Major disturbance to aquatic ecosystems in the South Island, New Zealand, following human settlement in the Late Holocene

Abstract

Lake sediment records from three lakes in the South Island of New Zealand were examined to determine the effects of human (Māori and European) impacts on the lake catchments during the Late Holocene. Major changes in lake biota occurred in the Early to Middle Holocene (11,000–6000 cal. yr BP), but there were no major changes between c. 6000 cal. yr BP and the time of human impact. Intensive Māori forest clearance occurred here between c. ad 1200 and 1600, which is consistent with other New Zealand records. Catchment erosion and increased sedimentation probably occurred in all of the studied lakes, but the most obvious changes occurred in Lake Clearwater and the Māori Lakes. There was evidence for gravity-induced slumping of the littoral sediments in Lake Clearwater due to increased sediment loading, and the outflow from the Māori Lakes was blocked by a migrating alluvial fan. The erosion of sediment (and nutrients) from the lake catchments led to eutrophication, but increases in lake depth were just as important in two of the lakes. Increased water depth was caused by damming of the Māori lakes outflow by a migrating alluvial fan. Reduced evapotranspiration following deforestation would also have led to increased water yield in lake catchments. European impacts were minor compared with the impacts of Māori deforestation, and all lakes display different levels of recovery towards pre-human impact conditions. Complete recovery is prevented by permanent changes in catchment hydrology and probable internal feedback mechanism such as wind-induced sediment re-suspension in the larger lakes.