Water column biogeochemistry in a tropical urban estuary

Abstract Tropical urban estuaries are understudied, where ecosystem health is often considered through a temperate lens. Here, we use stable isotopes and other biogeochemical parameters to begin to address the hypothesis that the biogeochemical drivers of water column chemistry in tropical urban estuaries are qualitatively different from those in temperate systems. We measured salinity, dissolved oxygen, temperature, turbidity, dissolved nutrients, enterococcus concentrations, and suspended particulate stable isotope and nitrogen (N) and carbon concentrations every two weeks for three years in the San Juan Bay Estuary, Puerto Rico. Enterococcus concentrations exceeded acceptable limits (35 CFU 100 ml -1 ) in all but three water samples measured in a densely populated channel within the estuary that receives urban runoff and untreated sewage ( n = 207). The δ 15 N values were lowest in this channel, highest in the portion of the estuary with open exchange with the sea, and negatively correlated with dissolved inorganic nitrogen concentrations. This observation, is contrary to current paradigms that were developed in temperate estuaries. The source of the low δ 15 N values is unknown, but consistent with observations in the sediment and consistent with high rates of N fixation. In addition to the need for a basic understanding of the biogeochemical and ecological processes involved, these data serve as a baseline to compare the effects of restoration efforts and climate change.