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THE OTHER NUTRIENT: NITROGEN PATTERNS IN LAKE ST. CROIX WATER AND BIOTA
Brenda Moraska Lafrancois, National Park Service, St. Croix Watershed Research Station
Nitrate loads from the upper Mississippi River Basin contribute to the Gulf Hypoxia problem, and nitrate management is an important concern in the Midwest and Central U.S. Monitoring data indicate that nitrate concentrations in the St. Croix have increased since the mid-1970s, particularly at downstream monitoring stations in Lake St. Croix. Various factors may contribute to these trends, including increases in fertilizer application and runoff, changes in wastewater treatment inputs, and other processes.
Nitrogen and oxygen stable isotope analysis can help identify sources of nitrate in aquatic ecosystems. We measured nitrate concentrations and investigated patterns in the δ15N signatures of nitrate, seston, and mussel tissue (Dreissena). We hypothesized that nitrate concentrations would increase from upstream to downstream in the mainstem St. Croix, and that the δ15N signals in water, seston, and Dreissena would reflect local and regional anthropogenic influences.
From May through October 2008, we collected water samples for nitrate concentration and δ15NNO3 analysis from seven mainstem St. Croix sites (from near Arcola to near Prescott), five tributaries (Brown's Creek, Valley Creek, Trout Brook, and the Willow and Kinnickinnic Rivers), and two wastewater treatment plants (Hudson and St. Croix Valley, near Bayport). Additionally, we collected seston from all mainstem St. Croix sites and Dreissena from the lowermost five mainstem St. Croix sites for analysis of δ15Nseston and δ15NDreissena.
Mean nitrate-N concentrations for mainstem St. Croix sites increased from 0.12 mg/l near Stillwater to 0.34 mg/l near Prescott; the largest increases occurred below Hudson, likely due to inputs from high-nitrate tributaries (e.g., the Kinnickinnic River, mean = 5.04 mg/l). Mean nitrate δ15N varied little among mainstem St. Croix sites, decreasing only slightly from upstream (Stillwater = 6.76 ‰) to downstream (Prescott = 5.11 ‰). In contrast, mean seston and Dreissena δ15N increased markedly from upstream to downstream, from 5-6‰ at upstream sites to near 10‰ at downstream sites, with the largest increases downstream of Hudson.
The pattern of increasing nitrate concentration and increasing δ15N in seston and Dreissena in the mainstem St. Croix likely reflects high nitrate, high δ15NNO3 inputs from increasingly urban and agricultural tributaries; this finding is consistent with similar studies nationwide. These results suggest that nitrate sources ranging from wastewater inputs to manure applications may contribute to the increasing nitrate concentrations found in the St. Croix mainstem; other processes such as denitrification in groundwater may also be important. We conclude that the δ15N of seston and/or mussel tissue may be an efficient, cost-effective means to monitor anthropogenic nitrogen inputs to Lake St. Croix into the future.