Nutrient discharges to Biscayne Bay, Florida: Trends, loads, and a pollutant index

Changes in land use, management practices, and environmental conditions may all lead to detectable differences in nutrients transported to aquatic systems. Biscayne Bay, an oligotrophic estuary in southeastern Florida, requires minimal phosphorus and nitrogen inputs and here we quantified the effects of continued watershed development. Nutrient (nitrate/nitrite–nitrogen [NOX–N], total ammonia nitrogen [NH3–N], and total phosphorus [TP]) water quality data (1992–2006) from six monitoring sites were evaluated using trend analysis, load estimation, and a new Pollutant Empower Density (PED) index. The PED index assesses the effect of discharged pollutants relative to the background productivity of aquatic environments. NOX–N, NH3–N, and TP concentrations declined or exhibited no change at most sites, with only six instances of significantly (p < 0.1) increasing trends. Load estimates revealed higher NOX–N loads in the southern, agricultural section of the watershed and higher NH3–N and TP loads in the urbanized northern and central areas. NOX–N loads from site MW04 (south) were the highest for all sites while site LR06 (north) had the highest NH3–N and TP loads. Of the evaluated canal discharges, PED index values also suggested that canal discharges from these two sites (MW04 and LR06) had the greatest potential for impact in the bay. Overall, water quality is generally improving but canal discharges are coupled with land use activities in adjacent drainage areas. Trend analysis, load estimation, and the PED index can be used together to provide a more holistic interpretation of water quality, which is necessary for optimizing resources to meet watershed management goals.

Research Highlights
► Based on the analysis of historical data (1992–2006), nutrient concentrations and loads are generally improving throughout the Biscayne Bay watershed, Florida.
► Canal discharges associated with two water quality monitoring sites had the greatest potential to impact the bay.
► A new Pollutant Empower Density (PED) index complements traditional trend analysis and load calculation techniques to aid watershed management decisions.