SEASONAL SUCCESSION IN THE PELAGIC ECOSYSTEM OF THE NORTH-ATLANTIC AND THE UTILIZATION OF NITROGEN

Observations during the Joint Global Ocean Flux Study (JGOFS) North At lantic Bloom Experiment in 1989 are compared with a mixed-layer model of phytoplankton seasonal succession in which the latitudinal variatio n of the succession is driven by physical forcing. In the model, the f irst phytoplankton groups to grow at the end of the winter are those w ith the fastest intrinsic growth rates: the phytoflagellates and picop hytoplankton. The increases of these groups are soon halted by the rap id growth of microzooplankton and heteroflagellates whereupon they are succeeded by diatoms, the next fastest growers. With depletion of sil icate, the diatom bloom ends and is briefly replaced by phytoflagellat es and picophytoplankton, before these in turn are replaced by slower growing dinoflagellates. Differences in the physical forcing cause the timings and magnitudes of these changes to vary with latitude. By sam pling the model results at the times and places of the JGOFS observati ons, it is shown that the major changes of populations and nutrients a re reproduced, as are many production and grazing rates. The model res ults suggest the nature of nutrient utilization in the region. While n itrate and silicate are both reduced to low values at 47-degrees-N, on ly silicate is depleted at 60-degrees-N. Nitrate is not depleted at no rthern latitudes due to the greater depth of the mixed layer, more int ense vertical mixing and the shorter season, so that phytoplankton hav e more nutrients to utilize and a shorter time in which to do it. Phyt oflagellates and picophytoplankton are unable to utilize all the inorg anic nitrogen because of the grazing by the micrograzers, and diatoms because of silicate depletion. Dinoflagellates are slow growing and on ly have sufficient time to deplete the nitrogen at low latitudes. Ther e is no need to invoke limitation by a trace nutrient, such as iron, t o reproduce the events in the NE Atlantic during 1989.