Water column biogeochemistry in a tropical urban estuary

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, 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 δ¹⁵N 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 which is contrary to current paradigms developed in temperate estuaries. The source of these low δ¹⁵N 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 from which to compare the effects of restoration efforts and climate change.