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.

A phytoplankton sampling device is described that collects samples at relatively close depth intervals (10 cm). This device is simple, transportable, easy to construct, and low in cost.

1. Use of a multi-column thin-layer pneumatic sampler and modified analytic procedures has enabled resolutions of chemical and biological strata at 2.5 or 5,0 cm depth intervals. Examination of meromictic Lake Fidler. Tasmania, indicates the presence of a thin, intense stratum of bacteriochlorohyll d in the upper monimolimnion. associated with a discrete stratum of Chlorobium cf, limicola at the microaerobic interface between the oxygenated and sulphide-rich zones.2. Algae included small populations of Chlorophyceae, Chrysophyceae, Bacillariophyceac and Cryptophyceac. Bacteria included microacrophils and obligate anaerobes, pigmented and colourless, in well-defined strata in the upper monimolimnion. A population of the microcrustacean Calamoecia tasmanica tasmatiica was present in the mixolimnion. Chaohortts larvae were concentrated within the Chlorohiutn layer.3. The chemical profile of Lake Fidler was stable, with a chemocline constant in position relative to the lake bottom. The surface water levels rose and fell through a distance of 1 m in conjunction with heavy rainfall in the rainforest, and with river level variation, but had no measurable effect on the absolute position of the chemocline. Marked heterogeneity of dissolved substances at depths in the vicinity of bacteria suggested endogenous influence on pH and gelbstoff ('gilvin" in Australia).4. Downwelling light attenuation was influenced primarily by gclbstolf ('gilvin") in the mixolimnion. with only red light (peak at 7(HI nni) measurable below 2 m. Light was absorbed mainly by Chlorobium in the monimolimnion. and was unmcasurable deeper than 3 m. 5. The absorption spectrum of the bactcriochlorophyll d in vivo, with a maximum absorbance at 721 nm. eorresponds with the available downwelling light penetrating the mixolimnion to the Chlorobium layer.

A natural metalimnetic population of the filamentous blue‐green Oscillatoria agardhii showed diurnal variation in net production of oxygen. The population was incubated during a series of 2‐hr periods during daylight hours on 7 August 1967, while suspended at several depths in Lake Itasca, Minnesota (47° N lat, 95° W long). The net rate of oxygen production varied with light intensity, reaching light saturation at near 10% of the maximum insolation (the incident intensity, I0, at solar noon). Productivity curves for depths corresponding to 100, 60, 30, 17, 11, 6, and 4% I° were obtained. High rates of production occurred during twilight at the surface, but near solar noon at the 6 and 4% levels. The variation, over the range of depths investigated, of oxygen per unit volume represents the shape of a photosynthesis‐depth profile for a homogeneously distributed population. The highest production is at 30% I0. The rate‐light intensity curve for the Oscillatoria population is similar in shape to that for certain cultures of marine algae, using the 14C technique. Distribution of points on the rate‐light intensity curve suggests that light quality, as well as intensity, is important, and that the filtering effect of lake water enhances subsurface oxygen production even at relatively high light intensities (80% of the solar noon I0 value).