Methods
Toxic algae sentinel site framework
A 2-tier sentinel site monitoring framework (implemented December 2018) targets monitoring efforts at known ‘sentinel’ sites, allowing for flexibility to increase surveillance and responsiveness in responding to current health risks.
Tier-1 Sentinel Sites
Classed as sites where toxic algae has been known to historically and predictively bloom under optimal weather and river flow conditions.
Monitoring of Tier 1 sites occurs if there had been an accrual period of seven days. An accrual period is defined as the number of days between a freshwater flushing flow (i.e. 6x median flow). At this flushing flow, toxic algae mats are generally washed away and the potential health risk due to the algae is negligible.
Tier-2 Sentinel Sites
Known to bloom when coverages at Tier-1 Sites reaches a critical threshold level of (i.e. >15%, defined for the Wellington Region only).
Monitoring of Tier 2 sites occurs if a toxic algae coverage greater than 15% is observed at a Tier-1 Sites. Tier-2 site monitoring is restricted to sites within the same Whaitua where a threshold exceedance was observed. This benchmark of 15% is regionally specific for Greater Wellington, based on analysis of historical toxic algae coverage across sites in relation to the probability of heightened risk alerts.
Action guidelines for the Wellington Region.
Greater Wellington Regional Council has adopted a more conservative action level guideline of 20%, driven by the rapid growth rates of toxic algae experienced in the region and the subsequent need to manage risks to the public. This approach is consistent with the procedures set out in Brasell and Conwell 2018.
In addition to the routine transect assessment as set out in the Interim Guidelines (see Benchmarking and Resources for further details about the Interim Guidelines), a bankside assessment was introduced to assess non-wadeable sections of a river site, and where the assessment from a transect method alone was insufficient to assess percentage toxic algae cover.
