Land‐use legacy and the differential response of stream macroinvertebrates to multiple stressors studied using in situ experimental mesocosms

Abstract

Summary The historical reduction in habitat heterogeneity and water quality of running waters is an ongoing challenge being addressed by resource managers worldwide. The persistent legacy of past land use, reflected in part by depauperate macroinvertebrate assemblages, is likely to affect the responses of macroinvertebrate assemblages to novel anthropogenic stressors and amendments (e.g. river restoration). The response of macroinvertebrate assemblages differing in the abundance of sensitive taxa to stressors was investigated using a mesocosm experiment conducted at five sites within two sub‐basins that differed in historic land‐use impacts. The sub‐basins were similar in morphology and water quality, but a higher dissolved organic carbon (DOC) level in the sub‐basin with a less sensitive assemblage reflected a greater agricultural influence. Macroinvertebrate assemblages were assessed monthly over an annual cycle in replicated in situ mesocosms manipulated with respect to substratum heterogeneity (fine, coarse) and water quality (nutrient amendments) following a full‐factorial design. Due to relatively high background nutrient levels, there was no effect of nutrient amendment on macroinvertebrate colonisation of mesocosms. In the sub‐basin harbouring a more sensitive macroinvertebrate assemblage, higher total and EPT taxa richness was found in coarse (heterogeneous) substratum than that in fine (homogeneous) substratum mesocosms, whereas this response was not observed in the sub‐basin with a less sensitive assemblage. In the former sub‐basin, coarse substratum mesocosms were more favourable for sensitive taxa, as they had more diverse flow patterns, less clogging of interstitial spaces and more benthic organic matter than fine substratum mesocosms. These results suggest that past land‐use legacies influencing current macroinvertebrate assemblages can underlie differential response patterns of macroinvertebrates to habitat mitigation measures in human‐dominated catchments.