Assessing impacts of harmful algal bloom events on South Australian benthic habitats

The extent and severity of benthic habitat impacts from the HAB is unknown due to access challenges and the bloom’s rapid development. Commonwealth and state agencies identified a need for repeated benthic habitat surveys to provide robust pre and post-bloom comparisons and assist in the development of HAB-related coastal habitat monitoring programs.

This project is integrating benthic habitat imagery, fish assemblage surveys and environmental data, to deliver the first comprehensive assessment of HAB impacts on benthic ecosystems in South Australia.

The assessment will contribute to the SA HAB Science Program and provide state and Commonwealth government agencies with an evidence base for prioritising management actions including monitoring and future research. It will also generate an enhanced national HAB impact assessment capability.

Approach

The project team will analyse data collected in pre and post-bloom towed camera and baited underwater stereo-video surveys to assess and quantify HAB impacts.

Towed camera

Towed camera surveys conducted from the MRV Ngerin have repeated a subset of 500-metre deep-water habitat transects in Gulf St Vincent and Spencer Gulf (100 of 294 original sites). This survey method provides data on substratum types, abundance and diversity of habitats and mobile species. The resurvey, in Spring 2025, has provided a visual archive allowing HAB-related impacts on habitat condition to be quantified.

The environmental sampling will help explain the observed impacts. For example, a key concern for benthic organisms is that Karenia species are hypothesised to reduce oxygen saturation.

Baited remote underwater stereo-video

Healthy benthic habitats support secondary biomass, which in turn supports marine food webs and commercial and recreational fishing. It is therefore necessary to assess the abundance and diversity of fish and larger invertebrate communities to understand the HAB impacts across multiple trophic (feeding) groups.

Multiple stereo-BRUV arrays and benthic trawls were completed in 2023 to early 2025 within replicate seagrass and sand habitats in northern Gulf St Vincent, an area of high Karenia concentration. A repeat survey will provide an indication of the post-HAB mortality of species that show high site fidelity/habitat association (via decreases in relative abundance and richness). It will also provide information on any patterns of immigration/emigration of mobile species within these habitat types.

The surveys and data assessment will generate:

• open-access post-HAB benthic habitat imagery and metadata;
• qualitative visual assessments of habitat condition;
• quantitative before/after impact assessments for benthic habitats and fish communities;
• peer-reviewed publications and technical reports for the SA HAB Science Program; and
• recommendations for monitoring and management.

Expected outcomes

Comparable pre and post-bloom survey data and assessments to assist the development of HAB-related coastal habitat monitoring programs will be provided to research users including the:

• South Australian Department of Environment and Water;
• South Australian Department of Primary Industries and Regions;
• South Australian Algal Bloom Taskforce; and
• Commonwealth Department of Climate Change, Energy, the Environment and Water.

Project location

Gulf St Vincent and Spencer Gulf, SA