Establishing pollutant load reduction targets for the Indian River Lagoon, Florida

Establishment of seagrass-based pollutant load reduction targets is a major aim of water quality management in the Indian River Lagoon (IRL) estuary. It is believed that light and water clarity are the Limiting factors affecting seagrass abundance and distribution in the Lagoon. Thus, targeted reductions of nutrients, dissolved organic matter and/or suspended sediments should achieve desired seagrass coverage goals. On an annual average, the Lagoon is receiving external loadings of 5,346, 771, and 54,408 metric tons per year of nitrogen (N), phosphorus (P), and total suspended solids (TSS), respectively. Most of the loadings are stormwater generated; thus, implementation of reduction targets will key in on major urban and agricultural stormwater systems. The process of developing targets requires the establishment of light requirements for seagrass. Two methods are being employed to develop such targets. One method is the application of a predictive, 3-D model that accounts for the essential interactive processes of the Lagoon: hydrology, hydrodynamics, water quality, nutrient uptake/release, seagrass growth rates, and light attenuation. The other method is based simply on the comparison of loading rates between: a developed sub-basin with associated seagrass impacts and an undeveloped sub-basin with adjacent healthy seagrass coverages. The latter method will provide the initial and preliminary targets that can be tested, confirmed, or modified by application of the 3-D model.