In situ growth rate and distribution of the ichthyotoxic dinoflagellate Gyrodinium corsicum Paulmier in an estuarine embayment (Alfacs Bay, NW Mediterranean Sea)

Gyrodinium corsicum Paulmier produced massive blooms in Alfacs Bay (Ebro Delta, NW Mediterranean Sea) from December 1994 to April 1995, a period characterized by an initial period of water stability and low outward Aux of water during subsequent months. The highest cell densities of G.corsicum were found at the bottom of the bay during the population maintenance period (from January to March). During the day, no differences in the vertical distribution of G.corsicum were observed and no diel in situ vertical migration of the population was recorded. The in situ growth rate of G.corsicum was estimated using the cell cycle method since the cell division cycle was well phased with the daily light cycle. The S phase fraction of G.corsicum reached a maximum near the middle of the dark period. The G(2)M peak usually occurred in the dark period. The duration of the S phase was 2.1-8.7 h and the duration of the G(2)M phase was 2.6-7.7 h. The estimated specific in situ growth rate of G.corsicum ranged from 0.94 day(-1) during the initial phases of the bloom to 0.3 day(-1) when the bloom was well developed. Growth rates, measured in different locations at the same time, varied by a factor of 1.5, suggesting that different parts of the same blooming population were at different stages of growth. The persistence of the G.corsicum bloom with high cell densities (10(5)-10(6) cells l(-1)) at the bottom of the bay is discussed in relation to the low cell losses by physical dispersion in this area and the lack of upward vertical migration of G.corsicum.