Predictive models for phosphorus retention in wetlands

Abstract

The potential of wetlands to efficiently remove (i.e., act as a nutrient sink) or to transform nutrients like phosphorus under high nutrient loading has resulted in their consideration as a cost-effective means of treating wastewater on the landscape. Few predictive models exist which can accurately assess P retention capacity. An analysis of the north American data base (NADB) allowed us to develop a mass loading model that can be used to predict P storage and effluent concentrations from wetlands. Phosphorus storage in wetlands is proportional to P loadings but the output total phosphorus (TP) concentrations increase exponentially after a P loading threshold is reached. The threshold P assimilative capacity based on the NADB and a test site in the Everglades is approximately 1 g m⁻² yr⁻¹. We hypothesize that once loadings exceed 1 g m⁻² yr⁻¹ and short-term mechanisms are saturated, that the mechanisms controlling the uptake and storage of P in wetlands are exceeded and effluent concentrations of TP rise exponentially. We propose a “One Gram Rule” for freshwater wetlands and contend that this loading is near the assimilative capacity of wetlands. Our analysis further suggests that P loadings must be reduced to 1 g m⁻² yr⁻¹ or lower within the wetland if maintaining long-term low P output concentrations from the wetlands is the central goal. A carbon based phosphorus retention model developed for peatlands and tested in the Everglades of Florida provided further evidence of the proposed “One Gram Rule” for wetlands. This model is based on data from the Everglades areas impacted by agricultural runoff during the past 30 years. Preliminary estimates indicate that these wetlands store P primarily as humic organic-P, insoluble P, and Ca bound P at 0.44 g m⁻² yr⁻¹ on average. Areas loaded with 4.0 g m⁻² yr⁻¹ (at water concentrations>150 μg·L⁻¹ TP) stored 0.8 to 0.6 g m⁻² yr⁻¹ P, areas loaded with 3.3 g m⁻² yr⁻¹ P retained 0.6 to 0.4 g m⁻² yr⁻¹ P, and areas receiving 0.6 g m⁻² yr⁻¹ P retained 0.3 to 0.2 g m⁻² yr⁻¹. The TP water concentrations in the wetland did not drop below 50 μg·L⁻¹ until loadings were below 1 g m² yr⁻¹ P.