Amy T. Hansen

Coupling freshwater mussel ecology and river dynamics using a simplified dynamic interaction model

Freshwater faunal diversity and abundance have declined dramatically worldwide, concurrent with changes in streamflow and sediment loads in rivers. Cumulative effects and interdependencies of chronic covarying environmental stressors can obscure causal linkages that may be controlling the population dynamics of longer-lived freshwater fauna, such as mussels. To understand changes in long-term mussel population density, we developed a dynamic, process-based interaction model that couples streamflow, suspended sediment, phytoplankton, and mussel abundance under the hypothesis that chronic exposure to increased suspended sediment and food limitation are the primary factors controlling native mussel population density in a midwestern USA agricultural river basin. We calibrated and validated the model with extensive survey data from multiple time periods and used it to evaluate changes in mussel abundance at a subbasin scale over decades. We evaluated sensitivity of simulated mussel densities across a range of mortality rates and initial population densities. In scenarios representing altered sediment concentrations, such as might occur with climate or landuse-induced changes in streamflow or sediment generation rates, mussel population density showed critical threshold responses to long-term changes in suspended sediment concentration. This model of mussel population density can be used to test hypotheses about limiting factors, identify priority locations for restoration activities, and evaluate the effects of climate- or landuse-change scenarios.

Contribution of wetlands to nitrate removal at the watershed scale

Intensively managed row crop agriculture has fundamentally changed Earth surface processes within the Mississippi River basin through large-scale alterations of land cover, hydrology and reactive nitrogen availability. These changes have created leaky landscapes where excess agriculturally derived nitrate degrades riverine water quality at local, regional and continental scales. Individually, wetlands are known to remove nitrate but the conditions under which multiple wetlands meaningfully reduce riverine nitrate concentration have not been established. Only one region of the Mississippi River basin—the 44,000 km2 Minnesota River basin—still contains enough wetland cover within its intensively agriculturally managed watersheds to empirically address this question. Here we combine high-resolution land cover data for the Minnesota River basin with spatially extensive repeat water sampling data. By clearly isolating the effect of wetlands from crop cover, we show that, under moderate–high streamflow, wetlands are five times more efficient per unit area at reducing riverine nitrate concentration than the most effective land-based nitrogen mitigation strategies, which include cover crops and land retirement. Our results suggest that wetland restorations that account for the effects of spatial position in stream networks could provide a much greater benefit to water quality then previously assumed.Depending on their connectivity to the river network, wetlands can be much more efficient at removing nitrate in a watershed than common nitrogen mitigation strategies according to an analysis of the Minnesota River basin.

Uptake of dissolved nickel by Elodea canadensis and epiphytes influenced by fluid flow conditions

Using a laboratory mesocosm consisting of live plants and epiphytes grown in a re-circulating flume, dissolved nickel uptake by Elodea canadensis Michaux is compared with nickel uptake by the associated epiphytic community under a range of flow conditions. A flux model was developed and applied to the measured tissue nickel concentration data and generated three parameters descriptive of nickel uptake: uptake rate, equilibrium concentration, and time to equilibrium. The relationship of these parameters to flow conditions, represented by the dimensionless variable Reynolds number, was compared between epiphytes and plants. Water flow was shown to have a stronger effect on the uptake performance of epiphytes than that of plants, implying that water-side mass transfer plays a more important role in epiphytic nickel uptake than it does in plant nickel uptake. Although nickel concentrations were much higher in the epiphyte community than in E. canadensis, more total nickel was sequestered in E. canadensis. This research indicates that fluid flow conditions alter nickel uptake by E. canadensis and the epiphytic community and that the two have different preferential flow regimes. It also suggests the promising bioremediation potential of both in moving fluids in aquatic environments.

Contextualizing Wetlands Within a River Network to Assess Nitrate Removal and Inform Watershed Management

This research was funded by NSF grant EAR-1209402 under the Water Sustainability and Climate Program (WSC): REACH (REsilience under Accelerated CHange)

High‐Frequency Sensor Data Reveal Across‐Scale Nitrate Dynamics in Response to Hydrology and Biogeochemistry in Intensively Managed Agricultural Basins

An edited version of this paper was published by AGU. Copyright 2018 American Geophysical Union.