Ultraphytoplankton succession is triggered by deep winter mixing in the Gulf of Aqaba (Eilat), Red Sea

The seasonal dynamics of ultraphytoplankton for the northern Gulf of Aqaba (29°28′N, 34°55′E) were investigated in detail. Monthly analysis of pigments by HPLC and cell abundances by epifluorescence microscopy showed large fluctuations in ultraphytoplankton community structure concurrent with strong seasonal changes in water‐column conditions. Following extensive winter mixing, ultraphytoplankton seasonal succession progressed rapidly as water‐column stratification strengthened. Eucaryotic algae dominated in nutrient‐replete winter mixing conditions, Synechococcus was the major component of the ultraphytoplankton during a spring bloom of massive proportions, and Prochlorococcus was dominant in nutrient‐depleted summer‐stratified waters. Over the fall–winter period, as water‐column mixing progressed, succession was in the reverse order but at a much slower rate. Prochlorococcus was the major component of the community during summer stratification, yet was not detected at the height of the mixing event in late winter. The reestablishment of the population occurred only 3 months after the onset of stratification. This is the first report on seasonal succession involving all three ultraphytoplankton groups. We suggest that water‐column stability is an important factor influencing seasonal variations in ultraphytoplankton community structure.