Mechanisms of Benthic Algal Succession in Lotic Environments

Abstract

Although seasonal patterns of algal succession have received considerable attention, few studies have attempted to elucidate the processes that contribute to short—term algal succession following disturbance. We framed the present study around four general mechanisms of succession in order to investigate the contribution of species—specific autecologies and interspecific interactions to observed short—term patterns of algal succession in a productive third—order stream in Kentucky, USA. Observations in different current environments in the stream during two consecutive winters were used to classify dominant species as early or late successional based on changes in relative abundance through time on newly exposed substrates. Assemblages of the same age but differing in the relative abundance of early— and late—successional species were developed in streamside channels recreating both current environments to measure growth parameters (e.g., reproduction) of dominant species and to test for interspecific interactions. Of the five dominant species of algae in the winter assemblage, three species were consistently classified as early successional (i.e., decreased in relative abundance with increasing assemblage age) and two species as late successional. Early—successional species differed in their growth form and strategy for initially dominating the substrate, having high densities in the water column, high probabilities of attaching, or fast early reproductive rates. Late—successional species exhibited a more extended growth form and had the highest per capita reproductive rates during later growth. Late—successional species reduced the reproductive rate of early species as succession proceeded in both current environments but were not themselves inhibited by biovolume increases. While differences in growth strategies between early— and late—successional species indicated the importance of passive tolerance mechanisms of succession, density—dependent interactions during community development were consistent with active tolerance mechanisms. As in terrestrial plant communities, successional patterns in benthic algal assemblages appear to result from several processes that defy explanation by a single mechanistic model.