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 Atlantic Bloom Experiment in 1989 are compared with amixed-layer model of phytoplankton seasonal succession in whichthe latitudinal variation of the succession is driven by physicalforcing. In the model, the first phytoplankton groups to growat the end of the winter are those with the fastest intrinsicgrowth rates: the phytoflagellates and picophytoplankton. Theincreases of these groups are soon halted by the rapid growthof microzooplankton and heteroflagellates whereupon they aresucceeded by diatoms, the next fastest growers. With depletionof silicate, the diatom bloom ends and is briefly replaced byphytoflagellates and picophytoplankton, before these in turnare replaced by slower growing dinoflagellates. Differencesin the physical forcing cause the timings and magnitudes ofthese changes to vary with latitude. By sampling the model resultsat the times and places of the JGOFS observations, it is shownthat the major changes of populations and nutrients are reproduced,as are many production and grazing rates. The model resultssuggest the nature of nutrient utilization in the region. Whilenitrate and silicate are both reduced to low values at 47N,only silicate is depleted at 60N. Nitrate is not depleted atnorthern latitudes due to the greater depth of the mixed layer,more intense vertical mixing and the shorter season, so thatphytoplankton have more nutrients to utilize and a shorter timein which to do it. Phytoflagellates and picophytoplankton areunable to utilize all the inorganic nitrogen because of thegrazing by the micrograzers, and diatoms because of silicatedepletion. Dinoflagellates are slow growing and only have sufficienttime to deplete the nitrogen at low latitudes. There is no needto invoke limitation by a trace nutrient, such as iron, to reproducethe events in the NE Atlantic during 1989.