Sea fan corals provide a stable isotope baseline for assessing sewage pollution in the Mexican Caribbean

We compared stable nitrogen isotope (delta(15)N) values from the common Caribbean sea fan Gorgonia ventalina, collected from a developed and undeveloped coastline, to test the hypothesis that sewage-derived nitrogen (N) inputs are detectable and more severe in developed areas along the Mesoamerican barrier reef of Mexico. The Akumal coast was selected as the developed site since this area is inhabited by thousands of local residents and has a significant flux of tourists; it was compared to a relatively undeveloped shoreline south of Mahahual, a small town with a few hundred residents and sewage treatment infrastructure. Gorgonians sampled from Akumal were relatively enriched in delta(15)N (as high as 7.7% nearshore) and were similar to 3.5% greater than sea fans from Mahahual collected at similar depths. While previous work has shown that water column N concentrations are uniform around Akumal, delta(15)N values of sea fans sampled parallel to shore were variable, indicating that sewage-derived N inputs are spotty along the coast. delta(15)N values were positively correlated with fecal Enterococcus counts from seawater, confirming that these enrichments are associated with sewage and not denitrification. We suggest that the data from Mahahual can be used as an isotopic baseline for monitoring the Mesoamerican barrier reef at sites where increased development is planned or underway.