Diatom communities along stream longitudinal gradients

1. Summer diatom communities on artificial substrates were sampled weekly for a month in three first- to sixth-order tributaries of the Kentucky River to determine how community structure varied with stream size. 2. Diatom cell abundances were generally higher in the headwaters. Species diversity increased in a downstream direction in two of the streams, and in an upstream direction in the third. However, diversity in general seemed more closely related to current regimes than to stream size per se, with highest species diversity at intermediate current velocities. 3. Variation in diatom accumulation rates was greater in downstream communities than in the headwater assemblages of two streams, suggesting that downstream communities may experience greater fluctuations in abundance, at least under low-flow conditions. 4. Patterns of species distributions suggested a relationship between morphological growth forms (guilds) and stream size, as well as the influence of current. Achnanthes spp., Eunotia spp., erect, and stalked taxa were more commonly associated with headwater assemblages. Filamentous and centric diatoms occurred with greater frequency downstream.