A record of natural and human-induced environmental change from Lake Horowhenua : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science, School of Agriculture and Natural Environment, Massey University, Palmerston North, New Zealand

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Massey University
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Lake Horowhenua is a hypertrophic turbid lake located in the western coastal plain of the lower North Island of New Zealand. In order to effectively restore or manage modified systems such as Lake Horowhenua, an understanding of past environmental change and natural variability is essential to provide a benchmark for ‘natural’ conditions. Cores from the bed of Lake Horowhenua have been analysed to reconstruct a detailed environmental record for the last c. 4,200 cal yr BP. Prior to lake formation, the area now occupied by the lake was subject to fluvial deposition from the Ohau River sometime prior to 7,500 cal yr BP. Dune transgression began in the region c. 7,700 cal yr BP at the very earliest, and drainage of the small streams and springs was impeded, allowing for the formation of a proto lake. A tidal surge up the Hokio Stream may have occurred c. 7,100 cal yr BP. Clastic delivery into the lake from the inflowing streams was high from 4,200 cal yr BP until sometime around 3,200 cal yr BP. The lake then transitioned into a phase of increased autochthonous sedimentation. During this phase, stable bottom water oxygenation, high water quality, and little catchment disturbance dominated. The pollen record indicates that the pre-human vegetation was a lowland podocarp-hardwood forest, dominated by Dacrydium cupressinum. There is some evidence of vegetation response to long-term climate change associated with increasing intensification of the westerly circulation regime through the mid to late Holocene. Proxy evidence implies human arrival to the Lake Horowhenua area occurred c. 519-486 cal yr BP, and land clearance was initiated on the plains inducing erosion within the catchment. However, sedimentation rates did not change from pre-human levels and water quality remained high. European arrival c. 1840 AD saw a further acceleration of land clearance on the plains and in the uplands, the introduction of exotic plant species for forestry, increased pastoral land for agriculture, and a reduction in water quality. Aquatic weed growth began at 1898 AD and was prolific from 1985 AD. High nutrient levels are evident from c. 1985 AD. Sedimentation rates and erosion increased significantly to 0.34 cm a⁻¹ compared to pre-human (0.08 cm a⁻¹) and Māori settlement (0.07 cm a⁻¹).
Horowhenua, Lake (N.Z.), Horowhenua District (N.Z.), Environmental conditions, History