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

Abstract

1. Lakes are becoming degraded at an accelerating rate owing to human activity, andunderstanding their past ecology is necessary for lake management andrehabilitation. Palaeolimnology provides numerous methods that enable thehistorical state of lakes to be determined. New Zealand provides an ideal settingin which to do this as human modification of the landscape occurred later herethan in most regions of the world (approx. 1300 CE. 2. Lake Oporoa is a shallow lake that is highly significant to the local indigenousM aori community. This study used multiple proxy palaeolimnology to explorehow lake ecology shifted following M aori and European settlement in thecatchment, and how palaeolimnological data can be used to inform lakerehabilitation and conservation measures, alongside the desires of the indigenouscommunity. Sedimentary pollen, diatoms, bacterial communities, and elementaland hyperspectral imaging scanning were used to infer ecological changes in thelake and catchment from pre-human times to present. 3. Following M aori settlement (approx. 1620 CE) there was gradual vegetationchange and a rapid shift in diatom and bacterial assemblages, but not inphytoplankton pigments or sediment geochemistry. An increasing abundance ofdiatom taxa Discostella stelligera and Staurosirella cf. ovata indicates early nutrientenrichment. European pastoralism from approximately 1840 CE resulted infurther deforestation, and all proxies show evidence for enhanced primaryproductivity driven by a combination of nutrient enrichment and changing lakelevels, particularly since the 1960s. This has caused degradation in water qualityand is likely to have contributed to the decline in populations of tuna (eel,Anguilla spp.). 4. Conversations with local M aori, together with the palaeolimnological results,indicate that a culturally acceptable and realistic rehabilitation target for LakeOporoa aligns with ecological conditions in the 1950s. The palaeoecological data provide information to guide catchment and lake revegetation and other methodsof nutrient abatement, with the eventual aim of restoring culturally importanttuna and native fish populations.