Nitrogen (N) is a key nutrient necessary to the primary production of the system, but can be devastating in excess, causing algae blooms and lowered dissolved oxygen in the water which chokes out life.
The Clean Water Act requires local municipalities to reduce the discharge of nitrogen. Twenty years of data show that these efforts are paying off, as nitrogen levels in some areas of the lagoon have significantly decreased over the past 20 years.
The table below shows how nitrogen scores were generally higher in the decade from 2005–2015, during which we also had sea grass increases. The nitrogen scores severely declined in 2016, the same year the brown tide caused an extensive fish kill in the Banana River and North IRL. In the southern most portion of the South IRL, all water quality monitoring was discontinued in 2013, thus the missing data thereafter.
Nitrogen enters the lagoon through many sources. Water contaminated with nitrogen from fertilizers, septic tanks, leaking wastewater infrastructure, and animal waste enters the lagoon through stormwater drains, canals and seepage. It is also being released by the black ooze known as “muck” that covers about 11% of the lagoon bottom.
There are several established targets for ambient total nitrogen concentrations established by the EPA, the SJRWMD, and the SFWMD for different regions of the IRL. A combination of the established targets wase used to be as conservative in protecting the lagoon’s health as possible. The EPA targets were used for the ML; the SJRWMD targets for the BRL, CIRL, and NIRL; and the SFWMD target for the SIRL.
For the ML, the targets established by the EPA were chosen as they were being used by the SJRWMD and no other targets were available. For the BRL and NIRL, there were three potential available targets to choose from: monthly maxima, annual medians, and annual geometric means.
The SJRWMD annual median target was chosen because geometric means (EPA recommended target) were typically less protective of the Lagoon than the District’s median targets. For the CIRL, the SJRWMD annual median targets specific to the north and southern regions were selected (Steward, Lasi, and Phlips, 2010). Finally for the SIRL, the SFWMD’s conservative criteria of annual median was used as target for all three sublagoon areas.
Targets for the ML have only been established by the EPA and are more restrictive than the currently proposed targets for the Reasonable Assurance Plan (RAP). Since the RAP is still undergoing review, the stricter targets were adopted for assessment purposes.
Total Nitrogen (TN) had site-specific targets that used medians and geometric means and the results are reported in offsets from the established target by year and twenty years for each sublagoon. Annual central tendencies (median and geometric mean) for each station were calculated and offsets from the respective targets.
Data were summarized by averaging offsets of each sublagoon or section of each sublagoon, per year and for the entire POR. The data distribution of the data shown in the figure provided guidance to create the classes to be used for TN scoring.
To learn more about how scores were calculated, to see the targets for each region, and to read other sciencey stuff, check out the 2020 Health Assessment Methodology Report.