Microbial characteristics and fate of stream bank legacy sediments in Mid-Atlantic streams
Date
2019
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Publisher
University of Delaware
Abstract
The mobilization of anthropogenic sediments has become an increasing concern in Mid-Atlantic watersheds. A large portion of annual watershed sediment exports in Mid-Atlantic streams could be composed of legacy sediments. Despite this, we know very little about the microbial communities in legacy sediment and how they may affect nutrient processes in stream banks. There is additional uncertainty whether legacy sediments will act as a source or a sink once deposited into the water column. Legacy sediment stream banks were sampled across 15 different sites in northern Delaware, eastern Maryland, and southeastern Pennsylvania, USA. Multiple analyses were performed on the sediments to determine nutrient, heavy metal content, microbial community and functional gene abundance, and the sorption properties. ☐ The focus of the first study was on the variation in microbial community structure and nitrogen transforming functional genes across four different land uses and by multiple stream bank depths. Results from the study indicated that variation in microbial community composition varied by land use and bank depth highlighting the complexity and spatial heterogeneity of each site. Nitrification genes were lower in abundance than the measured denitrification gene: nosZ which may influence nitrogen cycling within the stream banks. The interactions between microbes and the stream bank chemistry highlight the importance of looking at both aspects when concluding on the potential impacts legacy sediments have on water quality. ☐ The second studies focused on if legacy sediments acted as a source or a sink for nutrients once deposited into streams. A variety of laboratory experiments were conducted on coarse (>63µm) and fine (<63µm) fractions of legacy sediment collected from the fifteen sites. Sediments were incubated for 24 hours to determine the Phosphorus Sorption Index (PSI) and the Equilibrium Phosphorus Concentration (EPC0). Sediments were also incubated in anoxic and oxic conditions to measure the amount of phosphorus release in both conditions. Additionally, legacy sediment was incubated in modified milk crates in two small tributaries for eight months along the Big Elk Creek. Results indicated that while the fine fraction sediment has a high sorption capacity, whether the sediment acts as a source or a sink will depend upon the environmental conditions of the stream water. ☐ This thesis expands upon the current knowledge about legacy sediments and provides insight into microbial community variation in legacy sediment and their potential role in nutrient processing. Findings from this study also give insight into how legacy sediments may interact with the water column once deposited into streams. These findings will be of interest to natural resource managers, water quality scientists, TMDL planners, and the legacy sediment community.