Seasonal succession in the pelagic ecosystem of the North Atlantic and the utilization of nitrogen

• Published in: Journal of Plankton Research
• Main Authors: Taylor, A.H., Harbour, D.S., Harris, R.P., Burkill, P.H., Edwards, E.S.
• Format: Text
• Language: English
• Published: Oxford University Press 1993
• Subjects: ORIGINAL ARTICLES; North Atlantic
• Online Access: http://plankt.oxfordjournals.org/cgi/content/short/15/8/875; https://doi.org/10.1093/plankt/15.8.875

Observations during the Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment in 1989 are compared with a mixed-layer model of phytoplankton seasonal succession in which the latitudinal variation of the succession is driven by physical forcing. In the model, the first phytoplankton groups to grow at the end of the winter are those with the fastest intrinsic growth rates: the phytoflagellates and picophytoplankton. The increases of these groups are soon halted by the rapid growth of microzooplankton and heteroflagellates whereupon they are succeeded by diatoms, the next fastest growers. With depletion of silicate, the diatom bloom ends and is briefly replaced by phytoflagellates 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 sampling the model results at the times and places of the JGOFS observations, it is shown that the major changes of populations and nutrients are reproduced, as are many production and grazing rates. The model results suggest the nature of nutrient utilization in the region. While nitrate and silicate are both reduced to low values at 47�N, only silicate is depleted at 60�N. Nitrate is not depleted at northern latitudes due to the greater depth of the mixed layer, more intense vertical mixing and the shorter season, so that phytoplankton have more nutrients to utilize and a shorter time in which to do it. Phytoflagellates and picophytoplankton are unable to utilize all the inorganic nitrogen because of the grazing by the micrograzers, and diatoms because of silicate depletion. Dinoflagellates are slow growing and only have sufficient time to deplete the nitrogen at low latitudes. There is no need to invoke limitation by a trace nutrient, such as iron, to reproduce the events in the NE Atlantic during 1989.