In mid- to late June 1997, unusually low dissolved oxygen (DO) concentrations were found by federal and state agencies in a 150-mile reach of the Mississippi River during a period of warm water (25-27 deg.C) and below average river flow. Main channel DO concentrations below 4 mg/L were common in a reach extending from Pool 9 near Lynxville, Wisconsin to Pool 13 near Bellevue, Iowa (Figure 1). Dissolved oxygen concentrations increased in July in response to increased river flow, but values below 5 were still present at times. An investigation was made to determine if zebra mussels contributed to the low DO concentrations and influenced other water quality variables during the summer of 1997.
Zebra mussel spatial coverage and densities have expanded greatly in the Upper Mississippi River since their first discovery in 1991 (Cope et al. 1991). Previous research in zebra mussel-infested rivers have revealed low DO concentrations during summer periods as a result of zebra mussel activity (Effier and Siegfried 1994, Effier et al. 1996 and Whitney et al. 1995). These problems have been attributed to respiratory processes and biodegradation of waste products.
An evaluation of USGS's Long Term Resource Monitoring data (USGS, 1997) near Lock and Dams 8 (Genoa, WI), 9 (Lynxville, WI), 11 (Dubuque, IA) and 12 (Bellevue, IA) and our data from Lock and Dams (LD) 8 and 9 was conducted to help assess factors contributing to the low DO concentrations during June of 1997. We also conducted in situ measurements of benthic oxygen demand in July and August to evaluate the relative importance of zebra mussels as an oxygen sink in the Mississippi River (Sullivan and Endris, 1998).
Water temperatures rose quickly from about 15 to 25 deg.C during the first two weeks of June during a period of rapidly falling river flows (Figure 2a and b). Dissolved oxygen concentrations fell from about 10 to 12 mg/L in late May to less than 5 mg/L at LD 9, 11 and 12 in late June (Figure 2c and d) . Chlorophyll a concentrations, a surrogate for phytoplankton biomass, were high (60 to 90 ug/L) during mid- to late May but fell to low concentrations (< 10 ug/L) by late June at LD 9, 11 and 12 (Figure 2e and f). In contrast, DO and chlorophyll a concentrations were notably greater at LD 8 during this period.
Inorganic nitrogen, soluble reactive phosphorus and dissolved silica concentrations were depressed to very low levels in May and early June due to nutrient assimilation by the late spring phytoplankton bloom and reduced tributary inflows. These nutrients increased during mid- to late June following the decline of the spring algae bloom. A marked increase in ammonia nitrogen during mid-June at LD 9, 11 and 12 was closely associated with the onset of low DO concentrations at these sites (Figure 2g and h). It was believed the major source of this ammonia nitrogen was associated with zebra mussels. Specifically, zebra mussel waste excretions, decomposition of feces and pseduofeces and decaying zebra mussels in Pools 9 to 12 may have contributed to this ammonia increase. Nonpoint source inputs were not believed to be major source of this nitrogen since their relative contribution would have been low during the low flow period. Major inputs of ammonia from point source discharges to the affected river reach were not likely since their wastewater contributions are relatively small. Further, there were no major spills reported during or immediately preceding the low DO period.
Decreased DO concentrations during mid- to late June likely reflected increased water temperature (reduced DO saturation), reduced photosynthesis (lower phytoplankton concentrations) and increased benthic oxygen demand (freshly settled algae) during this period. However, these processes alone did not appear to explain the unusually low DO concentrations experienced in the main channel between LD 9 and 12 since similar low DO concentrations were not observed upstream of Pool 9 (Figure 1).
In situ sediment oxygen demand (SOD) measurements (Photo 1) made in areas infested with high zebra mussel densities (10,000 to 14,000/m2) revealed very high oxygen demands of 15-20 g/m2/day during July and August 1997. Benthic oxygen demand rates were substantially lower(3.6 to 5.0 g/m2/day) in silty substrates sites with moderate to high unionid mussel densities of 16.5 to 49.4/m2 where zebra mussels were absent. In contrast, silty sediments without any unionids or zebra mussels had an SOD of only 0.8 to 0.9 g/m2/day. Water temperatures ranged from about 21 to 25 deg.C during these measurements and played a minimal role in influencing the benthic oxygen demand rates. Maximum SOD was significantly correlated with zebra mussel density, volume and weight (including shells), (Sullivan and Endris, 1998).
Based on the benthic oxygen demand data and previous reports of low DO in rivers infested with zebra mussels, we believe the low DO in portions of the Mississippi River main channel during June and July was likely influenced by zebra mussel respiratory activity, decomposition of waste products and decaying zebra mussels. Reduced river flow and warm water conditions during mid- to late June would have contributed to a greater DO deficit during this time in reaches with high zebra mussel infestations.
Zebra mussel-induced water quality impacts were also believed to be responsible for unusually water quality conditions in the river during August and September of 1997. Unusually low chlorophyll concentrations and high light penetration was observed at LD 9 during this time. This was particularly apparent in early September when chlorophyll a concentrations were about 2 ug/L and light penetration (1% depth) was 3.5 m. In contrast, chlorophyll a was about ten fold greater upstream at LD 8 (Figure 2e) during this time and light penetration was substantially lower. Ratios of dissolved nutrients (soluble reactive phosphorus, ammonia nitrogen and silica) to chlorophyll a were also unusually high at LD 9 based on long term data for this site (Sullivan and Endris, 1998). Zebra mussel-induced nutrient conversions (particulate to dissolved forms) and phytoplankton consumption would be expected to contribute to high nutrient to chlorophyll ratios.
We believe more data are needed to verify zebra mussel-induced water quality impacts in the river. A detailed DO, suspended solids, chlorophyll and nutrient budget in a river reach with high zebra mussel-infestation may provide another means for assessing their impacts on water quality. River monitoring agencies need to better coordinate their monitoring efforts and quickly share information should unusually water quality conditions reappear. More pool-wide information is needed on zebra mussel densities and spatial coverage to better evaluate their impacts in the Mississippi River.
Cope, W.G., M.R. Bartsch, and R.R. Hayden. 1997. Longitudinal patterns in abundance of the zebra mussel (Dresissena polymorpha) in the Upper Mississippi River. J. Freshwater Ecol. 1 2(2): 235-238.
Effier, S.W., C.M. Brooks, K. Whitehead, B. Wagner, S.M. Doerr, M. Perkins, C.A. Siegfried, L. Walrath, and R.P. Canale. 1996. Impact of zebra mussel invasion of river water quality. Water Environ. Res. Vol 68 (2): 205-214
Effler, S.W. and C. Siegfried 1994. Zebra mussel (Dreissena polymorpha)populations in the Seneca River, New York: Impact on oxygen resources. Environ. Sci. Technol., Vol. 28 (12): 2216-2221.
Sullivan, J.F. and Mark B. Endris. 1998. Zebra mussel-induced water quality impacts in the Mississippi River observed during the summer of 1997. Wisconsin Department of Natural Resources, La Crosse, WI. 29p.
U.S. Geological Survey. 1997. Provisional data. Long Term Resource Monitoring Program. Environmental Management Technical Center. Onalaska, WI.
Whitney, S.D., K.D. Blodgett, and R.E. Sparks. 1995. Update on zebra mussels and native unionids in the Illinois River. Illinois Natural History Survey, LTRMP Field Station, Havana, IL.
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