Environmental conditions which lead to increase in cell density of the toxic dinoflagellates Dinophysis spp. in nutrient-rich and nutrient-poor waters of the French Atlantic coast

Abstract

From 22 April to 19 June 1990, 7 stations located off La Rochelle, France, from near-shore ('bassin Marennes-Oleron') to offshore waters (50 m depth contour) were visited 8 times, with the aim of determining whether the toxic dinoflagellates Dinophysis spp. increase in cell density in nutrient-poor offshore water, in nutrient-rich inshore waters, or both, and to determine their vertical distribution in relation to stratification. Temperature stratification developed offshore, but not inshore. In April, few Dinophysis spp. were present offshore and none inshore. In late May, up to 15 X 10(3) cells l(-1) were recorded in the 10 to 15 m layer offshore, whereas inshore waters contained only a few cells l(-1). In late May, a short period of wind partly modified the vertical structure, and Dinophysis spp. concentration fell, increasing again when marked stratification was again established. Through the strait, flood-tide currents moved Dinophysis spp. cells to the inshore area, and ebb-tide currents brought some of them back out. No relationship was found between Dinophysis spp. growth and availability of dissolved nutrients. We concluded that: (1) stratification of sufficient magnitude (DELTAt 5-degrees-C) and duration ( 2 wk) are the factors necessary for Dinophysis spp. cell increase, and (2) inorganic nutrient input of terrestrial origin does not directly promote their growth.