Water velocity is commonly accepted as a factor in the development of benthic algal mats in streams. Within a stream, two different zones of velocity are observed: the free-water velocity of the open water and the local velocity near the stream substrate. A closed laboratory flume system was used to observe the taxonomic composition of benthic algal mats and corresponding changes in the local velocities under different free-water velocities. As the algal mat developed under each experimental velocity, local velocities diminished and eventually became equal in all sections, while free-water velocities remained different. After a period of maximum taxonomic diversity during the first 2 wk of mat development, taxonomic composition, relative abundance of the taxa, and dry weight biomass became increasingly similar in the three velocity regimes, although the mats appeared different upon casual observation. Differences in composition and morphology in natural algal mats may not result from differences in current velocity, and the idea of a "closed monolayer" algal mat may not be appropriate in all situations.

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Recent research concerning benthic algae in streams suggests that the near-substrate hydrodynamic regime may affect the composition of benthic algal assemblages. Algae from local streams were allowed to colonize laboratory flumes containing substrates of different sizes under two velocity regimes to determine whether benthic algal assemblages were affected by substrate size or hydrodynamics. While all hydrodynamic parameters except for local velocity diverged among flumes with different substrate sizes, cell counts per unit area of substrate and the relative proportions of different forms of algae remained similar overall. Much of the similarity appears to be attributable to the dominant filamentous algae. "Blooms" of one or two species were detected on occasion, but did not affect overall similarity (though they may affect local recolonization). The results, if extrapolated, suggest that hydrodynamic conditions alone may not be responsible for the variation in algal assemblages seen in the field, and that the morphological form of the alga may be more important than the species of alga when determining its response to hydrodynamic conditions.