Effects of Irradiance on the Community Structure and Biomass of Algal Assemblages in Laboratory Streams

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Abstract
Studies in natural streams have implicated irradiance as a factor which has a strong influence on the dynamics of algal communities. In this study, laboratory streams were used in a replicated, experimental design to investigate whether differences in algal biomass, taxonomic structure, and physiognomy result from exposure to different irradiances. Effects of four photon flux densities (15, 50, 150, and 400 μE∙m−2∙s−1) on locally collected benthic stream algae were monitored over a 48-d period. Biomass increased in all streams during the experiment, but the streams exposed to the highest irradiance had 25 times more biomass at the end of the experiment than the channels exposed to the lowest irradiance. Although diatoms were the dominant algal class in all streams, the relative abundance of chlorophytes was much greater in streams exposed to 150 and 400 μE∙m−2∙s−1 than in channels treated with 15 and 50 μE∙m−2∙s−1 Detrended correspondence analysis indicated that the successional trajectories of assemblages exposed to low irradiances were quite distinct from those of assemblages treated with high irradiances. Observations of assemblage physiognomy by scanning electron microscopy revealed that at low irradiances, a densely packed understory of adnate diatoms, with a few overstory diatoms, covered the tile surface. At high irradiances, tiles were overlaid with thick algal mats, composed of filamentous and coenobic chlorophytes and diatoms of various growth forms (rosette, chain-forming, and solitary). The experimental results suggested that differences in biomass and community structure among the laboratory assemblages were a direct result of light energy, and that irradiance is a major factor influencing algal dynamics in lotic ecosystems.

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