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Unexpected Nitrogen Sources in a Tropical Urban Estuary
Oczkowski, A., E. Santos, R. Martin, A. Gray, Alana Hanson, E. Watson, E. Huertas, AND C. Wigand. Unexpected Nitrogen Sources in a Tropical Urban Estuary. Journal of Geophysical Research - Biogeosciences. American Geophysical Union, Washington, DC, 125(3):e2019JG005502, (2020). https://doi.org/10.1029/2019JG005502
Urban tropical coastal ecosystems are understudied. These areas tend to be low-lying, making them vulnerable to the impacts of tropical storms. Here we sought to understand the basic nutrient cycling processes and ecosystem dynamics in the urban, tropical San Juan Bay Estuary. Portions of the adjacent land area flood regularly with pollutant rich waters, creating unhealthy conditions for local residents. In this study, we found that, despite the high inputs from sewage, the flooded part of the system might be rich with the kind of microbes that convert nitrogen gas to forms that are available to plants, essentially creating unaccounted for fertilizer. It is surprising that these processes have not already been documented, but our observations underscore how little is known about these systems. This additional nitrogen fertilizer compounds the effects of the sewage, likely making water quality problems worse than if sewage were the only main nitrogen source to the estuary.
Tropical urban estuaries are severely understudied. Little is known about the basic biogeochemical cycles and dominant ecosystem processes in these waterbodies, which are often low lying and heavily modified. The San Juan Bay Estuary (SJBE) in San Juan, Puerto Rico, is an example of such a system. Over the past 80 years, a portion of the estuary has filled in, changing the hydrodynamics and negatively affecting water quality. Here we sought to document these changes using ecological and biogeochemical measurements of surface sediments and bivalves. Measurements of sediment physical characteristics, organic matter content, and stable isotope ratios (δ13C, δ15N, δ34S) illustrated the effects of the closure of the Caño Martín Peña (CMP) on the hydrology and water quality of the enclosed and semi‐enclosed parts of the estuary. The nitrogen stable isotope (δ15N) values were lowest in the CMP, the stretch of the SJBE that is characterized by waters with low dissolved oxygen and high fecal coliform concentrations. Despite this, the results of this study indicate that nitrogen (N) contributions from N‐fixing, sulfate‐reducing microbes may meet or even exceed contributions from urban runoff and sewage. While the importance of sulfate reducers in contributing N to mangrove ecosystems is well documented, this is the first indication that such processes could be dominant in an intensely urban system. It also underscores just how little we know about tropical coastal ecosystems in densely populated areas throughout the globe.