Reconstructing Ecological Change, Catchment Disturbance, and Anthropogenic Impact over the last 3000 years at Lake Pounui, Wairarapa, New Zealand
Increased eutrophication of freshwater lakes has been attributed to an intensification in agriculture, with global warming projected to further compound the problem. Determining pre-human conditions of a lake has major conservation implications for water quality management, such as setting evidence-based restoration targets. New Zealand’s historical monitoring records of lake water quality are short and typically only begin after the onset of deterioration. Paleolimnology offers a complementary means of evaluating historical trends that predate human impact. This thesis investigates the declining water quality of Lake Pounui. Lake Pounui possesses high ecological integrity, though the lake is experiencing an increased frequency of severe algal blooms. The primary aim of this thesis is to reconstruct the past environment of Lake Pounui using paleolimnological methods to extend the historical monitoring data beyond human arrival. The reconstruction is used to address whether algal blooms are a natural feature of the lake, and examine anthropogenic impact. This study then attempts to identify reference conditions and critical transitions within the lake environment. Using this information possible targets for lake health restoration are discussed. Based on elevated charcoal influx, palynological evidence, and catchment disturbance indicators, such as organic content (Loss-on-Ignition (LOI)), grain size, and micro-X-ray fluorescence (µ-XRF), Māori land clearance was identified at ~450-350 calendar years before present (cal yr BP) (95% confidence interval (CI): 515.2-202.3 cal yr BP). The appearance of Pinus pollen and the diatom Asterionella formosa placed European arrival at ~150 cal yr BP (95% CI: 243-39 cal yr BP). Pre- and post-human environments of Lake Pounui were characterised using diatom analysis, bacterial DNA analysis, and supporting evidence from µ-XRF data. It appears that the lake existed in both a higher nutrient (3000-2100 cal yr BP, 95% CI: 3210-1977 cal yr BP) and lower nutrient (1600-450 cal yr BP, 95% CI: 1737- 389 cal yr BP) state, separated by a period of natural disturbance, which could relate to a combination of earthquakes and increased storminess. During Māori occupation (450-150 cal yr BP, 95% CI: 515-57 cal yr BP) the quality of the lake remained relatively high; however from 150 cal yr BP, the lake appears to become more nutrient enriched, and the cyanobacteria responsible for today’s blooms (Dolichospermum and Phormidium) become abundant. Paleolimnological analysis identified that the decline in water quality seen at Lake Pounui is a trend that has occurred over the last 200 years. Dissimilarity and critical transition analysis support this finding and suggest that rapid decline began just prior to 1950 AD. Based on dissimilarity analysis, the period of Māori occupation provides the most realistic restoration target. Planting native vegetation, ceasing stock grazing, and removing perch should be investigated to control the phytoplankton biomass, at the owner’s discretion.