A paleo-environmental reconstruction of the sedimentation, productivity, hydrology and geochemistry of the Pouto Dune Lakes, Northland

Abstract

The dune lakes on the Pouto Peninsula, Northland, New Zealand are all showing signs of impact through catchment land-use change, with water quality of most lakes in a state of degradation. Most of these lakes are situated in catchments dominated by exotic pine forest and pasture and are observed with high sedimentation and having high nutrient levels with water quality ranging between mesotrophic and eutrophic. Sediment cores from three dune lakes displaying signs of water quality deterioration such as increased algae, were analysed using paleolimnological approaches to broaden our understanding of how shallow dune lakes function in New Zealand with respect to the hydrological, geochemical and biological processes that alter in response to resource and agricultural land-use change. A multi-proxy analysis of the biological productivity, water quality and catchment erosion of Lakes Rotokawau, Humuhumu and Rototuna was carried out to reconstruct the paleoenvironment back to a pre-human state. A reliable chronology was determined from AMS ¹⁴C and ²¹⁰Pb dating of the sediments, with sediment stratigraphy and composition, organic matter and sediment elemental geochemistry and carbon and nitrogen isotope geochemistry used to evaluate changes in sediment sources and lake conditions. The diatom sedimentary record from Lakes Rotokawau and Humuhumu were used to determine the algal productivity and trophic states of the lakes. The sediment and organic matter geochemistry of all three lakes reflect extensive fluctuations in water levels from the mid-Holocene, with lowering of water levels and wind induced re-suspension of sediments a major cause for increased turbid conditions and increased algal productivity. The first sign of land use changes and catchment disturbance is erosion, with sand and nutrient influx increasing in the upper sediments of Lake Rototuna, specifically in the southern basin of the lake which is dominated by pasture with livestock access to the water’s edge. Lakes Humuhumu and Rotokawau display similar sand influx in the upper sediments, first observed before ca. 1929 AD in Lake Humuhumu and after ca. 1968 AD in Lake Rotokawau. The diatom records for both these lakes display decreasing water quality from oligotrophic conditions prior to human impact of the two lakes to mesotrophic conditions after catchment disturbance had occurred.