Unravelling the Physical Drivers behind Initiation and Patchiness of the Spring Bloom in a Temperate Shelf Sea

Although the approximate timing of the spring bloom can be predicted following Sverdrup’s critical depth hypothesis the precise timing, intensity and evolution of this annual peak in primary production is determined by small scale and often incoherent, short and transient events. This is particularly true in shallow and highly dynamic temperate continental shelf sea environments.

Following an intense field campaign on the NW European Shelf during the transition from mixed to stratified conditions we are able to examine the physical drivers behind initiation of the spring bloom in unprecedented detail. A wave powered vertically profiling float co-located with two ocean gliders provided high resolution profiles of density, chlorophyll-a fluorescence and the rate of turbulent kinetic energy dissipation every 10-15 minutes for 21 days. Full water column currents, meteorological variables and near surface PAR are taken from additional moorings in the array.

After the onset of positive net surface heat fluxes, our data sets show how the timing and subsequent development of the bloom is determined by the available PAR and its recent history; the fine scale vertical hydrographic and turbulent structure of the water column that controls the residence time of phytoplankton at each depth; and the timing and intensity of wind and tidal mixing events. In April 2015 the main peak in depth integrated chlorophyll occurred almost a week after the main seasonal thermocline had started to form. It peaked following three consecutive sunny days and a reduction in wind stress that allowed a thin (10 m) near surface warm layer to be established and maintained overnight. There is significant semi-diurnal variability in the depth integrated chlorophyll demonstrating how small scale (< 10 km) incoherence in these physical drivers leads to strong gradients and patchiness in the bloom dynamics across a shelf.