Variations in the seasonal cycle of biological production in subarctic oceans: A model sensitivity analysis

The various hypotheses that have been proposed to explain the absence of a spring bloom at OWS P in the North Pacific have been evaluated by means of a sensitivity analysis of two seasonal mixed layer ecosystem models. The main model consisted of six compartments (phytoplankton, herbivorous and carnivorous zooplankton, nitrate, ammonium and detritus) forced by climatological mixed layer depths, photosynthetically available radiation and deep nitrate concentrations. It was concluded that the absence of a spring bloom at OWS P can be attributed to the limiting effect of herbivore grazing. In contrast at OWS I the deeper winter mixed layer depths means that net phytoplankton production in winter is insufficient to allow herbivore populations to reach high enough levels to exert a limiting grazing pressure at the time of the spring bloom. The high levels of summer nitrate at OWS P are mainly due to the limitation of new production by grazing coupled with a supply of nitrate from below the mixed layer; ammonium limitation of nitrate uptake is a secondary factor. The simulations at OWS P showed that small variations in the parameter values had only a marginal effect on phytoplankton biomass and this might explain the low degree of inter-annual variability of phytoplankton at OWS P. The possible role that iron deficiency might play in limiting production is discussed.