Harmful algal blooms (HABs) can restructure benthic communities in ways that are rapid, uneven, and often poorly captured by fisheries‑centric monitoring. As part of the South Australian Department for Environment and Water’s HAB impact assessment on nearshore marine biodiversity project, we used diver‑based Reef Life Survey (RLS) transects to examine changes in reef‑associated macro‑invertebrate assemblages following an extensive algal bloom event.
Across surveyed reefs, macro‑invertebrate records revealed pronounced shifts in community composition, with clear taxon‑specific responses. Several gastropod groups showed marked declines in abundance and occurrence, consistent with their vulnerability to bloom‑related toxicity and hypoxia. In contrast, hermit crabs appeared to fare comparatively well, may be benefitting from increased availability of empty shells and food resources generated by widespread invertebrate and vertebrate mortality. Other mobile macro‑invertebrates exhibited more muted or variable responses, suggesting differing tolerances or capacity for behavioural avoidance.
Notably, dramatic losses were observed among sessile and encrusting invertebrates such as sponges, ascidians, and bryozoans groups not directly quantified by standard RLS methods, but conspicuously absent from post‑bloom reef structure. Correspondingly, taxa that rely on these organisms as prey or habitat, including firebrick stars, were also largely missing from surveys.
These results highlight the value of diver observations for contextualising RLS data and underscore how HABs can differentially reshape benthic food webs, sometimes disadvantaging obvious losers while quietly creating opportunities for others.