The changing water quality of New Zealand’s lakes in response to anthropogenic activities

Abstract

Humans have interacted with landscapes for millennia. However, degradation of environmental systems now occurs at unprecedented rates damaging ecosystem functioning and biodiversity. Arguably, freshwater ecosystems, particularly lakes, are the most affected. Continuing anthropogenic pressures have made it necessary to manage and protect lakes effectively, as well as understand their long-term structure and function. New Zealand is in unique location for investigating these matters as it was one of the last substantial landmasses settled by humans, approximately 750–800 years ago. The human-settlement history of New Zealand creates an opportunity to examine changing water quality of lakes while comparing the timing of these shifts with changes in land use. Lakes in New Zealand are integral to the country’s freshwater landscape, and many have elevated nutrient concentrations. There is substantial effort to prevent further enrichment, and understanding past water quality could assist with this process. To provide these insights, the sediment records of five lakes were analysed using diatoms as indicators of past water quality. The primary focus was to examine the responses of these lakes to changes in land use following Māori settlement, and later after European-style agriculture commenced. Lake Oporoa, a small shallow lake in the Manawatū-Whanganui region of the North Island was the first study site. This lake has significant cultural values to iwi (Māori “tribe”) Ngāti Hauiti. A highresolution study provided suggestions for rehabilitating this lake to meet Ngāti Hauiti’s cultural aspirations. The diatom assemblages revealed that despite nutrient enrichment from the 1840s, rehabilitating Lake Oporoa using the 1950s as a target, would be sufficient for the cultural wishes of Ngāti Hauiti – this study highlights the importance of working with Indigenous peoples in managing lakes. The second study location was the Canterbury high country on the South Island of New Zealand. Regional and national governments have focused on conservation of the lakes in this region. Originally covered by native forest, the landscape in the area altered following Māori fires and is now dominated by tussock grasslands. Fossil diatom assemblages from four lakes, Lakes Ōtūroto, Ōpōrea, Kirihonuhonu and Emily, with interpretations aided by analysis of modern diatoms samples, were examined to identify significant points of change in water quality. Before human activity in the catchment, all lakes had extremely low nutrients. After first Māori activity in c. 1400 CE, all lakes experienced aquatic transitions with rapid changes in their diatom assemblages to taxa, Discostella stelligera, in particular, indicating cultural nutrient enrichment. All lake records indicated that European colonial activities triggered marked changes in water quality, albeit at differing times and magnitudes in each lake. There was a two-stage change in the three shallow lakes, Lakes Oporoa, Kirihonuhonu and Emily. They first responded rapidly to pastoral activity (c. 1840–50s) but became further enriched from the 1950s. Diatom assemblages are now dominated by species tolerant of nutrient-enriched conditions. By contrast, Lakes Ōtūroto and Ōpōrea, recorded minimal response to colonial activity and declining water quality occurred from approximately 1990 onwards. The sedimentary records of the five lakes, over 1000 years in length, clearly captured the legacy of two waves of anthropogenic activity, in addition to the natural pre-human water quality. These lakes, along with others in New Zealand, are of great socio-ecological value, and management of these sites is needed to prevent a further decline in water quality, and to protect the ecosystem services they provide.