The “red tides” of Malta

A 5 year survey of Marsamxetto Harbour, Malta, was carried out to investigate some factors contributing to red tides and to identify the causative organisms. Red tides occurred usually in May but, less predictably, also during June – August. Causative organisms included the diatom Chaetoceros sp., and the dinoflagellate genera Prorocentrum, Peridinium, Ptychodiscus, Gonyaulax, Ceratium and Cochlodinium. Early summer red tides occurred at a time of increasing insolation, and water temperature, prevailing onshore winds and salinities in the range 34.4 to 36.6 A consistent feature was a quick rise in sea temperature of more than 2 C^o during the week preceding red water. Red tides later in the year coincided with higher temperatures and salinities and calm weather.