Early warning of toxic dinoflagellate blooms of Gymnodinium catenatum in southern Tasmanian waters

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Abstract
Blooms of the toxic dinoflagellate Gymnodinium catenatum (a causative organism of paralytic shellfish poisoning) in the Derwent and Huon estuaries of southern Tasmania, Australia, are predictable, annually recurrent events in the period January to June (late summer to early winter). However, their spatial distribution, duration and magnitude exhibit significant interannual variability. High shellfish toxicities in 1986, 1991 and 1993 (>8000 μ.g paralytic shellfish poisoning per 100 g shellfish meat) also coincided with the greatest spatial extent of shellfish toxicity (up to 35 shellfish farms closed for periods up to 6 months). An exploratory analysis of the results of a shellfish toxin monitoring programme conducted from 1986 to 1994, and of available hydrological and meteorological data for the region, indicates that a significant G.catenatum bloom in Tasmanian waters can only develop within a permissive seasonal water temperature window (>14°C at the time of bloom initiation) requiring a rainfall event as a trigger (Huon River discharge, measured at Frying Pan Creek, must exceed 100 000 megalitres over a 3-week period) and a calm stable water column for sustained development (windspeed −1 for periods of 5 days or more). Once established, dinoflagellate populations are subject to disturbance by turbulence caused by high windstress; this explains the incidence in some years of multiple shellfish toxicity peaks. In winter months declining water temperatures (<10°C) and increasing windstress are responsible for the termination of seasonal dinoflagellate blooms.

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