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Could nitrogen fixation be an important source of nitrogen in tropical urban estuaries?
Citation:
Oczkowski, A., Alana Hanson, I. Cappielo Cosme, E. Huertas, D. Katz, M. Schwartz, S. Balint, AND M. Molina. Could nitrogen fixation be an important source of nitrogen in tropical urban estuaries? Joint Aquatic Sciences Meeting, Grand Rapids, MI, May 14 - 20, 2022.
Impact/Purpose:
Nitrogen is often considered to be a pollutant in coastal ecosystems, where human sources of nitrogen (mostly from sewage and fertilizer) have overwhelmed 'natural' nitrogen sources. As our understanding of nitrogen sources and sinks improves with more precise analytical capabilities, researchers are becoming more aware of the potentially important role that biological nitrogen fixation (the conversion of intert dinitrogen gas to bioavailable forms) may play in nitrogen dynamics. There is also preliminary evidence that nitrogen fixation could be a dominant source of nitrogen to tropical coastal ecosystems. Here we review and synthesize the pre-existing information to determine what we do, and don't know, about fixation in tropical urban estuaries.
Description:
Recent biogeochemical measurements from the urban San Juan Bay Estuary (SJBE) in Puerto Rico indicate that, despite abundant bioavailable nitrogen (N), N fixation may be an important source of new N to the coastal ecosystem. Nitrogen stable isotope data suggest that N contributions from fixation may be on par with those from sewage-dominated urban runoff. To assess the viability of the hypothesis that N fixation may be one of the top sources of bioavailable N to tropical urban estuaries surrounded by mangroves, and that it is produced during microbially facilitated sulfate reduction, we review what is known, and not known, about tropical estuarine N fixation. There is documentation that many sulfate-reducing bacteria in mangrove ecosystems are also N fixers and that reduction rates are positively correlated with the supply of organic matter. Further, others have shown that the rate of N fixation is not limited by N bioavailability. Overall, mangrove nutrient cycling is under-studied, and little is known about drivers and cycling in urban mangrove systems. But available information supports the theory that N fixation, as a byproduct of sulfate reduction, may be enhanced by urban carbon runoff, even in an N rich environment. Ongoing research in SJBE will directly test the mechanisms proposed in this assessment.
