How does geology influence the effect of land use on stream macroinvertebrate communities?

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Abstract Both catchment geology and land use surrounding a stream will affect the degree of fine sediment deposition from erosion and its consequent impacts on stream macroinvertebrate communities. While geology and land use have been shown to impact stream communities individually, their interaction has received less attention. We investigated the interaction between geology and land use on stream macroinvertebrate communities in the Tairāwhiti Region, New Zealand. The region has streams that drain harder sandstones and softer shales and mudstones, the latter having some of the highest erosion rates in the world. Macroinvertebrates were sampled each year between 2016 and 2018 at 80 sites in exotic forest, indigenous forest and pasture streams that drained hard or soft geology catchments. Conductivity in pasture and exotic forest streams was nearly twice as high as in indigenous forest streams and higher in soft than hard sediment streams. Deposited sediment cover was greater in soft than hard sediment streams, and greater in pasture and exotic forest streams than in indigenous forest streams. Periphyton cover did not differ between land uses but was higher in soft sediment streams. Ecological health was higher in the indigenous forest sites, intermediate in exotic forest sites and lowest in pasture streams. Biological indices were also higher within each land use in streams draining hard than soft sediment streams. Invertebrate communities appear to be determined primarily by the amount of deposited fine sediment and conductivity, both of which are higher in soft than in hard bottom streams, and in exotic forest and pasture streams. Synthesis and applications: Our results illustrate a complex and strong interaction between geology and land use and the consequent impact on stream macroinvertebrate communities. The combination of soft bottom streams with pasture or exotic forest land use is more detrimental than in similar hard bottom streams. Therefore, regulations for the management of any given land use need to be much stricter in catchments dominated by soft sediment geology.