Effects of the Dairyland Power Cooperative electrical generating facility on the phycoperiphyton in navigation pool no. 9, Upper Mississippi River

Abstract

A study was conducted in Navigation Pool No. 9 of the Upper Mississippi River to determine the impact of the thermal effluent from Dairyland Power Cooperative's Genoa, Wisconsin electric generating facility on the phycoperiphyton. The objectives of this study were: 1) to compare the taxonomic composition, density, biomass, and pigment concentration of the phycoperiphyton community upstream and downstream from the thermal effluent; 2) to monitor the physical and chemical characteristics of the upper portion of Navigation Pool 9; and 3) to establish baseline data for phycoperiphyton in channel areas of Navigation Pool No. 9 of the Mississippi River.

Sampling was conducted from 3 June 1980 to 1 June 1981. Phycoperiphyton samples were collected from artificial and natural substrates. Field determinations of temperature, dissolved oxygen, and current velocity were made at each sampling site. Water samples were collected and analyzed for turbidity, conductivity, pH, total hardness, total phosphorus, orthophosphorus, nitrate + nitrite-N, nitrite-N, ammonia-N, and silica.

The electric generating facility effluent increased the temperature of the Mississippi River in the vicinity of the discharge during the entire year. The temperature of the receiving water was elevated 4.3?C to 11?C above the ambient temperature. Increases in temperature were minimal at downstream sampling sites. Dissolved oxygen concentrations were similar to those recorded for other reaches of the Upper Mississippi River and were always >51% of saturation. Current velocity was low and mostly affected by local river morphometry and manmade structures. The chemical character of the water was similar among the four sites sampled. The water could be classified as hard and alkaline with sufficient nitrogen and phosphorus to support high rates of primary production.

A total of 259 species representing 52 genera were identified. Both green algae and blue-green algae were common but not abundant. The most common green algae were Stigeoclonium lubricum, an unidentified species of green algae, and several species of Scenedesmus. Oscillatoria sp. was the most prevalent blue-green alga and was observed at all sampling sites during most of the year.

Three major diatom assemblages were recognized: 1) a summer/autumn assemblage, 2) late autumn assemblage, and 3) a thermally influenced winter assemblage at the discharge. The dominant species in the summer/autumn assemblage was Cocconeis placentula var. euglypta and Navicula tripunctata var. schizonemoides. These species had an inverse dominance relationship at all the sampling sites except the site located at the discharge. The summer/autumn assemblage at the discharge site was dominated by Navicula tripunctata var. schizonemoides and several Cyclotella species. The late autumn assemblage persisted for less time than the summer/ autumn assemblage. Melosira varians was the most common species observed. Stephanodiscus rotula dominated at the discharge during the late autumn assemblage.

The thermal discharge was the only site sampled during the winter, and a thermally influenced winter assemblage was observed. Stephanodiscus rotula accounted for at least 70% of the phycoperiphyton density in January and early February. The density of Stephanodiscus rotula declined, but it remained dominant during the entire winter. Density was low in summer but increased during late autumn at the sites upstream and downstream of the discharge. In contrast, species diversity was lower in January and February but again increased in the spring.

The greatest total phycoperiphyton densities were observed during September and October. Cocconeis placentula var. euglypta was the most common alga observed during this time. The greatest density at the discharge site occurred in January and February and was associated with the high relative numbers of S. rotula.

The accumulation of chlorophyll a varied substantially, both temporally and spatially. In general, accumulation rates were low in the summer, increased in autumn, and declined in late autumn. Chlorophyll at the thermal discharge site accumlated more rapidly than at the other sites; the higest rates in the discharge occurred during the winter.

Accumulation of ash-free dry-weight was greatest in the summer and autumn, and declined in late autumn. The greatest production of organic matter was observed at the discharge site in January and February. This coincided with the greater densities of phycoperphyton.