Indicators of nutrient enrichment in coastal ecosystems of the northern Mexican Caribbean

We measured dissolved inorganic nitrogen (DIN = NH4+ + NOx [= NO3- + NO2-]), soluble reactive phosphorous (SRP), and ecological indicators (chlorophyll-a concentration, coverage of submerged aquatic vegetation, and %N, C:N and delta N-15 values in the seagrass Thalassia testudinum) to evaluate the effects of nutrient enrichment at three shallow coastal sites on the northern part of the Mexican Caribbean (Puerto Morelos, Puerto Juarez, and Isla Blanca) in two contrasting (cold-front or dry) seasons of the year. High nutrient concentrations prevailed in both seasons but were significantly higher during the cold-front season, probably as a result of sediment resuspension and groundwater discharges. The highest delta N-15 values were recorded in Puerto Juarez in both seasons (cold-front, 7.58 parts per thousand; dry, 7.35 parts per thousand). The highest macroalgae coverage and NH4+ concentration were also recorded in this site. These results, together with the high correlation between delta N-15 and NH4+ (r = 0.94) and DIN (r = 0.88) concentrations, suggest wastewater contamination in Puerto Juarez due to its proximity to the city of Cancun. In contrast, the lowest mean delta N-15 (-1.35 parts per thousand) and nutrient enrichment effects (i.e., highest coverage of T. testudinum and lowest macroalgae coverage) were observed in Isla Blanca, a site with no urban settlements. The low delta N-15 values recorded along the Puerto Morelos coastal transect suggest that a significant fraction of the nitrogen assimilated by T. testudinum may come from either exudates or the decomposition of drifting Sargassum stranded in the coast, as the delta N-15 values recorded in Sargassum (-0.64 parts per thousand) were similar to those found in the seagrass (-0.83 parts per thousand). The correlation between the DIN:SRP ratio and chlorophyll-a concentration in the study area may indicate N-limitation of phytoplankton growth, despite the high DIN concentrations. Beyond the differences observed between sites - which may indicate site-specific, long-term nutrient inputs - the indicators examined evidence environmental deterioration of the study area, likely due to a shift in the nutrient regime (away from oligotrophic), particularly in Puerto Juarez, the site with the heaviest load of urban and tourism activities.