Hydrology and substrates: determinants of oligochaete distribution in lowland streams (the Netherlands)

In most soft-bottomed, lowland streams in the Netherlands discharge regimes largely follow the precipitation pattern. Winter discharges are higher and much more dynamic then summer discharges, although rain storms throughout the year cause unexpected peak flows. Minimal precipitation, reduced stream flow and droughts can occur during the summer months. Lowland stream habitat, particularly in The Netherlands, is hydrologically dynamic, with substrates frequently moved or disturbed. Differences in discharge patterns in Dutch soft-bottomed lowland streams are expected to affect distribution patterns of macroinvertebrates and thus oligochaetes. Ten small to medium sized lowland streams, differing from one another in hydrological regime, were studied. Five major habitats in each stream were assessed on three occasions over a 15-month period. Each habitat sampled with a micro-macrofauna shovel; during each sampling period, several environmental parameters, especially hydrological and substrate parameterswere measured. Ordination (CANOCO) and statistical tests (chi-squared test) were used to determine the major oligochaete distribution patterns between streams as well as between habitats within each stream. Each stream was characterized by its oligochaete assemblage; general distribution patterns and individual oligochaete-substratum relationships were documented. Hydro-morphological parameters together explained the differences in major distribution patterns. Preferences of oligochaetes for specific structural habitats are discussed