Nutrient Release from a Recently Flooded Delta Wetland: Comparison of Field Measurements to Laboratory Results

Abstract

Wetland restoration is expected to reduce external phosphorous (P) loading to hypereutrophic Upper Klamath Lake in Oregon, USA, where P was identified as the primary driver of lake productivity. However, previous laboratory experiments showed that a large P release could occur when former agricultural land is re-flooded for restoration, thus presenting an initial challenge for wetland restoration projects. We tested whether results from those experiments were supported during the initial phase of restoration at the Williamson River Delta adjacent to Upper Klamath Lake. Our objectives were to document post-flood surface water nutrient concentrations, estimate the P mass released from the newly flooded wetlands, and compare these results to the laboratory experiments. Phosphorus concentrations in the wetlands ranged from 0.1 to 0.63 mg P/L and were up to six times greater than in the lakes, corroborating that the wetlands released P upon flooding. However, we estimated 2 Mg P released within three weeks of flooding, which is much lower than the anticipated 64-Mg release from these wetlands, and a fraction of the annual 21–25 Mg load from the Delta before reconnection. This pulse is expected to be short-term; longer term studies will address the role of these wetlands in retaining nutrients.