Relative importance of surface and subsurface movement on benthic community recovery in the Makaretu River, North Island, New Zealand

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Colonisation of macroinvertebrates following changes to the stream environment occurs rapidly through drift, oviposition, lateral movement, and vertical migration. To assess the influence of different sources of colonists on benthic macroinvertebrate community composition, a colonisation experiment using artificial substrate baskets was conducted in the braided Makaretu River in the southern Hawke's Bay (North Island, New Zealand). Both the numbers of taxa and individuals rapidly increased through time in baskets only allowing drifting and surface crawling invertebrates (surface), baskets only allowing invertebrates migrating from the hyporheic zone (hyporheic), and baskets that allowed both (controls). The numbers of taxa and density of individuals indicated that the artificial substrate baskets were similar to the benthos after 14 and 28 days, respectively. However, overall benthic community composition was not similar to the surrounding benthos until Day 42 in all three basket treatments. The colonisation rates identified from artificial baskets in the Makaretu River are consistent with other colonisation experiments undertaken in New Zealand and overseas. Elmidae (Coleoptera) was most abundant within surface treatments until Day 28 after which Deleatidium spp. (Ephemeroptera) became dominant. The abundance of Elmidae and Deleatidium in the control treatments was consistent with colonisation patterns in the surface baskets. Deleatidium dominated baskets allowing access from the hyporheic zone throughout the experiment. It appears that Elmidae colonise quickly in greater numbers by drifting and/or crawling and Deleatidium colonises more slowly from the hyporheic zone, eventually replacing Elmidae as the most common animal. This indicates that the hyporheic zone provides a source of invertebrate colonists later during community recovery, whereas drifting and/or crawling provides colonists early during recovery.