The Effects of Land Use and Novel Stream Pollutants on Extracellular Enzyme Activity in Baltimore County, Maryland Urban Streams
Urban streams are characterized by their impervious surface cover, lack of riparian vegetation, and pollutant loading. Studies have shown that urban streams uniquely cycle nutrients and pollutants leading to degradation of water quality throughout the watershed. The goal of my study was to understand the differences in aquatic ecosystem health and resilience between rural and urban streams in order to develop more effective ecological restoration practices. Previous studies within Baltimore County, Maryland, have found pollutants such as the antimicrobial triclocarban and the poly-aromatic hydrocarbons (PAH) phenanthrene, fluoranthene, and benzo(a)pyrene in the urban streams. Microbes assimilate dissolved organic matter (DOM) using extracellular enzymes, which have been studied as an indicator of microbial function. I measured extracellular enzyme activity (EEA) in the water of two rural and two urban streams focusing on the EEA of three enzymes, phosphatase (PHOS), beta-glucosidase (BG), and leucine amino peptidase (LAP) to account for phosphorus, carbon and nitrogen substrates, respectively, in the DOM. For each site I measured EEA under ambient conditions and with four pollutant treatments at low and high doses. Overall, the EEA for the untreated water did not show consistent similarities or differences between the rural and urban sites, which were categorized by amount of impervious surface cover in the watershed. This suggests that additional site characteristics, such as hydrologic base flow, temperature or density of septic systems, might be important for determining land use effects on water quality and microbial function. With the addition of pollutant treatments, I hypothesized that urban sites would have higher EEA than rural sites because the microbial communities would have adapted to the degraded conditions; but that all sites would decrease in enzyme activity as dose of the treatment increased. However, the results showed the rural sites generally had higher activity than the urban sites, and that neither the urban nor the rural sites consistently declined in EEA with an increase in treatment dose. This suggests that greater microbial biodiversity and competition in the rural sites may be as important as the assimilation period of the microbial community in the more degraded sites. In contrast to the PHOS and BG enzyme activity, LAP activity was not significantly affected by any of the treatments or doses, suggesting that the nitrogen sources in these sites may primarily be found in the soil and sediment rather than dissolved as gases in water. These results demonstrate that EEA is an important, but complex tool for understanding microbial function in an ecosystem.