Harmful Algal Bloom Field Study: Lake Waikare 2024
January 2025
Harmful algal blooms (HABs) are increasing across many locations globally. Cyanobacterial HABs in freshwater systems cause many environmental issues, and because many species produce natural toxins, known as cyanotoxins, they are also a risk to human health. Aerosols from lakes with cyanobacterial HABs could pose potentially serious health risks to people. However, the health risk posed by toxin aerosolisation, and the environmental factors that promote aerosolisation, are unclear.
This field study was a multidisciplinary initiative between air quality experts (Emission Impossible Ltd and Mote) and marine and freshwater ecosystem experts (Cawthron Institute).
The objectives of the study were two-fold; to investigate the environmental drivers that may influence aerosol production during a cyanobacterial bloom, and to measure cyanotoxins in aerosols downwind of a bloom.
A recent review in the Lancet (Lim et al., 2023) noted":
“The public health burden of HAB aerosols is likely to be considerable. One study estimated that approximately 15% of global asthma cases are attributable to the inhalation of aerosolized HAB toxins in coastal regions.”
JP (Cawthron Institute) happy with his scum sample.
Lake Waikare is the largest lake in the lower Waikato catchment, with 3,442 hectares of open water. A history of extensive land clearance within the catchment has caused increased sediment and nutrient loading in Lake Waikare. Combined with the decision to lower the lake for flood control purposes, Lake Waikare has become hypertrophic with very poor water quality.
Lake Waikare water sampling locations
What did the study find?
Our research into the aerosolisation of toxins from a cyanobacterial bloom measured in Lake Waikare did not detect any toxins in air samples collected daily for a week at 50 m, 100 m, 150 m and 200 m downwind. This was unexpected because detection limits were similar, or better, than other studies that had concurrently measured toxins in aerosols and in lake water samples.
If extracellular microcystins are more prone to aerosolisation than cellular / particulate microcystins, then the low levels of extracellular microcystins observed in Lake Waikare (max 0.8 µg/L) may have contributed to a lack of aerosolised microcystins during the current study. Alternatively, if the primary mechanism for aerosolisation is wind-driven wave action, it is hypothesised that sustained winds above a moderate breeze (> 3.3 m/s) are a requisite for aerosolisation of cyanobacteria to occur.
Full report located here.
The presence of cyanobacteria at levels exceeding current and revised Recreational Cyanobacteria Guidelines is consistent with a history of poor water quality in Lake Waikare. The non-detection of microcystins in aerosols collected downwind of the lake during a cyanobacterial bloom, may augur well for potential public exposure to aerosolised HABs. However, the physical and chemical mechanisms for aerosolisation and transport of toxins from HABs are complex and would require additional research before firm conclusions can be drawn.
Such hypotheses would not be applicable to HABs in the marine environment, for which wave break is more generally a persistent feature.
Paul Baynham (Mote) commissioning a met station and aerosol sampler in a downpour