Introduction
The results of numerous measurements of primary production in different kinds of environments have prompted speculation concerning the effects of current velocity on productivity in lotic ecosystems . ODUM (1956) suggested that the water flowing over stream communities renews the depleted requirements for life and removes accumulating by-products of metabolism . WHITFORD (1960) concluded that a swift current rapidly removes the relatively impoverished water near the community, thus bringing water of full nutrient concentration closer to the cell surfaces .
Experiments by WHITFORD & SCHUMACHER (1961, 1964) have demonstrated that lotic and lenitic species of Oedogonium and Spi- rogyra have higher rates of respiration and phosphorus uptake when subjected to a current of 15 cm/sec than in still water . In general, they attribute these higher rates to a steepening of the diffusion gradient near the cell surface . An analogous enhancement of photosynthetic rates was noted with land plants in rapidly circulating gas, and with pure cultures of algae in well-stirred acid or alkaline solutions (RABINOWITCH, 1951) . ODUM & HOSKIN (1957) found that the metabolism of periphyton communities developed in a glass condenser tube was often greater at higher current velocities .
Very little information is available in the literature concerning the