Eau colorée à Lingulodinium polyedrum (Stein) Dodge, dans une zone aquacole du littoral du Doukkala (Atlantique marocain)

Abstract

A red tide of Lingulodinium polyedrum occured along the moroccan atlantic coast in july 1999. Observed in the beginning of the month in the Kenitra area, it spreads with the coastal drift as far as the south of safi, passing on its way upon a sanitary monitored area. This area which is submitted to an all year round bimonthly monitoring analysis of phytoplankton, contains significant natural beds of oysters, clams and mussels, as well as shellfish farms in Oualidia and Sidi Moussa lagoons. However, during this period of bloom, there was weekly monitoring of hydrologic and climatic data associated with phytoplankton analysis to identifiy the causes of this bloom.

This bloom occured in 17–18°C surface waters, temperatures which are favourable to dinoflagellate encystment and development. The observation of two to three colored strips parallel to the coast suggests phytoplankton aggregation generated by internal waves created by moderate winds blowing constantly in the same direction and at constant speed at the time of the bloom. A change of climatic conditions might also be the cause of the dispersal of the phenomenon. We remark that this exceptional bloom has never reached the coastal shellfish sites, nor the monitored lagoons. The maximum Lingulodinium concentration observed was 79.10³ cell l^–1 at Sidi Moussa lagoon and minimum concentrations ranged from 1,6.10³ cell l^–1 to 18.10³ cell l^–1 at coastal stations. The diarrheic toxicity found in the shellfish (mussels, oysters) cannot be imputed to L. polyedrum but to associated species such as Dinophysis acuminata and D. acuta which were present, though in low density in these waters.

However, taking into account the controversy on the L. polyedrum toxicity and its frequent involvement in red tides, it is necessary to include it among the potentially harmful species that should be monitored along Moroccan coasts.