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

ABSTRACT

Pressure on lakes in Aotearoa New Zealand is increasing because of 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 because <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 3738 lakes in the dataset, 44% were eutrophic or higher trophic state, 22% mesotrophic, and 34% oligotrophic or lower trophic state. These data provide a benchmark to guide management and support the call for more resourcing to restore lakes in Aotearoa New Zealand. To identify lakes to prioritise for protection, we focused on the ∼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. We identified >170 lakes with limited catchment protection and easy human access. Immediate attention should be given to protecting these waterbodies to prevent the need for costly and resource-intensive remediation in the future.

KEYWORDS:

• Aotearoa New Zealand
• environmental DNA
• extreme boosted modelling
• lakes
• national-scale predictions
• trophic level index

Acknowledgements

We thank all members of the Lakes380 team for field assistance (www.lakes380.com/about-the-project/the-team). We thank Sophie Young and Eloise Beattie (Cawthron) for laboratory assistance. We thank Robyn Jones from “Friends of Mangarakau” for helping to sample Lake Mangarakau. We acknowledge the support of New Zealand regional authorities that provided data and permission to use it and assisted with access to the sampling sites: Northland Regional Council, Auckland Council, Waikato Regional Council, Bay of Plenty Regional Council, Hawkes Bay Regional Council, Taranaki District Council, Horizon Regional Council, Greater Wellington Regional Council, Marlborough District Council, Tasman District Council, West Coast Regional Council, Environment Canterbury, Otago Regional Council and Environment Southland. The authors thank iwi and landowners nationwide for their assistance with sampling, accessing sites and guidance throughout this work. The Department of Conservation is acknowledged for assistance with permitting.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by the New Zealand Ministry of Business, Innovation and Employment research programme – Our lakes’ health; past, present, future (Lakes380; C05X1707) and additional support from Cawthron Internal Investment Fund (2015–2021) and the Strategic Science Investment Funding to GNS Science in the framework of the Global Change Through Time research programme.