Zebra mussel spatial coverage and densities have expanded greatly in the Upper Mississippi River since their first discovery in 1991. Previous research in zebra mussel-infested rivers has revealed low DO concentrations during summer periods as a result of zebra mussel activity. 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 at Genoa, WI, 9  at Lynxville, WI, 11 at Dubuque, IA. and 12 at Bellevue, IA. our data from Lock and Dams 8 and 9 was conducted to help assess factors contributing to the low DO concentrations during June of 1997. We also conducted 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. 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. 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. 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 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. It was believed the major source of this ammonia nitrogen was associated with zebra mussels. Specifically, zebra mussel waste excretions, decomposition of feces 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.