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UNEXPECTED BIOGEOCHEMISTRY IN A TROPICAL URBAN ESTUARY SHOWS HOW LITTLE WE KNOW.
Citation:
Oczkowski, A., Alana Hanson, E. Huertas, AND C. Wigand. UNEXPECTED BIOGEOCHEMISTRY IN A TROPICAL URBAN ESTUARY SHOWS HOW LITTLE WE KNOW. Coastal & Estuarine Research Federation 26th Biennial Conference 2021, NA, Virtual, November 01 - 11, 2021.
Impact/Purpose:
The science and management communities know little about how tropical ecosystems function. Tropical urban estuaries in particular are severely understudied. Here we present preliminary data from the San Juan Bay Estuary in Puerto Rico. The results of our studies were surprising and suggested that the processes driving water quality in tropical, urban systems may be very different from how temperate systems function. If nothing else, the results presented here illustrate just how little we know about these vulnerable and important coastal regions.
Description:
The city of San Juan, Puerto Rico is built around the San Juan Bay Estuary (SJBE), a complex of lagoons and channels. A critical stretch of the estuary, the Caño Martín Peña (CMP), has filled in with sediment and debris and now frequently floods the surrounding environmental justice communities with water contaminated with sewage, causing human health and ecological problems. Fecal coliform levels in the SJBE typically exceed authorized limits, often by several orders of magnitude. But, dissolved inorganic nitrogen (N) concentrations are lower than expected, even in areas with nutrient loading from sewage. We conducted a preliminary assessment of N in the SJBE using stable isotopes. As sewage and other urban runoff has high N stable isotope values (δ15N, >8 ‰), we were surprised to find that surface sediments from the CMP had low δ15N values (~2 ‰). Sulfur isotope values (> 0 ‰) were high, consistent with the presence of pyrite and reduced conditions. Similarly, peat from the adjacent mangroves had high nitrous oxide fluxes, high N in the microbial biomass, and high N storage in mangrove soil. Sediment cores from the area document decreasing δ15N values over the past century. Collectively, these results suggest that N fixation may be a dominant biogeochemical process in this urban, tropical system. Such estuaries are understudied, so prior research provides little context for our surprising observations. We are currently testing the hypothesis that microbial activity may yield additional unaccounted for N that further contributes to eutrophied conditions and poor water quality.
