Alexandrium fundyense vertical distribution patterns during 1982, 2001 and 2002 in the offshore Bay of Fundy, eastern Canada

Abstract An understanding of population dynamics of individual species and strains of Alexandrium spp. is important to achieving the greater knowledge needed for forecasting occurrences, predicting consequences and determining mechanisms for bloom initiation and growth. Alexandrium fundyense populations were observed during July in the offshore waters of the Bay of Fundy (eastern Canada) at: 3-h intervals during a 54-h period in 1982; 2-h intervals during 30 and 22-h periods in 2001; and 2 h intervals for a 26 h period in 2002. Results suggest that A. fundyense vegetative cells (including duplets) and planozygotes concentrate in the upper layers, with highest concentrations observed in most surface samples. Concentrations decreased with depth. Cell concentrations of A. fundyense greater than 10 5  cells L −1 were detected, and concentrations varied considerably over the sampling periods. CTD data indicated that the water column was weakly stratified throughout each sampling period. Nutrient analysis suggests that silicates and phosphates in surface waters were not limiting, but nitrate values were lower in the upper layers than at depth. Statistical analyses of the profile data indicated that the observed counts were over dispersed or patchy. The pairwise comparison of the profiles did not support a diurnal vertical migration of cells over the depth range sampled in any of the surveys. Shifts in density were detected across the two sampling sessions of 2001, but these differences were unrelated to an effect of photoperiod. Analyses of grouped profiles also failed to detect changes in the daytime versus nighttime distribution of cells with depth.