The role of calcium and alkalinity on phosphorus removal by submerged aquatic vegetation in hardwater wetlands

The Everglades Stormwater Treatment Areas (STAs) are a 25,000 ha (61,000 acre) complex of constructed wetlands built to remove phosphorus (P) from agricultural and urban runoff in support of Everglades restoration. These STAs treat inflow waters that contain P in concentrations from 100 to 200 mu g L-1, average calcium (Ca) concentrations of 60-90 mg L-1, and alkalinity (Alk) in the range of 150-350 mg L-1 as calcium carbonate (CaCO3). Photosynthesis, primarily from submerged aquatic vegetation (SAV) within these STAs, increases water pH resulting in CaCO3 formation, which is thought to contribute to P removal through coprecipitation and other processes. Since 2002, the STA complex has removed over 3000 MT of P and over 400,000 MT of Ca. Because the STAs at times treat Lake Okeechobee waters that are lower in Ca and Alk (35-50 mg L-1 and 95-125 mg L-1 as CaCO3, respectively), we conducted two experiments to explore the role of Ca and Alk on wetland P removal.First, in outdoor flow-through microcosms (0.18 m2) containing Najas guadalupensis and treating artificial inflow solutions, Ca/Alk at typical STA inflow concentrations (77 mg Ca L-1/316 mg L-1 as CaCO3) resulted in significantly lower outflow P concentrations compared to lower Ca/Alk treatments (28 mg Ca L-1/67 mg L-1 as CaCO3). Based on sediment organic carbon to total P (TP) ratios, organic matter production and sedimentation was the P removal mechanism under low Ca/Alk levels, where CaCO3 precipitation did not occur, and also under high Ca/Alk, low P (31 mu g L-1) conditions, despite CaCO3 formation. High Ca/Alk increased organic matter sedimentation and associated P burial at low and high (168 mu g L-1) water P concentrations. Additional P removal associated with CaCO3 precipitation occurred only under high Ca/Alk, high P concentrations, and accounted for about 47% of P sedimentation in those microcosms.Second, in flow-through mesocosms (4.75 m2) containing STA muck soil and common STA SAV species, elevated Ca/Alk (94 mg Ca L-1/185 mg L-1 as CaCO3) produced significantly lower outflow TP concentrations than treatments receiving unamended Lake Okeechobee inflow water with lower Ca/Alk concentrations (44 mg Ca L-1/112 mg L-1 as CaCO3). In the same experiment, Ca/Alk levels had no effect on P removal in mesocosms containing Typha domingensis, a common STA emergent macrophyte species. SAV mesocosms produced lower outflow TP concentrations than Typha, regardless of Ca/Alk levels. In a follow-on laboratory incubation, low Ca/ Alk concentrations in incubation waters did not increase P release from sediments formed under prior low or high inflow Ca/Alk concentrations in the SAV mesocosms, but anoxia sharply increased P release from both.These experiments suggest that coprecipitation with CaCO3 is a meaningful P removal process in hardwater wetlands where high P concentrations and SAV coincide. Conversely, in STA outflow regions, where surface water P has already been treated to a low level, coprecipitation with CaCO3 probably does not contribute to further P reductions. However, Ca/Alk also may contribute to inorganic-C availability which influences sub-merged aquatic productivity, so the role of inorganic C limitation on STA vegetation communities and P removal merits further study.

Automated summary

The Everglades Stormwater Treatment Areas (STAs) are constructed wetlands built to remove phosphorus (P) from agricultural and urban runoff in support of Everglades restoration. Research shows that the levels of calcium (Ca) and alkalinity (Alk) have an impact on phosphorus removal in these wetlands.