Nitrate pollution and expansion of free-floating plants in 3 lower Wisconsin River oxbow lakes

Abstract Marshall DW, Wade K, Unmuth JL. 2023. Nitrate pollution and expansion of free-floating plants in 3 Lower Wisconsin River oxbow lakes. Lake Reserv Manage. 39:88–100. The Lower Wisconsin State Riverway (LWSR) in southwest Wisconsin is one of the highest quality large river floodplain ecosystems in the Midwest and is designated a Ramsar Wetland of International Importance. Oxbow lakes are important features of this floodplain ecosystem but many had become highly eutrophic by about 2011. Free-floating plants (FFP), comprised of duckweeds and filamentous algae, expanded in many groundwater-fed oxbow lakes along the LWSR. Beginning in 2013, we investigated possible causes of eutrophication. Our water pollution investigation indicated the primary oxbow water source is groundwater that flows beneath the cropland intensive Pleistocene sand terrace. The sand terrace aquifer delivers large amounts of NO3-N with concentrations that exceeded the federal and state Drinking Water Enforcement Standard (10 mg/L). Aquifer total phosphorus concentrations were orders of magnitude lower. NO3-N:total phosphorus (TP) ratios in terrace groundwater often exceeded 500:1. Estimated NO3-N loading rates were variable and ranged from 2989 kg/yr discharged to a 5.5 ha oxbow lake up to 33,091 kg/yr discharged to a 35 ha floodplain flowage. The results suggest coordinated groundwater and surface water quality protections are needed, and potentially Clean Water Act enforcement. Strategies designed to protect domestic water supplies beneath the sand terrace can reduce NO3-N pollution in the oxbow lakes. Addressing this issue could begin with adopting recommended surface water nitrogen criteria and developing FFP impairment criteria for LWSR oxbow lakes.