Most lakes in the Manawatū-Whanganui region have been affected to some degree by changes in water quality and through the introduction of invasive aquatic plants and animals, or by the clearance of indigenous vegetation and draining of wetlands. Early monitoring of the region’s lakes focussed on identifying the submerged plant species that were present. The aim was to prevent the spread of introduced aquatic plants between lakes, primarily through public education.
Lakes in the Manawatū-Whanganui region are unique and incredibly diverse, making them complex systems to understand. They are also vulnerable to the pressures of land use, water use, recreation and a changing climate. Over time these pressures have resulted in degraded water quality and the spread of invasive weeds in some of our lakes. This is particularly notable in our dune lakes, where around 68 per cent of the surrounding catchments have been converted to pastoral agriculture. In some cases, intervention is now necessary to remediate degraded lakes, and ensure those lakes in good health are managed and maintained.
Because every lake behaves in a slightly different way, no one solution will be suitable for every lake. Effective management requires sound understanding of ecological risks and cultural sensitivities, as well as logistical and financial challenges. Horizons Regional Council is actively working with local iwi, landowners, territorial authorities and lake experts from around New Zealand to improve our understanding of our region’s lakes, inform current management, and support future decision making.
During the development of the One Plan, Horizons’ focus shifted to further understanding the current state of health of the region’s lakes. At that time, the health of some of the region’s lakes, such as Lake Horowhenua, was known to be poor; however, water quality data was limited and very little was known about other lakes in the region.
Monitoring of the water quality of Lake Horowhenua was reinstated in 2013 with the installation of a permanent monitoring buoy. In 2014, Horizons commenced water quality monitoring by boat at three additional coastal dune lakes (Dudding, Pauri and Wiritoa) on a quarterly basis (monthly since 2019). By 2015 another 11 coastal dune lakes were added to the water quality monitoring programme on a quarterly basis by helicopter. These coastal dune lakes were initially selected due to their identification as target catchments for nutrient management, as set out in Horizons Regional Council’s One Plan. Two of these dune lakes (Dudding and Wiritoa) and one riverine/oxbow lake (Hokowhitu Lagoon) are monitored for public contact recreation on a weekly basis from 1 November through to 30 April for the presence of bacteria (measured as E. coli) and cyanobacteria (blue-green algae).
Since 2015, Horizons has contracted NIWA to implement a regular monitoring programme for the monitoring of lake macrophytes, known as LakeSPI (Lake Submerged Plant Indicators). LakeSPI is a bioassessment method that uses the degree of development by native submerged plants and level of impact by non-native invasive weeds to indicate an ecological condition. This monitoring provides information on the spread of introduced macrophytes and also provides a measure of ecological condition of the lakes monitored. Increased sediment and nutrient loading from catchment activities and displacement of native vegetation by invasive alien plant species are major influencers of lake ecology and condition. LakeSPI provides an effective means of assessing these impacts and compliments traditional water quality monitoring, such as the Trophic Level Index method. 45 lakes in the Manawatū-Whanganui region have been assessed or reassessed, and the majority of these lakes have been recorded as having at least one introduced macrophyte species present.