SUMMARY. Temperature and current discharge regulated phytoplanktonic concentration, chlorophyll‐a concentration, the light‐saturated rate of photosynthesis (Pmax), and photosynthetic capacity (Pcap) in the Mississippi River at Prairie Island, Minnesota. The chlorophyll‐a maximum was 48 mg m−3 in 1975, a wet year with a high current discharge, and 190 mg m−3 in 1976, a relatively dry year. The highest values of Pmax were 0.37 (mgO2 I−1h−1) in 1975 and 1.60 in 1976. Pcap varied from 3 to 21 (gO2 per g chlorophyll‐a h−1) both years, and its value was highly correlated with temperature. The temperature optimum shifted from 16°C for Pcap in the spring, to greater than 28°C in the summer. Multiple regression analysis indicated a second‐order relationship of Pcap in the spring to temperature. Other independent variables explained only negligible variation of Pcap.

Species composition and seasonal succession of the phytoplankton were investigated on the upper Mississippi River at Prairie Island, Minnesota, U .S .A . Both the numbers and volume of individual species were enumerated based on cell counts with an inverted microscope . A succession similar to algal succession in the local lakes occurred . The diatoms were dominant during the spring and fall and blue-green algae were dominant during the summer . The algal concentrations have increased up to 40 fold the concentrations of the 1920's, since the installation of locks and dams . The maximum freshweight standing crop was 4 mg . 1 -1 in 1928 (Reinhard 1931), 13 mg . 1-1 in 1975 a wet year, and 47 mg . 1 -1 in 1976, a relatively dry year with minimal current discharge . The diatoms varied from 36-99%, the blue-green algae from 0-44% and the crypt6monads from 0-50% of the total standing crop . The green algae were always present but never above 21% of the biomass . The dominant diatoms in recent years were centric -Stephanodiscus and Cyclotella spp. (maximum 50,000 ml-1 ) . The dominant blue-green algae were Aphanizomenon flos-aquae (L .) Ralfs ex Born . et Flahault and Oscillatoria agardhii Gomont (maximum 800 ml -1 ) . These algal species are also present in local lakes . Shannon diversity values indicated greatest diversity of algae during the summer months .

For freshwater cyanobacteria, the autofluorescence of phycocyanin is quite high while the in vivo fluorescence (IVF) yield of chlorophyll-a is relatively low, apparently because of low chlorophyll concentrations associated with photosystem 11. In eucaryotic phytoplankton, even those with phycobiliprotein accessory pigments (e.g. some cryptophytes), the opposite is true. Thus, an IVF ratio which relates phycocyanin to chlorophyll-a signals could be a good index of relative cyanobacterial abundance in the field. Spectrofluorometric scans of whole cells from laboratory cultures indicated that the ratio Em,,, @ Ex,,,/Em,,, @ Ex,,, could be a very sensitive cyanobacterial indicator. Tandem flowthrough fluorometers were then fitted with the appropriate interference filters and their discriminatory power was evaluated with mixtures of cyanobacterial and eucaryotic phytoplankton. Although subject to many of the constraints of other IVF assays, tandem fluorometry should be particularly appropriate for real-time mapping of the relative spatial and temporal distributions of broad phytoplankton taxa in continuous vertical of horizontal profiles in lakes.

A series of inexpensive, pneumatic thin layer water samplers is described. They can be operated from small boats, and permit sampling at 2.5 cm intervals with little or no disturbance of stratified systems such as oxyclines or redoxclines of meromictic lakes, or microstratification of flagellates in sheltered epilimnia. Some models permit replicate sampling at closely-spaced intervals in a two-dimensional array. Their performance abilities are illustrated with examples of microstratification.

Studies of the eflects of passage through a power plant on river phytoplankton have shown that chlorination depresses rates of photosynthesis and respiration to a much greater extent than does heating.