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Sediment nitrification and denitrification rates in a Lake Superior estuary
Citation:
Bellinger, B., T. Jicha, L. Lehto, L. Seifer-Monson, Dave Bolgrien, M. Starry, T. Angradi, M. Pearson, C. Elonen, AND B. Hill. Sediment nitrification and denitrification rates in a Lake Superior estuary. JOURNAL OF GREAT LAKES RESEARCH. International Association for Great Lakes Research, Ann Arbor, MI, 40(2):392-403, (2014).
Impact/Purpose:
In this study, we measured nitrogen transformation and removal rates within sediments of the St. Louis River Estuary. The St. Louis River is the largest U.S. tributary to Lake Superior, and supports a thriving shipping industry as well as a renowned fishery. Through decades of anthropogenic influence, the estuary has been identified as an area of concern, and remediation and restoration efforts are ongoing to restore degraded system attributes. Nutrient cycling and habitat are two impaired beneficial uses. We found, in general, nitrogen transformation and removal rates were higher in deeper waters of the Duluth/Superior harbor. Highest denitrification rates in the harbor were tied to the nitrate pulses provide by Lake Superior. Nitrogen oxidation rates between years of study were generally similar upriver of the harbor, but were higher in the deep waters of the harbor than in the vegetated shallows. Monthly patterns in nitrogen cycling rates were unclear. Water nitrate was most limiting to denitrification rates. In general, the nitrogen produced through denitrification was inert, with typically less than 20% in the form of the potent greenhouse gas nitrous oxide. However, greater nitrate and carbon availability has the potential to increase nitrous oxide emissions from sediments. Our findings suggest coastal systems of Lake Superior are not only serving to mitigate nitrogen pulses from the watershed, but can serve as nitrogen sinks, slowing down the century long trend of rising nitrate concentrations. Coastal ecosystems serve as a final means for reducing nutrient loading from the watersheds. Removal of nitrogen is viewed as an especially important ecosystem service, not only because of the links with eutrophication of systems, but also due to health risks to humans. The costs of removing nitrogen from waters by natural means, i.e., plant and microbial transformation, are only a fraction of those associated with water treatment plants. Therefore, it is important to quantify the capacity of our coastal ecosystems to mitigate nitrogen export, and to identify the environmental drivers related to nitrogen cycling rates. An understanding of the conditions that promote nitrogen removal can aid in the restoration of systems to enhance nitrogen removal.
Description:
Microbially-mediated nitrogen (N) cycling in aquatic sediments has been recognized as an ecosystem service due to mitigation of N-transport to receiving waters. In 2011 and 2012, we compared nitrification (NIT), unamended (DeNIT) and amended (DEA) denitrification rates among spatial and depths zones and in relation to site physicochemical characteristics in the St. Louis River Estuary (SLRE) of western Lake Superior. Among vegetated habitats in 2011, NIT rates were highest in deep (>2 m) waters (249 mgN m-2 d-1) and in the upper estuary (>126). DeNIT rates were highest in deep waters and the harbor (2,111 and 274, respectively). DEA rates were similar among habitats. In 2012, we observed highest NIT (223 and 287) and DeNIT (77 and 64) rates in the harbor and from deep waters, respectively. Highest rates for NIT, DeNIT, and DEA were in July, May, and June, respectively. Individual site characteristics were weakly related to N-cycling rates, but water and sediment N-concentrations were identified as significant predictors in multiple linear regression models. NO3- was most limiting to sediment denitrification rates. The SLRE acted as a net source of NO3- to the water column, but had the potential to act as a sink. Average N2O production in 2011 was half that of 2012, with production during DEA (23-54%) being higher than DeNIT (0-41%). SLRE N-cycling rates were spatially and temporally variable, but our results give an indication of how alterations of water depth and nutrient mitigation activities may impact sediment N-transformation rates.
