WATERSHED SCALE ASSESSMENT OF NITROGEN AND PHOSPHORUS LOADINGS IN THE INDIAN RIVER LAGOON BASIN, FLORIDA

Authors: Sigua, Gilbert; TWEEDALE, WENDY - ST. JOHNS RIVER WATER MGT

Submitted to: Journal of Water Resources Planning and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2002
Publication Date: 2/24/2003
Citation: Sigua, G.C., Tweedale, W.A. 2003. Watershed scale assessment of nitrogen and phosphorus loadings in the Indian River Lagoon basin, Florida. Journal of Water Resources Planning and Management Vol.67:pp 363-372.

Interpretive Summary: The decline in biological integrity from a seagrass-based ecosystem to a phytoplankton- or algal-based system of the Indian River and Banana River Lagoons reflects the importance of assessing the cycling of nutrients to water quality in these enclosed water bodies. Sources of nutrients can include surface water runoff, and the urbanization around the lagoon appears to have increased the load of nutrients from this source. Atmospheric deposition is another important source that has been measured directly from two locations in the lagoon. Internal cycling and mineralization of organic matter in the sediment provides another source of nutrients to the coastal water. Like in other coastal areas and estuaries, groundwater seepage may provide a mechanism for transportation of nutrients and other dissolved species to the IRL waters.

Technical Abstract: 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 832,646 and 94,475 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 and 1,050 kg of TN and TP per year. Indirect evidence suggests that atmospheric deposition has played a role in the ongoing nutrient enrichment in the IRL. Estimated total atmospheric deposition of TN and TP was about 32,940 and 824 kg per year, while groundwater contribution was about 84,920 and 24,275 5kg per year, respectively, to the surface waters of the IRL. The estimate annual contribution of point effluent discharge was about 60,411 kg TN and 7,248 kg TP. In total, the IRL basin is receiving an annual loading of about 1,053,557 kg TN and 127,872 kg TP.