Understanding Causes of Water Quality Decline in an Aotearoa New Zealand Ecosanctuary through Paleoenvironmental Analysis
Authors:   , Greenaway Katherine 
Published: July 2024

<p dir="ltr"><b>Lake eutrophication, specifically as a result of anthropogenic activity and industrial land management, is a significant global and local issue, impacting water quality in lakes worldwide. In Aotearoa New Zealand, eutrophication is widely acknowledged and increasingly being recognised as a critical issue. Short monitoring records, which typically only begin after a lake has started to deteriorate, prevent us from understanding the triggers of eutrophication and therefore requirements for lake restoration. Employing paleolimnological techniques allows for the reconstruction of high-resolution time-series data and is the only approach permitting extensive investigation and analysis of lake and catchment ecosystem change where historical data or oral histories are unavailable. This thesis focuses on Rotokare, a eutrophic lake in Te-Ika-a-Māui/the North Island located within a stable, forested catchment, and uses paleolimnological techniques with particular consideration of how changing landuse has impacted water quality. This thesis aimed to establish if Rotokare has always existed in a highly eutrophic state, or if not, if poor water quality was triggered by changing land management or an alternative catchment disturbance. Reconstruction was achieved via a multi-proxy approach, incorporating hyperspectral imaging, cladocera sampling, micro-XRF scanning, eDNA analysis, geochemistry, palynology, and temperature modelling.</b></p><p dir="ltr">Water quality at Rotokare experienced pronounced changes around ~1970 CE, transitioning from a stable lake characterised by low productivity and biomass to a system dominated by cyanobacteria blooms, high productivity and biomass, and changing in-lake communities. The timing of this transition was unusual as it did not correlate well with human activity in the Taranaki region. The pre-human environment was dominated by mature forest, with vegetation communities undergoing slight changes following Māori arrival at ~1400 CE, demonstrating some subsistence horticulture in the catchment. Following European arrival (~1880 CE), vegetation communities underwent further modification, with some reduction in sub-canopy tree populations. Similarly, minimal evidence of intensive farming (post-1950 CE) was found in the lake sediment. This suggests that, unlike many lakes in Aotearoa, the onset of poor water quality at Rotokare was triggered by changes in the catchment unrelated to changes in land management. Both fertiliser (superphosphate) application and sudden sediment influxes (e.g., following storm events) are unlikely triggers for observed poor water quality. The introduction of Eurasian perch in 1900 CE is the probable trigger for poor water quality, with the fish disturbing in-lake communities and increasing turbidity. However, an observed rise in temperature from ~1950 CE onwards is likely entrenching poor water quality and contributes to the severity of algal blooms. This research demonstrates that water quality degradation can occur even in lakes with relatively pristine catchments, demonstrating the impact of the introduction of new species to ecosystems.</p>