ABSTRACT The larval chironomid community of the bed sediment surface and the hyporheic interstitial was examined in a gravel stream between September 1984 through August 1985 and between March and June 1993.

Spatial and temporal species turnover between horizontally adjacent sampling sites fluctuated distinctly in all sediment depth layers. The species composition showed a significantly lower spatial turnover in the upper 10 cm of the bed sediments than in deeper layers (P<0.05). Moreover, species abundance patterns of a five-species assemblage implied random assortment (sensu Tokeshi, 1990) and indicated a high probability for a species assembly to be dictated by environmental stochasticity. Neutral models were developed to evaluate the significance of observed overlap values in spatial distribution amongst abundant chironomid species (sensu Schmid, 1993). The spatial organization of a larval chironomid assemblage in the gravel stream Oberer Seebach seemed to be governed by coexistence due to random colonization processes, which reduce the probability of strong competitive interactions. Larval species colonization into open microhabitat patches of interstitial space (47.8 cm3) was rapid with representatives of a species assemblage arriving within less then 24 hours of the start of the colonization experiment. Moreover, possible random movement across and between sediment depth layers of larval chironomid species may promote a rapid colonization in this gravel stream.