The influence of abiotic factors and density-dependent mechanisms on between-year variations in a stream invertebrate community

Samples of invertebrates were taken in two seasons (October and April) over 5 years in a trout stream in Belgium. Diversity, total benthic invertebrate density, and densities of 27 of 34 taxa tested showed significant differences between years. Significant seasonal effects and interactions were observed in 18 and 7 taxa respectively. The most common pattern of between-year variation was related to hydrological events (flooding) which caused low densities for many species. Most species recovered rapidly, but the recovery of some predators seemed to show a delay of one year. The amphipod Gammarus fossarum appeared to respond positively to flooding as well as to certain post-flood conditions. A significant positive correlation between mortality and the logarithm of initial density was obtained for 6 of 11 species tested. The relationships between mortality and abiotic factors such as water temperature, mean and maximum water level, were very poor. A hierarchy of environmental factors in the regulation of stream macroinvertebrate populations is suggested with: (1) exceptional abiotic events acting in a density-independent manner; and (2) biotic factors that may account for density-dependent regulation.