Assessing small-scale directional movements of benthic invertebrates in streams by using a multidirectional cage trap

A multidirectional cage trap (MDC-Trap) was developed in order to assess small-scale directional movements of benthic invertebrates in streams. The trap can be operated with causing minimal disturbance to the stream bed and enables repeated sampling of identical patches of stream bottom. The MDC-Trap combines characteristics of net samplers with features of colonization-substrate samplers. It is made out of a perforated plywood box which is divided into nine compartments. Each compartment is loaded with a gravel-filled directional cage that can be colonized from one direction only. The modular design of the MDC-Trap allows assessing of up to nine different types of invertebrate movement simultaneously: upstream, downstream and lateral on the substrate surface as well as subsurface movements upstream, downstream and lateral. If necessary, distinctions can be made between left and right oriented movements. Furthermore, movements vertically upward from within the hyporheos can be recorded. The MDC-Trap was designed to be used in streams with coarse sand, gravel or pebble substrates. The device is constructed so that there is no clogging and, once exposed, the trap can be left in the stream indefinitely. The results of first field tests show that the MDC-Trap is suitable to characterize small-scale movements of invertebrates in gravel streams. Net longitudinal movement was in the downstream direction for most species, although the taxon-specific patterns varied considerably. In particular, Baetis rhodani and Hydropsyche spp. exhibited a clear preference for downstream movement by drifting or crawling and seldom displayed upstream or lateral movement. Preference for downstream movement was less pronounced for chironomids, Eucyclops serrulatus and Gammarus pulex. Other taxa were about equally distributed among the direction-sensitive subunits. No taxon exhibited large-scale upstream movement. These findings are in contrast to the idea of compensatory migrations.