The influence of oceanic convection in primary production

Abstract

The influence of Oceanic Convection in Primary Production was investigated by both, numerical model studies and field observations. Cruises with RV VALDIVIA were conducted in the North Atlantic/Nordic Seas in Spring with the purpose of investigating a hypothesised relationship between Oceanic Convection and phytoplankton dynamics in winter in open oceans. In the ocean phytoplankton would sink down into deep waters after the breakdown of the seasonal thermocline in autumn. The mechanism that in spring reliably initiates phytoplankton blooms by bringing cells back to the surface has yet only rarely been investigated. The orbital motions of the water caused by Convection carries plankton up and down. Cells may therefore intermittently get back to the surface to receive enough light to stay alive during a winter. During VALDIVIA Cruise 176 (March 1999) a YOYO-station, covering 16 profiles measured every hour were conducted, which showed a high temporal variability of convective penetration. A phytoplankton model was coupled to a 2.5 dimensional non-linear, non-hydrostatic Convection model. In the first numerical study plankton was simulated by an Eulerian approach, in which plankton was introduced as an Eulerian tracer to investigate the temporal variability of the water column observed during the YOYO-station. The aim of the second numerical study is the provision of quantitative estimates of CONTACT times of water parcels, i.e. plankton that was carried by the water motions, with the uppermost water column, where light is available for growth. This was done by a Lagrangian approach in which plankton ensembles are simulated by Lagrangian tracers and a realistic stratification obtained during RV VALDIVIA Cruise 141 is prescribed as background field. In both cases the model domain is a vertical ocean slice. Horizontal dimensions of the model domain are chosen in accordance with expected convective aspect ratios, which vary between 1 and 3. Special attention was paid on the influence of Convection on the temporal variability of Chlorophyll a content in the water column.