Abstract:
The dune lakes on the Poutō Peninsula, Northland, New Zealand are all showing signs of impact due to anthropogenic catchment land-use change, with the water quality of most lakes in a state of degradation. Most of these lakes are situated in catchments dominated by exotic pine forest or pasture and are observed to have high sedimentation rates and nutrient levels with water quality ranging from mesotrophic to supertrophic. Eight sediment cores from four dune lakes displaying signs of water quality deterioration were examined using paleolimnological approaches to broaden the understanding of how shallow dune lakes function in New Zealand with respect to sedimentation, hydrological and geochemical processes that alter in response to anthropogenic land-use change. A multi-proxy analysis of the catchment erosion, water quality and biological productivity of Lakes Kahuparere, Mokeno, Roto-otuauru and Waingata was undertaken to reconstruct the paleoenvironment to a pre-human state. A reliable chronology was determined from AMS¹⁴C and ²¹⁰Pb dating of the sediments, with sediment stratigraphy, composition, index properties and elemental geochemistry used to examine changes in the sediment sources and lake conditions. The sedimentary indices and geochemistry of all four lakes reflect high levels of variability since the early to mid Holocene, with periods of fluctuating water levels, catchment erosion, biological productivity present in the pre-human lake condition. However, significant changes in the lake condition since European settlement has occurred. The first sign of land-use change is catchment disturbance through erosion causing large influxes of sand and nutrient loading, as demonstrated in the upper sediments of Lake Kahuparere (ca. AD 1943 to present), Mokeno, Roto-otuauru (ca. AD 1888 to present) and Waingata. Lakes Kahuparere, Mokeno and Roto-otuauru all show signs of lake condition change due to the cultivation of exotic Pinus on the Poutō Peninsula (ca. AD 1968) as demonstrated by the increase in sand influxes after commencement while nutrient loading is predominately present in pasture-dominate catchments such as Lakes Roto-otuauru and Waingata.
