Benthic communities of the Whanganui River catchment : the effects of land use and geology : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University

Abstract

The response of macroinvertebrate communities and freshwater mussels to variation associated with land use and geology was investigated in small headwaters of the Whanganui River catchment in the North Island of New Zealand. Conductivity and water clarity were higher in streams with soft tertiary/quaternary sedimentary geology, independent of land use. These soft geology catchments, when forested, had distinctive community structure often involving high relative abundances of Ephemeroptera. Pastoral agriculture resulted in a lower diversity and abundance of pollution sensitive taxa. Impacts of pastoral land use were accentuated by soft sedimentary geology culminating in low diversity and abundance in pasture streams with soft geology. Multivariate analysis showed that community structure varied significantly depending on the combination of land use and geology type present, with land use as the most significant factor. However, variations in geology may mask the effects of land use between catchments. In the main Whanganui River, taxonomic diversity and numbers of pollution sensitive taxa decreased downstream. This was correlated with reduced periphyton biomass and increased suspensoids. Estimates of macroinvertebrate community structure differed between artificial substrate and kick samples collected from the same sites. Shell morphology of the freshwater mussel Hyridella menziesi was not correlated with water chemistry. One site near the northern boundary of the Whanganui River catchment contained mussels with distinct shell morphology. A possible explanation involving stream capture by tectonic movements is considered. Shell erosion was correlated with channel width suggesting shell erosion is greater in larger waterways. Lack of demarcation in shell growth annuli meant accurate estimates of mussel age were not possible. Poor demarcation is likely to result from non seasonal patterns of environmental variation.