'Phyto-convection': The role of oceanic convection in primary production

Abstract

Typical sinking rates of marine phytoplankton cover a range extending from a few meters up to several hundred meters per day. If it were not for a process which maintains plankton near the sea surface, in the euphotic layer, it would sink to depths of thousands of meters in the deep ocean during the winter season. Consequently, plankton would not be available for the next spring bloom. In shelf seas and coastal areas, as well as in fjords, deep sinking is prohibited by the proximity of the sea bed. The mechanism which reliably initiates a spring bloom is generally not considered in models of marine primary production. Such models generally rely on the assumption that a very small background concentration of plankton is available to initiate a bloom. Penetrative oceanic convection in the open ocean forms the perennial thermocline in winter in mid and high latitudes. The thermocline is situated at depths of several hundred meters. On a shelf, or in a fjord, convection may penetrate to the seabed, thereby affecting the entire water column. We argue that oceanic convection in winter accounts for the availability of plankton in the euphotic layer in spring. In support of this hypothesis a coupled phytoplankton convection model was developed. In this model plankton, i.e. resting spores and vegetative cells, is simulated by Lagrangian tracers moving within the flow predicted by the convection model. For each tracer a simple phytoplankton model predicts growth dependent on light conditions. Plankton spores sink with a prescribed velocity of 120 m d-1. Growing vegetative cells have a sinking rate of only 1 m d-1. The model operates in a vertical ocean slice covering the water column. The width of the slice is typically 1 to 3 km, and it is resolved by an isotropic grid size of 5 m. The phyto-convection model was applied to a region in the Barents Sea shelf and to a coastal fjord in the north of Norway. It was run over winter/spring periods under realistic meteorological forcing. Tracers representing resting spores were initially introduced into a thin bottom layer of the model domain, which constitutes the worst case in terms of maximum sinking. The water column, apart from the bottom layer, was assumed to be void of plankton. In both cases convection eroded the initial stratification and dispersed plankton over the entire water column. The onset of a phytoplankton bloom coinciding with the establishment of a (weak) seasonal thermocline in spring was predicted, which agrees with observations from both regions considered. The simulations support the hypothesised role of oceanic convection in primary production.