Phosphorus (P) is a key nutrient for lagoon health that can be harmful in excess. The table at the right shows phosphorus scores declined in most of the lagoon in 2010, at the same time the lagoon started to experience persistent algae blooms. Although the Clean Water Act requires phosphorus reductions, concentrations have significantly increased in many lagoon regions. Reducing phosphorus concentrations in the lagoon will improve lagoon health.
Phosphorus comes from many sources such as wastewater, wash water of all kinds (car washing, pressure washing, etc.), fertilizers, and sediments. It enters the lagoon through groundwater, stormwater, and canal discharges. Most cities and counties in the lagoon watershed passed fertilizer ordinances that do not allow phosphorus to be applied to the lawn without a soil test confirming it is needed. Check the BeFloridianNow.org website for more information.
The available target metrics for total phosphorus are identical to those used for total nitrogen. After a comparison of the criteria from each source, it was decided that a combination of the established targets would be used in the Indian River Lagoon Health Assessment.
The EPA targets will be used for the ML; the SJRWMD targets for the BRL, CIRL, and NIRL; and the SFWMD target is to be used for the SIRL. 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.
Total Phosphorus (TP) had site-specific targets that used medians and geometric means, and the results are reported as offsets from the established target by year and twenty years for each sublagoon. The central tendencies (median and geometric mean) for each station was calculated from time-series data for each individual year as well as the twenty years.
Offsets from the target data were calculated from the central tendency and data were averaged for each sublagoon annually and for the entire twenty years. The frequency distribution of the data, as shown in the figure, provided guidance to create TP scores.
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.