Facteurs hydroclimatiques et apparitions d'eaux colorées, en baie de Vilaine, durant l'année 1983

Analysis of local hydrological and climatic factors help to determine basic conditions for blooms and red tides in Vilaine Bay during summer 1983. Spring or late summer heavy rainfalls produce nutrients increase and low salinities in coastal surface waters. It causes sharp saline stratification, enhanced or destroyed by winds, according to their direction and intensity. Summer thermal stress associated with low turbulence (no wind, neap tide, quiet waters...) produce strong thermal increase in surface layers. The thermal stratification thus obtained emphasizes saline stratification. When all favourab1e conditions are fulfilled increaeing development of diatoms lead to brown discolored waters. Decrease in diatoms communities occurs as nutrients crop is consumed. Such hydroclimatic factors are : thermal decreases, strong winds, high turbulence, spring tide may favour it. At consequent poor nutrient levels other motile and more tolerant phytoplankton1c organisms develop successfully : dinoflagellates, as, for instance, Dinophysis acuminata, a dirrhetic toxin producing one. [NOT CONTROLLED OCR]