Watershed scale assessment of nitrogen and phosphorus loadings in the Indian River Lagoon basin, Florida

There is a growing evidence that the ecological and biological integrity of the lagoon has declined during the last 50 years, probably due to the decline in water quality. Establishment of a watershed scale seagrass-based nutrient load assessment is the major aim of water quality management in the Indian River Lagoon (IRL). Best estimate loadings incorporate wet and dry deposition, surface water, groundwater, sediment nutrient flux, and point source effluent discharge data. On the average, the IRL is receiving annual external loadings of 832, 645 and 94,476 kg of total nitrogen (TN) and total phosphorus (TP), respectively, from stormwater discharges and agricultural runoff. The average internal cycling of TN and TP from sediment deposits in the IRL was about 42,640 kg TN and 1050 kg TP yr−1. Indirect evidence suggests that atmospheric deposition has played a role in the ongoing nutrient enrichment in the IRL. The estimated total atmospheric deposition of TN and TP was about 32,940 and 824 kg yr−1, while groundwater contribution was about 84,920 and 24,275 kg yr−1, respectively, to the surface waters of the IRL. The estimated annual contribution of point effluent discharge was about 60,408 kg TN and 7248 kg TP. In total, the IRL basin is receiving an annual loading of about 1,053,553 kg TN and 127,873 kg TP. With these results, it is clear that the current rate of nutrient loadings is causing a shift in the primary producers of the IRL from macrophyte to phytoplankton- or algal-based system. The goal is to reverse that shift, to attain and maintain a macrophyte-based estuarine system in the IRL.