Wetland phosphorus dynamics and phosphorus removal potential

Wetlands are typically defined as inundated areas with hydric soils forming a transitional zone between terrestrial and aquatic systems. Wetlands have numerous ecosystem benefits, one of which is the potential to mitigate or reverse eutrophication of surface water bodies. The physical, chemical, and biological processes governing phosphorus cycling in wetlands are nuanced and complex; understanding these has direct relevance to the restoration of wetlands, particularly for projects aimed at improving water quality in adjacent water bodies. This literature review summarizes these processes and provides recommendations relevant to restoration of permanent and semipermanent flow-through wetlands, such as those in the Upper Klamath Basin of Oregon. It also reviews several wetland restoration studies assessing phosphorus removal. In summary, appropriately designed and managed wetlands can remove 25% to 44% of inflowing total phosphorus. Deposition of particulate matter, adsorption, uptake by biomass, and peat accretion are the primary phosphorus sequestration mechanisms in wetlands, depending on site-specific conditions (e.g., growing season length, vegetation communities, and soil type). In areas with relatively short growing seasons and where wintertime loads are targeted for treatment, as in the Upper Klamath Basin, deposition of particulate matter will be the primary mechanism for phosphorus sequestration in wetlands given that two of the three remaining processes occur during the growing season. Recommendations to maximize phosphorus sequestration in wetlands include the following: designing wetlands for hydraulic residence time of several days to weeks, managing wetlands for rapid establishment of wetland vegetation with limited decomposition potential (e.g., tule [hardstem bulrush] to facilitate peat accretion), and flooding during periods with low water temperatures and initially isolating restored wetlands from adjacent water bodies (both to minimize diffusive flux of phosphorus from wetland sediment to the water column). Relevant to the Upper Klamath Basin, there is also justification to prioritize areas with relatively high particulate phosphorus load given the potential limited capacity for phosphorus treatment associated with other sequestration mechanisms. Finally, a combination of mitigation and restoration strategies is necessary to achieve water quality objectives, meaning that wetland restoration alone may not be sufficient. Monitoring is advised to facilitate application of adaptive management principles. PRACTITIONER POINTS: Appropriately designed and managed wetlands can remove 25% to 44% of inflowing total phosphorus. Deposition of particulate matter, adsorption, uptake by biomass, and peat accretion are the primary phosphorus sequestration mechanisms in wetlands, depending on site-specific conditions (e.g., growing season length, vegetation communities, and soil type). Recommendations to maximize phosphorus sequestration in wetlands include designing wetlands for hydraulic residence time of several days to weeks; managing wetlands for rapid establishment of wetland vegetation with limited decomposition potential (e.g., tule [hardstem bulrush], to facilitate peat accretion); and flooding during periods with low water temperatures and initially isolating restored wetlands from adjacent water bodies (both to minimize diffusive flux of phosphorus from wetland sediment to the water column). A combination of mitigation and restoration strategies are necessary to achieve water quality objectives, meaning that wetland restoration alone may not be sufficient. Monitoring is advised to facilitate application of adaptive management principles.