Detection of harmful algal species in a survey of eight near pristine fjords in Fiordland Marine Area, Aotearoa-New Zealand

Marine harmful algal blooms (HABs) appear to be increasing in their prevalence and severity globally, and this is commonly associated with anthropogenic impacts and global climate changes. There is often uncertainty as to whether bloom-forming species are new arrivals, or if they have always been present and are responding to environmental changes. Surveys of HABs in pristine systems are not rarely undertaken because of resources associated with carrying out surveys in areas with no apparent issues. Fiordland, in southwest of Aotearoa-New Zealand, hosts 14 fjords distinguished by steep walls, deep basins, and freshwater inputs from pristine rainforest catchments. We extracted environmental DNA (eDNA) from 91 surface sediment samples collected across eight fjords to investigate microalgal community composition and identify HAB taxa. The samples were analysed using 28S ribosomal RNA metabarcoding. Multivariate analyses revealed distinct communities among fjords. A total of 17 HAB taxa were detected, ranging from known toxin producers (e.g., Alexandrium ostenfeldii and Amphidoma languida ) to fish-killing genera (e.g., Chaetoceros spp.). Diatoms (particularly Skeletonema spp.) were dominant in the single fjord with a significant volume of freshwater input from a hydropower station. Apart from this observation, there was no relationship between human impact in the catchment and HAB taxa presence or abundance, with near-pristine fjords containing multiple potentially harmful taxa. These findings confirm that HAB species can naturally exist in remote, near pristine marine habitats and highlights the need for ongoing monitoring of Fiordland’s marine ecosystems, as they could accumulate and/or impact many seafood species thriving in these pristine waters and increase food safety risks .