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 bloomsin Alfacs Bay (Ebro Delta, NW Mediterranean Sea) from December 1994 toApril 1995, a period characterized by an initial period of water stabilityand low outward flux of water during subsequent months. The highest celldensities of G.corsicum were found at the bottom ofthe bay during the population maintenance period (from January to March).During the day, no differences in the vertical distribution ofG.corsicum were observed and no diel insitu vertical migration of the population was recorded. Thein situ growth rate of G.corsicumwas estimated using the cell cycle method since the cell division cycle waswell phased with the daily light cycle. The S phase fraction ofG.corsicum reached a maximum near the middle of thedark period. The G2M peak usually occurred in the dark period. The durationof the S phase was 2.1-8.7 h and the duration of the G2M phase was 2.6-7.7h. The estimated specific in situ growth rate ofG.corsicum ranged from 0.94day^-1 during the initial phases of the bloom to 0.3day^-1 when the bloom was well developed. Growthrates, measured in different locations at the same time, varied by a factorof 1.5, suggesting that different parts of the same blooming populationwere at different stages of growth. The persistence of theG.corsicum bloom with high cell densities(10⁵-10⁶ cellsl^-1) at the bottom of the bay is discussed inrelation to the low cell losses by physical dispersion in this area and thelack of upward vertical migration of G.corsicum.