Seasonal cycles of nutrients and dissolved inorganic carbon at high and mid latitudes in the North Pacific Ocean during the Skaugran cruises: determination of new production and nutrient uptake ratios

The results of a 2-yr monitoring program in the North Pacific (March 1995-April 1997) using the commercial cargo carrier M/V Skaugran are presented in this paper. Seasonal changes in macronutrients (nitrate, phosphate and silicate), dissolved inorganic carbon (DIC) and total alkalinity (TA) were recorded in the surface waters of 12 major provinces of the North Pacific Ocean (35-55degreesN). North of the subarctic boundary (40degreesN), nitrate rarely decreased to below detection limit except in the more near-shore provinces, with nitrate concentrations remaining as high as 8-9 mumol kg(-1) in the summer in the northern provinces of the Bering Sea, the central and western subarctic Pacific. There were also strong latitudinal nutrient gradients. In most provinces of the subarctic Pacific, there was little interannual change in the nutrient cycles, except in the western subarctic Pacific where nitrate concentration fell to 2.6 pmol kg(-1) in the summer of 1996. We suggest that this large nitrate depletion was due to enhanced diatom production triggered by the supply of iron from the Asian continent. The relationship between salinity-normalized TA and DIC revealed that calcification was low or negligible in the surface waters of the Bering Sea and the western subarctic Pacific. In contrast, the percent of CaCO(3) production to organic carbon production was found to be at times as high as 70-75% in the eastern subarctic Pacific and the southern region off east Japan. This study also revealed east-west contrasts in the seasonal nutrient-depletion ratios. In the east, the elevated DIC:NO(3) ratios and low Si(OH)(4):NO(3) ratios were due to high CaCO(3) production and low diatom production in surface waters. On the contrary, in the western subarctic Pacific and the Bering Sea, the low Redfieldian DIC:NO(3) ratios and the high Si(OH)(4):NO(3) ratios were due to low calcification and high diatom production. The annual new production was estimated from the seasonal drawdown of nitrate at the surface. It ranged from 10.1 g C m(-2) yr(-1) in the subtropical gyre to 76-83 g C m(-2) yr(-1) in the western subarctic Pacific. Overall, horizontal advection was the major source of uncertainty in the estimation of new production. Finally, the net removal of carbon from the surface by the biological pump was then estimated to be 0.58 Gt C yr(-1) north of 35degreesN in the Pacific Ocean. The subarctic North Pacific and the Bering Sea accounted for 93% of the carbon removal. (C) 2002 Elsevier Science Ltd. All rights reserved.