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Page 16 Attachment B: Supporting Research November 24, 2020 Update 2 — Research Review Phase of IRL Plan concentrations is not a realistic goal to have. Studies have shown that seagrass growth is limited by the amount of nutrients in the water compared to carbon concentrations in their environment. There is reasonable evidence to indicate that there is a lower limit to the nutrient content needed for seagrass survival, meaning seagrass cannot grow and survive without appropriate levels of nitrogen and phosphorus in their ecosystem. It is difficult to determine critical nutrient concentrations required for seagrass growth since this will vary among species and with changes in growth conditions. Studies of tributaries conducted throughout the Lagoon system show that within the Central Indian River Lagoon area, the Sebastian River produces the highest annual loadings of TN and TP into the central Lagoon basin. This is believed to be caused by the rapid increase in human population growth and activities throughout the Sebastian River drainage basin. Another source for TN and TP concentrations comes from groundwater seepage from the surficial aquifer. Seepage rates are variable throughout the Lagoon and vary spatially and temporally across the Lagoon's basin. Finally, nutrient flux rates, or the rate of transfer of nutrients from sediments to the water, vary based on materials present on the Lagoon's floor. One study found that muck fluxes for nitrogen can be 3,372% greater when compared to nitrogen fluxes from sandy sediments. Similarly, phosphorus fluxes from muck can be 1,566% more when compared to phosphorus fluxes from sandy sediments. The County has three major stormwater canals that transfer runoff and stormwater from within the County, out to the Lagoon. The canals are situated with one in the northern portion of the County, one in the central portion of the County, and one in the southern portion of the County. The County began sampling water from the Indian River Farms Water Control District's Main Relief Canal, North Relief Canal, and South Relief Canal on May 22, 2014. Water samples are collected weekly in each canal near their crossing under U.S. Highway 1. At these locations, additional downstream stormwater input into the canals is considered negligible. All three canals discharge directly in the Lagoon, and collectively, represent the majority of groundwater and stormwater flow into the Lagoon. In general, except after heavy rainfall events, the nutrients remain relatively low. Average results from May 22, 2014 through July 22, 2020 are listed below: Average Nitrogen and Phosphorus Levels in IRFWCD Relief Canals (May 22, 2014 through July 22, 2020) Parameter. North Relief Canal Main Relief Canal South Relief Canal Nitrate/Nitrite Nitrogen (mg/L) 0.27 0.19 0.19 Total Kjeldahl Nitrogen (mg/L) 0.73 0.66 0.71 TN (mg/L) 0.99 0.85 0.89 TP (mg/L) 0.17 0.11 0.12 117