The spring phytoplankton bloom is a major, extensively studied phenomenon in temperate and high latitude seas. Much less information isavailable on blooms in subtropical oligotrophic seas, where the water column is usually stratified. Yet, even in temperate seas the processes determining phytoplankton dynamics during the mixed-layer deepening and the factors triggering the initiation of the bloom are controversial. Here we use long-term measurements of chlorophyll concentration, nutrients, mixed-layer depth and grazing rates to examine the validity of three bloom-initiation processes for the Gulf of Aqaba (northern Red Sea): the Critical Depth Hypothesis, the Dilution-Recoupling Hypothesis, and the Critical Turbulence Hypothesis. The Gulf is a unique water body in the subtropics, where convective mixing during winter reaches hundreds of meters in depth, leading to conspicuous spring blooms. Here we show that neither the critical depth mechanism nor the dilution-recoupling hypothesis explain the phytoplankton dynamics during the winter and spring in the Gulf. Instead, our findings indicate that this dynamics is governed by the interplay between three main processes: (1) nutrient-driven primary production in the upper, illuminated layer; (2) physical ‘homogenization’ of phytoplankton by convective mixing; and (3) accumulation of phytoplankton in the upper layer after the termination of sea-surface cooling. The latter mechanism is responsible for the onset and magnitude of the spring bloom.
