The Parque Nacional Sistema Arrecifal Veracruzana (Veracruz Coral Reef System National Park) consists of 19 reef components surrounding the Port of Veracruz, Mexico, and encompasses a total of 52,000 hectares. Past research by the authors found significant reef fish distributional differences as well as distributional differences in stony coral density and prevalence of disease. The determinants of these distributional differences are not known. The purpose of this project is to correlate fish and coral assemblage distributions within the Park with two parameters (water temperature and salinity) that may identify freshwater effluent from neighboring river drainages. We will test the hypothesis that differences among the fish and coral assemblages within the Park correlate to site-specific differences in freshwater input. These parameters will serve as variables to provide important information on where, and to what extent, freshwater runoff impacts the reef system. Data collection will be accomplished using, in-situ continuous temperature monitoring, monthly salinity readings and non-destructive visual surveys of fishes and corals on widely separated reefs within the Park. Rigorous parametric and non-parametric data analyses will be accomplished. Project results will increase knowledge of the distribution of fishes and corals within the Park, previously initiated, and the impact of land-based freshwater runoff on these communities. This information is critical for establishing and evaluating resource management strategies as well as evaluating the extent of anthropogenic, and natural, impacts. Although salinity and temperature are important factors determining fish and coral distribution it may well be that some other factor(s) associated with the runoff is the major impacting factor: e.g. heavy metals, agricultural pesticide, sediment, fertilizer. The proposed research will provide the necessary documentation to pursue the extensive out-of-house funding required to parse out the specific factors negatively impacting the reef biota.