A national scale trophic state analysis to prioritize lakes for restoration in Aotearoa New Zealand

Pressures on lakes in Aotearoa New Zealand are increasing due to elevated catchment nutrient loads, establishment of non-native species, and climate change. Current government legislation requires that pressures are managed to avoid eutrophication and degradation of lake health. This approach requires information on the state of lakes at regional and national scales, which is challenging as less than 5% are currently monitored. In this study, we (1) modelled lake trophic status at a national scale using a highly representative dataset and lake characteristics, land-use, and environmental parameters as predictor variables, and (2) identified lakes that should be prioritized for protection to prevent further degradation. Six statistical models were evaluated, with extreme boosting producing the highest predictive power and lowest error. This model indicated that for the 3,738 lakes in the dataset, 44% were eutrophic or of higher trophic state, 22% mesotrophic, and 34% oligotrophic or of lower trophic state. This data provides a benchmark to guide management and supports the call for more resourcing to restore lakes in Aotearoa New Zealand. To identify lakes to prioritise for protection we focused on the approximately 800 mesotrophic lakes. We used (1) the portion of lake catchment not within conservation estate, and (2) road access as proxies for the likelihood of land-use intensification, and the introduction of non-native species, respectively. Over 170 lakes with limited catchment protection and easy human access were identified. Immediate attention should be given to protecting these waterbodies to prevent the need for costly and resource-intensive remediation in the future.