Invertebrate drift during in-stream gravel extraction in the River Baca , Slovenia

Abstract

Spatial and temporal variation in invertebrate drift during in-stream gravel extraction in a gravel-bed river in south western Slovenia (SE Europe) was investigated. Drift samples were collected at three sites (an upstream control site and two downstream impacted sites) on the first and third days of gravel extraction. Drift measured during gravel extraction was compared to the hyporheic invertebrate community at the site of gravel extraction. Significant differences in drift were observed between the sites (ANOVA, n = 18, F = 23.29, p <0.001 for the number of individuals h -1 , and F = 13.15, p <0.001 for the number of taxa h -1 ). Two way analysis of ANOVA revealed the significance of site (p < 0.01) for number of individuals h -1 , and of site (p <0.001) and site * date interaction (p < 0.001) for taxa number h -1 . Insect larvae dominated drift at the control site, while crustaceans dominated the drift on the third day at both impacted sites. PCA of drift and hyporheic samples explained 37.8 % of the variance in data by the first two ordination axes. Drift composition was most similar to the shallow hyporheos on the third day of excavation. The study demonstrates that gravel extraction induces drifting of meiofauna which normally do not occur in the drift and that mass drift can occur up to 200 m from the site of excavation. Drift rates were much higher (approximately 30 fold) than the rates observed during increased discharge (from 2-5 fold).