Using palaeolimnology to guide rehabilitation of a culturally significant lake in New Zealand

Abstract

Lakes are becoming degraded at an accelerating rate owing to human activity, and understanding their past ecology is necessary for lake management and rehabilitation. Palaeolimnology provides numerous methods that enable the historical state of lakes to be determined. New Zealand provides an ideal setting in which to do this as human modification of the landscape occurred later here than in most regions of the world (approx. 1300 CE). Lake Oporoa is a shallow lake that is highly significant to the local indigenous Māori community. This study used multiple proxy palaeolimnology to explore how lake ecology shifted following Māori and European settlement in the catchment, and how palaeolimnological data can be used to inform lake rehabilitation and conservation measures, alongside the desires of the indigenous community. Sedimentary pollen, diatoms, bacterial communities, and elemental and hyperspectral imaging scanning were used to infer ecological changes in the lake and catchment from pre‐human times to present. Following Māori settlement (approx. 1620 CE) there was gradual vegetation change and a rapid shift in diatom and bacterial assemblages, but not in phytoplankton pigments or sediment geochemistry. An increasing abundance of diatom taxa Discostella stelligera and Staurosirella cf. ovata indicates early nutrient enrichment. European pastoralism from approximately 1840 CE resulted in further deforestation, and all proxies show evidence for enhanced primary productivity driven by a combination of nutrient enrichment and changing lake levels, particularly since the 1960s. This has caused degradation in water quality and is likely to have contributed to the decline in populations of tuna (eel, Anguilla spp.). Conversations with local Māori, together with the palaeolimnological results, indicate that a culturally acceptable and realistic rehabilitation target for Lake Oporoa aligns with ecological conditions in the 1950s. The palaeoecological data provide information to guide catchment and lake revegetation and other methods of nutrient abatement, with the eventual aim of restoring culturally important tuna and native fish populations.

Resolving 500 Years of Anthropogenic Impacts in a Mesotrophic Lake: Nutrients Outweigh Other Drivers of Lake Change

“…The extracts were dried under a stream of nitrogen gas at 40 °C and stored at −20 °C and protected from light between extraction and analysis (several days). On the day of analysis, the dried extract was resuspended in acetone (0.5 mL) and transferred to a septum-capped amber vial for chlorophyll- a analysis by high-performance liquid chromatography (HPLC) with diode array detection using the methodology described in Short et al…”

“…The core was split and subsampled at Te Pu ̅ Ao/GNS Science in Wellington, New Zealand. One half of the core was subsampled for chronological, pollen, diatom, pigment, and DNA analyses following the methods described by Short et al 33 The other half of the core was used for X-ray fluorescence analysis.…”

“…The extracts were dried under a stream of nitrogen gas at 40 °C and stored at −20 °C and protected from light between extraction and analysis (several days). On the day of analysis, the dried extract was resuspended in acetone (0.5 mL) and transferred to a septum-capped amber vial for chlorophyll-a analysis by highperformance liquid chromatography (HPLC) with diode array detection using the methodology described in Short et al 33 To test for the potential degradation of chlorophyll-a in the sediment core samples, 48 the concentration of chlorophyll-a was assessed against the total concentration of chlorophyll-a derivatives determined by UV/vis spectrophotometry measured immediately after HPLC analysis by Pearson's correlation.…”

An integrative approach to assess the impact of disturbance on native fish in lakes

“…and traditional food-gathering practices complement each other to provide an enriched understanding of past fish populations and drivers of decline. Within a paleolimnological reconstruction of environmental change in Lake Oporoa (New Zealand), Short et al (2022) incorporated indigenous oral histories describing changes in tuna (freshwater eel) harvesting in response to lake degradation including lowered lake levels and reduced outlet flow, with the goal to inform culturally relevant rehabilitation goals for the lake and native fish.…”

Reconstructing the response of native fish in lakes to historical anthropogenic disturbances

“…and traditional food-gathering practices complement each other to provide an enriched understanding of past fish populations and drivers of decline. Within a paleolimnological reconstruction of environmental change in Lake Oporoa (Aotearoa New Zealand), Short et al (2022) incorporated indigenous oral histories describing changes in tuna (freshwater eel)…”