Denitrification and nutrient cycling dynamics over a trajectory of increasing eutrophication: evidence of change in a shallow coastal ecosystem
Foster, Sarah Quinn
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The natural gradient of eutrophic conditions that exists in Waquoit Bay, (Cape Cod, Massachusetts) allowed us to examine how key biogeochemical processes respond to eutrophication over time. Using a space-for-time substitution we measured oxygen (O2), nutrient, and di-nitrogen (N2) gas fluxes from sediments collected at four stations. The objective of this study was to assess how sediment metabolism, nutrient cycling, denitrification and the balance between N and P limitation may change over a trajectory of increasing eutrophication. In addition, for two sites we compared our more recent measurements to those made in the bay nearly 20 years ago (1992-1994). While we did not find a spatial pattern that was linearly linked to nitrogen (N) loading, our results show characteristics of a system in change. Sediment oxygen consumption was measured at 45% of its historic rate and ammonium flux at only 30%. The difference in net denitrification rate was particularly large, as our mean rate (29.9 µmol N2-N m-2 h-1) was considerably lower than the mean historic value (172 µmol N2-N m-2 h-1). This 83% reduction represents the substantial dampening of a key microbial process for the removal of reactive nitrogen from the ecosystem. Additionally, at our most impacted site, North Basin, we measured significant rates of net N fixation, indicating that the sediments are becoming a net source of reactive N. In another important example of change, we observed an anomalous efflux of phosphate (PO4 3-) from the sediments during the summer and high ratios of both oxygen uptake to phosphate release (O2:PO4 3-) and ammonium to phosphate release (NH4+:PO4 3-). This unexpected result suggests that, at least in some times of year, Waquoit Bay may be co-limited by both nitrogen (N) and phosphorus (P). If these observations of N and P co-limitation hold into the future, it will have important consequences for the ecology and management of this coastal system.
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