A quantitative survey of the phytoplankton and water quality of the La Crosse, "Black" and Mississippi Rivers

by Cary, George A.

A seven-month study of the phytoplankton populations and water quality of the La Crosse and Mississippi Rivers and the back water slough of the old Black River channel below navigation pool #7, hereafter referred to as the "Black" River, was initiated on May 19, 1971 and terminated on November 20, 1971. Nine sampling stations were chosen in such a manner as to monitor the different channels above and below their point of convergence. Phytoplankton samples were collected at 3 depths, while samples used in the determination of chemical parameters were obtained at the 2 m depth. Six major groups of algae were found in the study area. These were the Chrysophyceae, Pyrrophyta, Euglenophyta, Bacillariophyceae, Chlorophyta and Cyanophyta. The latter three made up the major portion of the phytoplankton, both in numbers and diversity. The "Black" and Mississippi Rivers were found to be similar with regard to phytoplankton composition. The chemical parameters of the two study areas were also similar, with the exception of total hardness. It was observed that the total hardness concentration was lower in the " Black " River than in the Mississippi River. The La Crosse River presented a seemingly unfavorable set of conditions for the maintenance of phytoplankton populations. The major organisms of this river were various species of Ulothrix. This was also the only genus to occur in bloom proportions in the La Crosse River. The La Crosse River, besides being distinct from the "Black" and Mississippi Rivers with regard to phytoplankton composition, was also distinct from the latter two on the basis of turbidity and settleable matter.