Micro‐environmental characteristics of filamentous algal communities in flowing freshwaters

SUMMARY. 1. Filamentous algae in flowing freshwaters can represent a spatially and temporally distinct sub‐habitat for epiphytic diatom communities. This sub‐habitat is described in a low discharge, spring‐fed stream with extensive filamentous green algal mats, and in a tuft of Cladophora glomerata (L.) Kützing from a large river. 2. Oxygen micro‐electrodes, a thermistor current velocity probe, a standard pH probe and water chemistry were employed to assess spatial heterogeneity. Temporal patterns of epiphyte colonization were evaluated on filamentous artificial substrates. 3. There were steep spatial gradients in the low discharge stream. At mid‐day, O 2 ranged from 0–1.5 times air‐saturated O 2 concentrations, pH varied from 7.25–8.0, and current velocity spanned 0–0.5 m s −1 . Areas near the surface of algal mats had high O 2 , pH and current velocity. These patterns were correlated with epiphyte community structure. 4. In the interior of C. glomerata tufts O 2 concentration was raised and current velocity depressed compared to the surrounding water, even when external current velocity was as high as 0.4 m s −1 . 5. After thirty‐five days of colonization of artificial substrate in the low discharge environment, epiphyte communities were similar to those on filamentous atgae. Epiphyte diversity on artificial substrates subsequently decreased compared to natural substrates as did the similarity between the types on substrates, suggesting that microscale renewal of epiphyte habitat (growth of filamentous algae) maintains high epiphytic diversity.