Distribution of Insects within Riffles of Streams

Extensive quantitative sampling of aquatic insects and factors known or suspected to affect their local distribution in streams was performed in three consecutive riffles in a hardwater stream of north central Texas, USA. This study revealed a strong upstream-biased distribution pattern of lotic insects within riffles. Twelve of sixteen taxonomic categories analyzed, representing 93.4% of the total numbers, showed a distinct pattern of distribution with greater abundance toward the heads of riffles. Density of insects was statistically correlated with several physical and chemical characteristics measured including quantity of coarse particulate organic matter on the substrate, quantity of fine particulate organic matter in transport, several substrate particle size classes, current, depth, dissolved oxygen and temperature. However, none of these variables were meaningfully correlated with distance from the heads of riffles, and therefore did not sufficiently explain distribution of the insect groups. Positive rheotaxis could have significant influence on distribution of insects in riffles. The observed distribution pattern indicates that filter-feeding riffle insects compete for high quality food items produced in upstream pools, and that density of macrobenthos within these riffles may be limited by the amount of high quality food available to them rather than by space.