The Bio-Physical and ecological response of Lake Okataina, Bay of Plenty, New Zealand following the natural disturbance of the 1886 eruption of Mt Tarawera

The health of New Zealand’s lakes is central to the environmental, social and cultural wellbeing of the country and its people. However, less than 5% of New Zealand lakes are monitored, and of those that are, monitoring records are often short in duration and only begin once a lake has already begun to deteriorate. This makes it challenging to determine the natural or undisturbed condition of a lake. Paleolimnological reconstructions from sediment cores have the potential to allow the development of high-resolution time series records that can be used to enhance understanding of how and why lakes and the land around them have changed over many centuries. While the impacts of press disturbances, such as land-use change, have been well studied using paleolimnological approaches, there are only a few studies on the effects of natural pulse disturbances, such as earthquakes, storms and volcanic eruptions and most have focused on a single proxy or only explored recovery over a short time period. This study uses a paleoecological approach to investigate how geochemical proxies and biological communities of Lake Okataina (Bay of Plenty) responded to a significant natural perturbation – the 1886 Mt Tarawera eruption. Lake Okataina is a deep (max. 70 m), oligotrophic, crater lake. Unlike most other lakes in this region, its 69.2 km2 catchment is largely vegetated by native podocarp forest (approximately 80%). This makes it an ideal location to study the impact and recovery of a lake to a pulse disturbance in the absence of other major land-use effects. A 127-cm sediment core was collected from the lake and was analysed using a multi-proxy approach that incorporated palynology, hyperspectral and ITRAX imaging, DNA bacterial metabarcoding and the morphological analysis of chironomids. Prior to the eruption, primary productivity was relatively high, geochemical proxies revealed an oxygen-rich environment and the bacterial and chironomid communities were stable and settled. Post-eruption the palynological data illustrated a major decrease in the abundance of species of green algae (Boytrococcus) and an increase in the abundance of tree ferns. The ITRAX data highlighted a series of major geochemical fluxes to the system following the eruption, including the addition of new soluble elements such as Ca, K and S as well as increases in elemental ratios such as Fe/Mn and Ti/Mn, suggesting the water column become relatively depleted in oxygen. The hyperspectral data outlined a significant decrease in primary productivity post eruption and the community composition of bacteria also shifted considerably. Chironomid species switched from a Paucispinigera-dominated community to a more diverse one, where species of Chironomus, a lowoxygen tolerant taxon, also thrived. The state of Lake Okataina following the initial disturbance appears to be returning to something similar to its pre-eruption state; however, many of the ecological communities (particularly bacteria) remain different. Collectively these data show that the eruption initiated a major regime shift in the system of Lake Okataina across multiple different trophic levels. Geochemical characteristics within the water column shifted dramatically and indicated an almost instantaneous switch from an oxygen rich, aerobic environment to a more anoxic state. Biological communities also shifted considerably to adapt to the new abiotic conditions they faced. This study highlights how pristine lakes respond to pulse disturbances and how when a system is perturbed, it can trigger a regime shift to a new state. Further research should look to expand the suite of paleoecological evidence (from Lake Okataina) to add additional understanding of the changes identified in this study. Studies on a wider range of lakes in the region would assist in ascertaining if the patterns observed in Lake Okataina also occur in other lakes which experienced varying degrees of impact from the 1886 Mt Tarawera eruption.