Effects of deposited sediment on invertebrate drift: an experimental study

The effect of deposited sediment on invertebrate drift was experimentally assessed by adding fine sediment to three of six cobble-lined artificial channels. One invertebrate species, Deleatidium spp., was added to the channel after the addition of sediment, whereas other taxa had colonised the channel naturally before the addition of sediment. Invertebrate drift was collected from each channel after the added sediment had worked its way into the interstitial spaces of the cobbles. Drift was collected every evening and morning for 3 days, after which time benthic densities in the channels were assessed. The percentage of benthos in the drift was significantly higher from sediment-treated channels than from untreated channels for Paracalliope fluviatilis, Oxyethira albiceps, Hydrobiosis sp., and chironomid larvae. Drift of these animals from sediment-treated channels was more than double that from untreated channels, with an average of over 5% of the benthos drifting from the sediment-treated channels per day. Although there was no difference in the percentage of Deleatidium in the benthos that drifted from treated or untreated channels, significantly more individuals drifted from treated channels. Deposition of fine sediment had no significant effect on chironomid emergence or diurnal drift patterns for the taxa in the experiment. Periphyton communities in sediment-treated channels were characterised by reduced chlorophyll a and increased inorganic content. We could not determine whether increased drift caused by deposited sediment reflected decreases in suitable habitat, reductions in food quality of the algae, or both.