Factors controlling silicon and nitrogen biogeochemical cycles in high nutrient, low chlorophyll systems (the Southern Ocean and the North Pacific): Comparison with a mesotrophic system (the North Atlantic)

A 1-D coupled physical-biogeochemical model is used to study the seasonal c ycles of silicon and nitrogen in two High Nutrient Low Chlorophyll(HNLC) sy stems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model c onsists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic aci d concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a "top-do wn" control by microzooplankton. Although nanophytoplankton sustain more th an 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences i n the seasonal plankton cycle among these regions come mainly from differen ces in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m(-3) as an unfavourable light/mixing r egime and a likely trace-metal limitation keep diatoms from blooming. In th e northeast Pacific, trace-metal limitation seems to keep diatoms from bloo ming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a sub sequent silicic acid limitation during the summertime. Ln the North Atlanti c, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chll-a m(-3). Thereafter, mesozooplankton grazing pressure and silicic a cid limitation induce the collapse of the chlorophyll maximum and the persi stence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica p roduction (1.9 and 1.07 mol Si m(-2) yr(-1)) and export flux (0.79 and 0.61 mol Si m(-2) yr(-1)), compared to the North Atlantic (production: 0.23 mol Si m(-2) yr(-1), export: 0.12 mol Si m(-2) yr(-1)). The differences in Si production and export between the HNLC systems and the mesotrophic North At lantic come from both higher Si concentrations and Si/N uptake ratios in th e HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibiti on of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the effic iency of the silica pump for the three sites: about 50% of the biogenic sil ica synthesized in the euphotic layer is exported out of the first 100 m, w hile only 4-11% of the particulate organic nitrogen escapes recycling in th e surface layer. (C) 1999 Elsevier Science Ltd. All rights reserved.