Heterotrophic aquatic metabolism and sustained carbon dioxide emissions in a mineral-soil wetland restored with treated effluent

Multi-decadal impacts of effluent loading on phosphorus sorption capacity in a restored wetland

Natural wetlands are widely used and cost-effective systems for the passive remediation of phosphorus (P)-rich surface waters from various effluent sources. Yet the long-term biogeochemical impacts of effluent loading on wetland P retention capacity are unclear. Here, we had a unique opportunity to document the spatio-temporal evolution of sediment P sorption over a ∼25-year period of constant municipal and industrial effluent loading, as part of a wetland restoration and wastewater treatment strategy in one of the largest restored wetlands in Canada. Sediment P sorption experiments across Frank Lake's three basins revealed a wide spatial variation in sorption capacity, closely linked to sediment geochemistry gradients (Ca, Fe, and Mn). Relative to a similar study ∼25 years prior, P sorption capacity has become exhausted near the effluent inlet, but remarkably, remains elevated throughout the rest of the wetland. Compared to other prairie wetlands and global aquatic ecosystems, Frank Lake has a greater capacity overall to retain P through sediment sorption. Given the paucity of long-term (multi-decade) data on wetland response to effluent loading, we provide key insights into the dynamics of wetland P cycling in human-dominated watersheds.

Experimental Ecosystem Eutrophication Causes Offsetting Effects on Emissions of CO2, CH4, and N2O from Agricultural Reservoirs

Despite decades of research and management efforts, eutrophication remains a persistent threat to inland waters. As nutrient pollution intensifies in the coming decades, the implications for aquatic greenhouse gas (GHG) emissions are poorly defined, particularly the responses of individual GHGs: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The biogeochemical controls of each gas can differ, making it difficult to predict the overall effect of nutrient pollution on the net radiative forcing of aquatic ecosystems. Here, we induced eutrophication of small nitrogen (N)-limited agricultural reservoirs and measured changes in diffusive GHG emissions within a before-after-control-impact (BACI) study design during June to September 2021. Each gas exhibited a unique response to 300% increases in primary production, with a shift from an overall CO2 source to a sink, a modest increase in N2O flux, and, unexpectedly, no significant change in CH4 emissions. The lack of net directional change in CO2-equivalent GHG emissions in fertilized reservoirs during the summer contrasts findings from empirical studies of eutrophic lakes. Our findings illustrate the difficulty in extrapolating among different sized ecosystems and suggest that forecast 2-fold increases in agricultural N fertilization by 2050 may not result in consistently elevated GHG emissions during summer, at least from small reservoirs in continental grassland regions.