The large bloom of Karenia cristata in South Australia in 2025/26 has caused widespread ecological impacts, yet the drivers and dynamics of realised toxicity remain poorly resolved. Here I present ecotoxicological observations from field-collected samples across the bloom and laboratory cultures indicating toxicity is largely waterborne and does not require direct contact with dinoflagellate cells. Rapid and strong toxic effects were observed across multiple taxa, including rotifers, mussels and macroalgae, with rapid tissue degradation in bryozoans and vacuolisation in microalgae, suggesting mechanisms extending beyond brevetoxin-mediated neurotoxicity. Bioassays indicate high potency, with LC50 values from several assays lower than those reported previously for other harmful algal taxa. Notably, substantial biological impacts occurred at concentrations orders of magnitude lower than peak bloom levels, including losses of sensitive taxa (e.g. bryozoans) and near-complete loss of sessile invertebrate assemblages on jetty pylons at affected sites. I further demonstrate that effective toxicity can decline under natural light exposure, with experimentally derived measures allowing quantification of degradation rates. Exposure to bright sunlight reduces toxicity, consistent with labile compounds sensitive to photodegradation. Together, these findings indicate that ecological risk during blooms depends not only on cell abundance, but also on production, persistence, and transformation of extracellular toxic compounds.