AbstractsGeography &GIS

Lakes change over time in response to climate and catchment disturbance, even in the absence of anthropogenic stressors. This temporal change may impact the setting of reference conditions for lakes. Many sources of information including monitoring data, historical information, modelling and paleolimnology can be used to understand temporal dynamics of lakes. In this thesis, the contribution of catchment disturbance by volcanic eruptions, introduction and proliferation of invasive mammals, and climate variability, to temporal variability in lake reference conditions was studied in a deep lake (Lake Okataina) in the Central North Island in New Zealand that is close to reference state. The lake and its catchment have been subject to anthropogenic and natural biotic and abiotic disturbances during the last millennium. Polynesian settlements were thought to have been established in and around the lake catchment in the early 14th century following the Kaharoa eruption of AD 1314 ± 12. Mt Tarawera erupted 10 June 1886 and damaged large tracts of vegetation in the Okataina catchment. Following European settlements around the catchment in the early 20th century, mammals which were hitherto not present, were liberated into the forest and rainbow trout (Oncorhynchus mykiss) were released into the lake. Large populations of the dama wallaby (Macropus eugenii), wild boar (Sus scrofa), red deer (Cervus elaphus) and the Australian brushtail possum (Trichosurus vulpecula) exist in the catchment. These mammals have become the “new normal” in many New Zealand catchments. To understand the role of volcanic eruptions and invasive mammals in shifting baseline conditions of lakes, two 49 cm cores were retrieved from a central deep location at Lake Okataina. The core was sliced at 1 cm intervals and slices dated using 210Pb, with the tephra from the Kaharoa and Tarawera eruptions used to validate the dates and benchmark events. Dating indicated the length of the core encompassed the period between the Kaharoa eruption of AD 1314 ± 12 and 2009. Various inorganic and organic geochemical proxies in the sediment core slices were analysed to determine: changes in catchment erosion, phosphorus speciation, phytoplankton community assemblage, primary productivity, sources of organic matter loading, redox potential, and internal phosphorus loading to the lake. A suite of statistical techniques, including structural equation modelling (SEM), general linear modelling (GLM), nonmetric multidimensional scaling (NMDS) and correlation analysis was used to interpret vertical changes in the composition of the core slices and relationships amongst analytes. SEM was used to compare the relative importance of climate and catchment disturbance in changing the primary productivity of Lake Okataina after the Tarawera eruption (1886) and European settlement (1900). Changes in historical primary productivity inferred from total organic carbon in sediment core slices were modelled as a function of geochemical proxies for erosion (Al and Ti) and records of… Advisors/Committee Members: Hamilton, David P (advisor), Laughlin, Daniel C (advisor), Hartland, Adam (advisor), Lusk, Christopher H (advisor).