Oral Presentation Freshwater Sciences 2023

Does geology influence the effect of land use on stream macroinvertebrate communities? (#405)

Russell Death 1 , Harriet Hunter 2 , Ian Fuller 3
  1. Pohangina Environmental Consulting, Ashhurst, New Zealand
  2. 4Sight Consulting, Gisborne, New Zealand
  3. Innovatve River Solutions, Massey University, Palmerston North, , New Zealand

Although geology is considered to have a critical influence on the effects of land use change on stream communities there have been few studies to quantify this effect.      We investigated the interactive effect of 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 which has some of the highest erosion rates in the world. Macroinvertebrates were sampled each year between 2016-2018 at 80 sites in exotic forest, indigenous forest and pasture streams that drained hard or soft geology. The 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 in the pasture and exotic forest streams was also twice as high as in the indigenous forest streams. Periphyton cover did not differ between land uses but was higher in soft sediment streams. Biological indices indicated that ecological health was higher in indigenous forest sites, intermediate in exotic forest sites and lowest in pasture streams. Over all three land uses, indices were also higher in streams draining hard than soft sediment streams. Boosted regression tree models revealed conductivity, stream slope and habitat quality were among the most influential predictors of index scores. Although there was considerable variability, macroinvertebrate communities were statistically distinct between land uses and geologies. Our results illustrate that there is a complex and strong interaction between geology and land use and the impact on stream macroinvertebrate communities. This needs greater consideration when contemplating alternative land use options. Ecological communities in soft sediment streams are a quarter to a third more sensitive to land use change than similar hard sediment streams.