| Summary: | Blooms of the dinoflagellate <i>Dinophysis acuminata</i> occur every year in an important mussel cultivation area in Port Underwood, Marlborough Sounds, New Zealand. Annual maximum cell numbers range from 1500⁻75,000 cells L<sup>−1</sup> and over 25 years of weekly monitoring the <i>D. acuminata</i> bloom has never failed to exhibit peaks in abundance at some time between spring and autumn. During winter (June⁻August) the dinoflagellate is often undetectable, or at low levels (≤100 cells L<sup>−1</sup>), and the risk of diarrhetic shellfish poisoning (DSP)-toxin contamination over this period is negligible. Bloom occurrence may be coupled to the abundance of <i>D. acuminata</i> prey (<i>Mesodinium</i> sp.) but the mechanism by which it maintains its long-term residence in this hydrologically dynamic environment is unknown. The toxin profile of <i>D. acuminata</i> is dominated by pectenotoxin-2 (PTX-2) and dinophysistoxin-1 (DTX-1), but the cellular toxin content is low. It is rare that free DTX-1 is detected in mussels as this is invariably exclusively present as fatty acid-esters. In only five out of >2500 mussel samples over 16 years have the levels of total DTX-1 marginally exceeded the regulated level of 0.16 mg kg<sup>−1</sup>. It is also rare that free PTX-2 is detected in mussels, as it is generally only present in its hydrolysed non-toxic PTX-2 seco acid form. The <i>D. acuminata</i> alert level of 1000 cells L<sup>−1</sup> is often exceeded without DTX-1 residues increasing appreciably, and this level is considered too conservative.
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