Diatom colonization dynamics in a lotic system

A series of three overlapping sets of slides were exposed in riffle and pool habitats in a fourth order river in the Upper Peninsula of Michigan. Each set was exposed for 6–8 weeks and overlapped the preceeding set by 2 weeks. Diatom cell densities and community structure were determined after daily or weekly exposure periods for each set. The first set was placed on August 4, 1982, and the third set removed on October 16, 1982. Species diversity and evenness peaked quickly during colonization. Both indices decreased as the length of exposure increased. Early colonizing diatom species that occasionally accounted for large proportions of the total diatom community were soon replaced by other diatom species that tended to persist through time. Major dominant species were well established by day 28. Severe net cell losses (up to 17% of the total density) were recorded after only an 8–9 day exposure in both pools and riffles. Pool slides showed greater cell densities during the first few day's exposure than did slides exposed in riffle zones. After this brief conditioning period, however, the riffle slides showed more rapid cell growth and/or accumulation rates. Mean cell densities were similar between pool and riffle slides after 6–8 week exposures.Seasonal changes appeared to strongly influence diatom species succession. Seasonal changes in water velocity, temperature, or light may have the same effect as the more dramatic flood events which reset periphyton to earlier successional stages, resulting in increased major changes in species composition of the periphyton diatom community.