Total nitrate removal by flocculent organic sediments in shallow freshwater ecosystems

Kincaid, Dustin W. and Stephen K. Hamilton
Michigan State University

Presented at the All Scientist Meeting and Investigators Field Tour (2017-10-06 to 2017-10-07 )

Anthropogenic activities have increased the availability of nitrogen (N) to the biosphere. Some proportion of this N enters aquatic ecosystems, mainly as nitrate, via runoff, groundwater, and/or atmospheric deposition. Relative to their areal coverage, a disproportionate amount of anthropogenic N is processed by these recipient ecosystems. Nitrogen cycling in small waterbodies, given their shallow nature, is strongly influenced by the sediment-water interface. In these ecosystems, thick layers of flocculent organic sediments (floc) have a tendency to accumulate. Although floc is nearly ubiquitous, little is known about its biogeochemical importance in shallow freshwater ecosystems. The objective of this study was to measure in situ nitrate removal rates by floc in a variety of shallow waters by enriching overlying water in mesocosms with nitrate and bromide and monitoring changes in concentrations over three diel cycles. We predicted that because floc has high levels of organic matter, these accumulations would promote rapid removal of nitrate from the overlying water relative to other sediment types found in the literature. We found that while floc layers have the potential to remove nitrate from overlying water columns, we could not conclude that removal rates were much greater than those measured with other sediment types.