Evidence and implications of the background phosphorus concentration of submerged aquatic vegetation wetlands in Stormwater Treatment Areas for Everglades restoration

The limits of phosphorus (P) removal from the 18,120 ha Stormwater Treatment Areas (STAs) for Everglades restoration depend largely on the performance of submerged aquatic vegetation (SAV) wetlands, as SAV treatment cells now provide final stage treatment for 85% of the STA project. A long‐term internal P profile in STA‐2 cell 3 (STA2C3), one of the longest‐running and best performing SAV cells, demonstrated no further net removal in the back quarter of the cell once total P (TP) levels approached 15 μg L−1. Inflow‐outflow performance data from STA2C3 were analyzed at monthly and annual scales and were pooled with data from an additional eight STA SAV treatment cells. The pooled data allowed inference of background TP concentrations in SAV treatment cells using existing Bayesian methods. Results showed a central tendency of 16 μg L−1 (13–17, 90% bounds), insensitivity to P loads less than ∼1.7 g m−2 yr−1, and interannual variability outside these bounds. Internal data from the STA2C3 profile provided validation. Background P concentrations of 7 and 6 μg L−1 were identified for dissolved organic and particulate P fractions in the data pool, respectively, again similar to values in the STA2C3 gradient. Existing simulation modeling approaches for STA evaluations were identified as ineffective at or near background TP concentrations. Instead, we use an empirical and probabilistic approach based on full‐scale data from STAs that produces annual risk of exceedance statistics and is easy to update. The current analysis suggests tangible risks for exceeding proposed annual discharge criteria from the STAs in the range of 16–20 μg L−1.