Relationship between water quality, δ15N, and aspergillosis of Caribbean sea fan corals

It has been hypothesized that excess nitrogen (N) contributes to coral reef decline by exacerbating the impact of disease. We tested the relationship between N concentration and the prevalence and severity of aspergillosis of the sea fan coral Gorgonia ventalina at 9 reefs along the Florida Keys Reef Tract. Quarterly water quality data, averaged over 1 and 4 yr prior to a disease survey, were used to examine whether aspergillosis dynamics reflected short- or long-term N concentrations. A positive relationship was detected between prevalence of aspergillosis and long-term total nitrogen (TN) concentration; in contrast, disease severity was positively related to the ratio between dissolved inorganic nitrogen and total phosphate (DIN:TP) over both short- and long-terms. These results may reflect the differential influence of N on the host and pathogen. We also tested whether N isotope analysis (delta N-15) of sea fan coral tissue and skeleton could be used to assess the relative exposure to anthropogenic N inputs and its impact on disease. There was no relationship between delta N-15 and aspergillosis (either prevalence or severity). Furthermore, there was no relationship between delta N-15 and environmental concentrations of N. It is possible that the source of N (e.g. anthropogenic) does not affect the dynamics of sea fan aspergillosis, or that the delta N-15 signatures were suppressed by agricultural effluents and other N sources, thus confounding our analyses with disease.