The interaction of flow regimes and nutrient fluxes on the water quality and ecosystem health of a clear, freshwater wetland

Across the globe, the hydrology and ecology of wetland systems have been altered by anthropogenic activities, sometimes leading to regime shift or even ecosystem collapse. Often, it is not only the impact of one stressor, but the combination of multiple stressors interacting that ultimately leads to adverse ecological impact in wetland systems. However, because of the difficulty in measuring the combined, dynamic effects of multiple stressors, relatively few studies estimate the relative importance of multiple stressors on wetland ecosystems. We combined controlled laboratory and field experiments with a modeling exercise to examine the relative importance of flow and nutrient loads on the resilience of a clear, groundwater-fed wetland dominated by macrophytes. We examined the potential for a combination of lower inflow and higher nutrient loads to increase phytoplankton growth and reduce light availability, culminating in a reduction in macrophyte growth due to the shading of the phytoplankton. This combination of events could result in a collapse of this endemic ecosystem, including local extinction of several endangered species. We found that the resilience of the macrophyte-dominated wetlands is maintained by preserving high flow even under increasing phosphorus concentrations. Nutrient availability increases as flow decreases, favoring pelagic algal development and inducing a shift in the ecosystem conditions. This shows that focusing only on input nutrient levels, as is often done in open waters of concern, is not sufficient to preserve the native ecosystem and highlights the need to consider multiple factors when assessing anthropogenic impacts on wetlands.