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

Seven stations located off La Rochelle, from near-shore to offshore waters (50m depth contour) were visited. Temperature stratification developed offshore, but not inshore. In April, few Dinophysis spp. were present offshore and none inshore. In late May, up to 15 × 103 cells l-1 were recorded in the 10-15m 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. The authors conclude that: 1) stratification of sufficient magnitude (Δt>5°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. -from Authors