Nuclepore filters are better than cellulose filters for the direct counting of bacteria because they have uniform pore size and a flat surface that retains all of the bacteria on top of the filter. Although cellulose filters also retain all of the bacteria, many are trapped inside the filter where they cannot be counted. Before use, the Nuclepore filters must be dyed with irgalan black to eliminate autofluorescence. Direct counts of bacteria in lake and ocean waters are twice as high with Nuclepore filters as with cellulose filters.

Growth rates, accumulation dynamics, and species succession of periphytic diatom communities were examined in the presence and absence of natural ultraviolet (UV) radiation using a series of outdoor, continuous‐flow experimental flumes located on the South Thompson River, British Columbia. In a short‐term experiment (2–3 wk), log‐phase growth rates of naturally seeded diatom communities comprised of Tabellaria fenestrata (Lyngb.) Kütz., T. flocculosa (Roth) Kütz., Fragilaria crotonesis Kitton, and F. vaucheriae (Ehr.) Peter. exposed to 90% ambient photosynthetically active radiation (PAR) + UV were 30–40% lower than growth rates under 90% PAR alone. UV inhibition of growth rate was independent of the degree of P limitation within the range of relative specific growth rates (μ:μmax‐P) of 0.5–1.0. In a long‐term trial, inhibition of attached diatom accumulation under 90% PAR + UV during the first 2–3 wk was corroborated. Reduction of full sunlight to 50% PAR + UV prevented the initial inhibition phase. The initial inihibitory effect of 90% PAR + UV on algal accumulation was reversed after 3–4 wk, and by 5 wk total diatom abundance (chlorophyll a, cell numbers and cell biovolumes) in communities exposed to PAR + UV were 2–4‐old greater than in communities protected from UV. Under 90% PAR + UV and 50% PAR + UV, a succession to stalked diatom genera (Cymbella and Gomphoneis) occurred. Species succession under UV radiation doubled the mean cell size of the diatom communities. The shift from inhibition to a long‐term increase in the autotrophic community under PAR + UV compared to PAR alone provides further evidence against the use of short‐term incubation experiments to define the long‐term implications of increases in UVB. These results suggest that the ecological effects of present‐day levels of UVB and UVB:UVA ratios on autotrophic communities are not well understood and might be mediated through complex trophic level interactions.

Various excitation and emission filters, microscope lamps, membrane filters, and field storage techniques used for direct counting of bacteria by epifluorescence microscopy have been compared. A rapid, simple modification of the Acridine Orange procedure using a new light filter combination, Sartorius cellulose membrane filters, and distilled water rinse gave the brightest cell fluorescence, darkest background, least fading, and highest counts for both estuarine and freshwater samples. Field fixation with Formalin permits storage of bacterial samples for 1 to 2 weeks without changes in bacterial numbers.

The physical effects of a large river (Thompson River) entering a deep, intermontane lake (Kamloops Lake, British Columbia) suggest that, depending upon its temperature relative to that of lake water, river water moves through the lake as a surface overflow, an intermediate depth interflow, or a near-bottom underflow. Circulation is further influenced by the earth's rotation so that the incoming river flows preferentially along the right-hand shoreline of the lake. Convective overturn in autumn and spring is influenced by cabbeling, which occurs than 4°C and one colder, combine to form a whenever two parcels of water, one warmer mixture whose temperature is at or near 4"~.

The photosynthetic response of tundra pond algae to various combinations of temperature, light intensity, and phosphate concentration was measured at weekly intervals during the 1973 summer. In these small (50 m diam by 20 em deep) ponds near Barrow, Alaska the epipelic algae had a higher temperature optimum (> 20°C) for photosynthesis than did the phytoplankton (14°C) but the epipelic O,o (2.5°-l2.5°C) for photosynthesis was only 2.2, compared to a value of 3.0 for the planktonic algae. Thus the epipelic algae seemed to be adapted to the sediment environment where temperatures were usually higher than temperatures in the water. The plankton algae, in contrast, appeared to consist of species which photosynthesized more efficiently at the lower temperatures of the pond water. The photosynthetic half-saturation light intensity, lo.s, was temperature dependent, increasing as much as threefold in the epipelic experiments (from 0.04 to 0.12 ly · min-1 ) over a tO-degree temperature range. No diurnal change in lo.s was observed but it declined steadily for both algal groups throughout the summer in response to declining illumination. This decrease in Io.• probably resulted from an increase in the chlorophyll-to-carbon ratio of the algae. Light inhibition was common at low temperatures in the planktonic experiments but never occurred in the epipelic experiments. Short-term phosphate enrichment experiments resulted in no detectable response in either plankton or epipelic algal photosynthesis. However, after whole pond fertilizations large increases in algal biomass and productivity occurred within several days. Thus, the ponds were phosphate-limited but there was a time lag in the response to the nutrient addition.