Source or sink? Meta-analysis reveals diverging controls of phosphorus retention and release in restored and constructed wetlands

Abstract

Wetland restoration is a popular nutrient management strategy for improving water quality in agricultural catchments. However, a wetland’s ability to retain phosphorus is highly variable and wetlands can sometimes be a source of phosphorus to downstream ecosystems. Here, we used a meta-analysis approach to explore the source and sink capacity of 139 wetlands for both total phosphorus (TP) and the more bioavailable form, phosphate (PO43-), at seasonal and annual timescales. Median retention efficiency across all studies is 32 % for TP and 28 % for PO₄³⁻, however the range is extremely broad. We found that wetlands are often sinks for TP (84 % of site-years) and PO₄³⁻ (75% of site years). The median TP retention within wetlands that are sinks (2.0 g∙m-2∙year-1) is greater than release by wetlands that are sources (-0.5 g∙m-2∙year-1). In contrast, for PO₄³⁻, median retention within wetlands that are phosphorus sinks (0.8 g∙m-2∙year-1) is of similar magnitude to that released by wetlands that are phosphorus sources (-0.7 g∙m-2∙year-1). We found that phosphorus release from wetlands coincides with higher hydraulic loading rates, lower influent phosphorus concentration, and legacy soil/sediment phosphorus. Phosphate releases were especially common in wetlands used for treating municipal wastewater, as well as restored and constructed wetlands with flashy, precipitation-driven flow. We found that experimental design may inherently bias our understanding of wetland performance for phosphorus retention as studies conducted in mesocosms outperform other wetland types. Analysis of monthly data demonstrated significant temporal variability in wetland phosphorus dynamics, often switching from retention to release many times within a year, but with no generalizable seasonal trends. Our results highlight the value of restoring wetlands for phosphorus retention and point to ways of furthering their utility towards improving water quality by simultaneously targeting retention enhancing measures and release avoidance.

Modeling Phosphorus Retention and Release in Riparian Wetlands Restored on Historically Farmed Land

“…Our findings highlight the importance of considering both particulate and dissolved P in P mass balances for restored wetlands, building on prior studies (Walton et al 2020;Ury et al 2023). Basing estimates of TP retention on P deposition (i.e., particle trapping) alone will likely lead to overestimates of net TP retention.…”

“…Generalizations about P retention in wetlands are difficult to make, especially for restored riparian wetlands (Fisher and Acreman 2004;Hoffman et al 2009;Land et al 2016;Walton et al 2020;Ury et al 2023). Measuring or modeling P retention in riparian wetlands is complicated by unconstrained hydrology and spatiotemporal variability in water and soil that affect P dynamics (Underwood et al 2017;Wiegman et al 2022).…”

“…Measuring or modeling P retention in riparian wetlands is complicated by unconstrained hydrology and spatiotemporal variability in water and soil that affect P dynamics (Underwood et al 2017;Wiegman et al 2022). A recent meta-analysis of P retention in restored and constructed wetlands excluded riparian and floodplain wetlands because inflow and outflow are not easily defined (Ury et al 2023). Some studies of restored riparian wetlands report net total P (TP) retention driven by particulate P deposition (Kronvang et al 2009;Noe et al 2019), while others have found potential for P export (Hoffmann et al 2009;Jones et al 2015) or decreasing TP retention over time (Mitsch et al 2012).…”

“…In riparian wetlands, P retention is also linked to local hydrology, which influences P loading rate, sedimentation, contact between water and soil/sediments, P uptake by vegetation, and decomposition (Hoffmann et al 2009). The complicated hydrology of riparian wetlands (e.g., absence of well-defined inflows and outflows, networks of barriers/berms/roads and crevasses/ ditches/culverts, variable depth/velocity across the floodplain) commonly precludes the ability to measure influent and effluent P loads in the field (Ury et al 2023). In such cases, a combination of field measurements and modeling is required to estimate net P retention.…”

Water Renewal Time in Lakes with Transformed Water Distribution in the Catchment Areas

“…Residence time determines both mixing and transport processes and the time available for biochemical transformations [11]. Flushing time is considered to be a strong predictor of phosphorous retention in lakes [12,13]. There are many ways to estimate water exchange ratios, such as hydrodynamic models [14], traces [15], or measured values of water balance elements.…”