Mesocosm studies clarify effects of external and internal phosphorus (P) loads to treatment wetlands at lower limits of P removal

This study evaluates wetland phosphorus (P) removal where P concentrations can be extremely low (-20 & mu;g/L or less). Stormwater Treatment Area (STAs) are large water quality improvement projects constructed for Ever-glades restoration, where research is underway to understand the processes that control P removal at already low P concentrations, particularly in the lower reaches of the flow-ways, upstream of the discharge structures. One concept that is being researched consists of scraping the P-enriched muck soil from the discharge region of STAs to expose the underlying limerock (LR) substrate. This has been investigated in numerous subscale platforms and results in proliferation of periphyton and submerged aquatic vegetation (SAV) communities. In principle, it also reduces internal P loads to the water column, although this point has lacked direct data support. One 40-ha field-scale periphyton-based STA (PSTA) cell has operated since 2008 and has regularly reduced-15 & mu;g/L inflow TP to-10 & mu;g/L (long-term flow-weighted means). This study documents two mesocosm experiments aimed at better understanding PSTA systems. The first experiment investigated the response of outflow P to external P loading rates (PLR = 0.3-1.3 g/m2/yr). Together with the field data from full-scale STAs, the results establish that PSTA systems are responsive to upstream P inputs (a reduction in external P inputs results in lower outflow P), whereas adjacent muck-based STA treatment cells receiving similar P inputs are not. The second experiment investigated differences in discharge TP and plant tissue and sediment P. Seven substrate-vegetation combinations were used: 0, 5, and 15 cm of limerock (LR) cap over indigenous muck with and without SAV found in the STAs; and a LR-only system inoculated with SAV but heavily dominated by periphyton. Rooted SAV produced internal P loads to the water column at a magnitude that is dependent upon access to the soil-P pool (muck only > LR cap over muck > LR only). Thus, the likely reason why PSTA achieves lower P discharges and exhibits external PLR influence is very low internal P loading rates (iPLR), where more elevated iPLR in muck-based systems appears to mask the effects of reducing external loading under low-P conditions. These results improve our understanding of external and internal P load contributions in low-P wetlands.

Automated summary

This study evaluates the removal of wetland phosphorus (P) in extremely low concentrations. It investigates the concept of scraping P-enriched muck soil from the discharge region of Stormwater Treatment Areas (STAs) to expose the underlying limerock substrate, which promotes the growth of periphyton and submerged aquatic vegetation (SAV) communities. The study also examines the impact of external and internal P inputs on P discharges in low-P wetlands.