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EFFECTS OF NUTRIENT ENRICHMENT ON THE GROWTH AND SURVIVAL OF JUVENILE LAMPSILIS CARDIUM IN THE ST. CROIX (SACN) AND UPPER MISSISSIPPI RIVERS (MISS)
Michelle Bartsch, USGS Upper Midwest Environmental Sciences Center, La Crosse WI
Considerable nutrient-related research in the St. Croix (SACN) and the Upper Mississippi River (MISS) suggests that both parks suffer from nutrient enrichment from agricultural runoff, wastewater inputs, and storm water drainage. However, the degree and impact of nutrient enrichment varies in each. Nutrient concentrations in the SACN are generally lower than those found in MISS, and this contrast provides an excellent opportunity to assess how excess nutrients affect key indicator biota. The SACN contains a historically rich assemblage of ~ 40 freshwater mussel species. Mussel fauna in the MISS were nearly decimated by pollution in the early 1900s, but has recently shown signs of recovery. Ammonia is one of the most common pollutants in aquatic systems and can be delivered via anthropogenic inputs, but also generated through natural bacterial processes. Given that juvenile mussels have been shown to be sensitive to ammonia in laboratory assays, we assessed its effects on the growth and survival of juvenile Lampsilis cardium during the summers of 2008 and 2009. Mussel survival and growth assays were conducted in both parks at each of six sites; 2 main channel (MC), 2 flowing backwater (FBW) and 2 isolated backwaters (IBW). At each site, we deployed 3 chambers at the sediment water interface for 30 d, each containing 20 juveniles. Sedimentary ammonia (µg NH3-N/L) was characterized using cores (top 5 cm sections) and averaged 13 and 23 in the MC, 4 and 19 in the FBW, and 13 and 11in the IBW of SACN and MISS, respectively. Ammonia (µg NH3-N/L) was also measured in individual mussel chambers and averaged 1 and 6 in MC, 2 and 4 in FBW, and 2 and 5 in IBW in the SACN and MISS, respectively. Survival of mussels was variable across habitats within the SACN (averaged 92% in MC, 54% in FBW, and 72% in IBW) but more similar within MISS (averaged 89% in MC, 89% in FBW, and 77% in IBW) and was not related to ammonia concentrations. Likewise, growth rate (µm/d) in 2008 was highly variable across habitats within the SACN (averaged 30 in MC, 8 in FBW, and 15 in IBW) and was more similar within MISS (averaged 21 in MC, 16 in FBW, and 19 in IBW), but generally unrelated to ammonia concentrations. In both parks, the ammonia concentrations observed in sediment pore water and mussel chambers were below the LC50 (116 µg NH3-N/L) and EC50 (64 µg NH3-N/L) values reported to cause effects on survival and growth of L. cardium from laboratory sediment toxicity studies.