMMUNITY STRUCTURE AND ECOLOGICAL FUNCTION IN MARYLAND COASTAL PLAIN STREAMS

Abstract

Abiotic conditions within streams, especially those conditions impacted by human activities, often influence the community structure and ecosystem function. While coastal regions have been strongly impacted by urban development and agriculture, little research has focused on characterizing the biotic community structure and function in these Coastal Plain streams. Such watersheds are characterized by low gradient, blackwater streams with sandy and silty substrate, coupled with large amounts of human disturbance. The objectives of this research were 1) to characterize the macroinvertebrate community and the chemical and physical characteristics of two Coastal Plain watersheds with differing landuse practices, 2) to examine appropriate macroinvertebrate sampling protocol comparing leaf pack and Maryland Biological Stream Sampling (MBSS) methods, 3) to compare these community structure measures with the functional measure of leaf decomposition, and 4) to investigate potential mechanisms for shifts in decomposition due to elevated nutrients and dissolved organic carbon (DOC) concentrations in mesocosms. Results of three years of monthly sampling showed differences between watersheds in a number of chemical parameters, including nutrient concentrations. However, structural differences between the macroinvertebrate communities, using both three years of leaf pack sampling and a MBSS survey, were only identified for certain taxa and depended on the taxa resolution used. Also, two in situ leaf decomposition experiments using leaf decomposition tubes showed no differences in the macroinvertebrate or microbial contribution to detrital processing. Correlation analyses demonstrated that rates of decomposition were negatively associated with macroinvertebrate predator abundance and positively associated with the abundance of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa in the community. Lastly, a laboratory mesocosm experiment illustrated the strong effect of DOC from blackwater streams in reducing rates of leaf decomposition and processing efficiency by a macroinvertebrate shredder. While abiotic measurements of Coastal Plain stream sites varied greatly both spatially and temporally, and the field experiments yielded little consistent pattern with the macroinvertebrate community, the mesocosm experiment demonstrated the strong inhibitory effect of DOC on detrital processing and processing efficiencies of a macroinvertebrate shredder. Thus, while measuring rates of decomposition may not be suitable as a biomonitoring tool to differentiate already nutrient enriched Coastal Plain streams, it can add to stream assessments by providing a measure of ecosystem function where impacts are less subtle.