Abstract
Streams in Hawaii are experiencing increasing anthropogenic impacts due to agriculture and urbanization via nutrient enrichment and riparian canopy removal. The effects of these factors on stream algal communities in Hawaii have rarely been studied using manipulative experiments. A study was conducted in a slightly impacted forested stream on Oahu, Hawaii, to examine the effects of nutrient enrichment and canopy cover on the benthic algal community, especially diatoms. Benthic algae were colonized onto nutrient‐diffusing substrata in a factorial design with nitrate and phosphate enrichments, at high and low light levels at similar elevations. Algal productivity as measured by ash‐free dry mass and chlorophyll a levels increased significantly with nitrate enrichment at the high light site. Pulse‐amplitude modulated (PAM) fluorescence was used to estimate photosynthetic parameters in situ, and indicated a higher maximum photosynthetic rate for the algal community with nitrate enrichment at the high light site. Significant differences in diatom community composition were observed at low and high light sites, and several diatom species also responded significantly to either nitrate or phosphate enrichment at both sites. These results suggest that benthic stream algal communities in Hawaii may be significantly affected by human activities within watersheds.
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