Assessing anthropogenic climate change and land use impacts on lake ecosystems in Hawke’s Bay, Aotearoa-New Zealand

Lakes across Aotearoa-New Zealand carry strong cultural, ecological, and economic value but are currently threatened by anthropogenic climate change and land-use pressures (MfE, 2021). Anthropogenic climate change is having rapid effects on lacustrine environment, with temperatures expected to increase 1-2ᵒC over the next century (MFE 2020, ). The last 1,000 years of climate history has however experienced weather conditions similar to those predicted during the Medieval Climate Anomaly, (MCA) which may provide a useful analogue of ecosystem structures. Aotearoa-New Zealand also has a unique history of land use due to the relatively recent migration of humans experiencing three major land-use changes: Pre-human impact environment (≤1250 CE), subsistence agriculture (~1250-1820 CE) and intensification of agriculture (~≥1820 CE). With these distinct phases it allows for the identification of how ecosystems adapt to varying degrees of catchment pressures.

This study focuses on Kaweka Lake in the Hawke’s Bay region of the north island, Aotearoa-New Zealand. The two aims of this thesis are to identify evidence of the anthropogenic climate change within an lake of Aotearoa-New Zealand, how ecosystems change in response to previous climate shifts, with providing possible analogues for predicted future conditions, as well as catchment pressures on biodiversity. These aims were achieved by using a multi-proxy approach of the pre- and post-human environment of Lake Kaweka utilizing paleolimnological and environmental reconstruction methods of using fossilised chironomid head capsules, palynology, eDNA and hyperspectral scanning.

The pre-human environment was characterised by a closed catchment of tall trees with an ecosystem which was. the steadiest observed, producing an environment with naturally high levels of nutrients and algae growth, likely driven by high rates of natural erosion of the region. The beginning of subsistence agriculture is identified by the introduction of charcoal in the record and rapid decline of tall trees, which is replaced by shrubs and grasses, particularly Pteridium esculentum. There is also a change of the primary production within the lake to be primarily dominated by macrophytes as indicated by the increase abundance of Naonella kimihia. (Boothroyd, 2012). The intensification era is defined by the decline of indigenous fauna and introduction of exotics, specifically Pinus and where biodiversity experiences the most instability with a transition to more environmentally tolerant species. Temperature reconstruction shows evidence of anthropogenic climate change as well as the MWA, which may provide an analogue for an ecosystem which may be more resilient to future warming under anthropogenic climate change. However, this may be affected by the culminated effects of historic and current land use practices.