DIURNAL-VARIATIONS OF CONVECTIVE MIXING AND THE SPRING BLOOM OF PHYTOPLANKTON

Turbulent stirring of the surface mixed layer extends to shallower depths during the day than in the night because the increased buoyancy resulting from solar heating inhibits the mixing associated with wind action and surface heat loss. Calculations using a simple model in which a mixed layer of constant depth is more strongly coupled to deeper layers at night than during the day indicate that the reduction of mixing by day may be critical to the onset of the spring phytoplankton bloom, for this occurs at a time when there is increasing solar warming in the day and yet considerable heat loss and wind stirring at night. If the Kraus-Turner model of the mixed layer is used to estimate the mixing rates occurring during darkness, the values obtained agree with those that give realistic simulations of the spring bloom. Diurnal observations of chlorophyll a, pCO2 and oxygen saturation made at 60-degrees-N during the Lagrangian experiment carried out in 1989 as part of the U.K. Biogeochemical Ocean Flux Study can be modelled more successfully if the day-night changes in vertical mixing are included in the same manner as the single layer model. The calculations indicate that these changes may shift the timing of the bloom by about 1 week and may account for the depth of penetration of some spring blooms. This process needs to be considered when modelling the coupling between climate and phytoplankton.