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 .

Sediment oxygen demand SOD measurements 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 lower3.6 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. 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 was 3.5 m. In contrast, chlorophyll “A” was about ten fold greater upstream at LD 8 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. Zebra mussel-induced nutrient conversions and phytoplankton consumption would be expected to contribute to high nutrient to chlorophyll ratios. We believe more data is 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.   

  A recent survey by the Corps of engineers, in the East Channel of the Mississippi River at Prairie du Chien has revealed a substantial reduction in the diversity and density of native mussels. The decline was likely the result of zebra mussels whose densities reached over 10,000 per square meter in 1998. The East Channel was one of the best mussel beds in the Upper Mississippi River. Future efforts are being considered to relocate native mussel beds to other waters that are less likely to be impacted by zebra mussels.