Benthic Algal Community Dynamics in a Stream during and after a Spate

Abstract

Changes in a benthic algal community were observed during and after a major storm. Algal abundance, mostly diatoms, increased slightly during a 3-d interval that included the storm on day 1. Positive effects of the storm on benthic algae were indicated by decreases in the proportions of diatoms that were dead, but not in the dead cell numbers, and by decreases in the proportions of sexually reproducing cells. Increases in current or nutrient supply to periphyton during the storm may have stimulated growth of live cells and retarded sexual reproduction. During two 3-d growth periods after the storm, benthic diatoms grew rapidly from 1.1× 10⁷ cells/cm² and seemed to reach a carrying capacity near 5.2× 10⁷ cells/cm² in only 8 days. Density-dependent decreases in growth rates were accompanied by increases in the proportion of diatoms that were dead or in stages of sexual reproduction, indicating nutrient limitation. Immigration was not an important mechanism of accumulation immediately after the storm. However, immigration/colonization rates increased during community development and indicated substantial turnover by cell transport during late succession. Benthic diatom communities are well adapted to storms in temperate climates in streams with gravelbed channel morphology. I hypothesize that benthic diatoms are actually affected positively by most storms and are negatively affected only by the most severe storms that overturn substrates in the stream.