Interannual variability in nitrate supply to surface waters of the Northeast Pacific Ocean

Abstract

In the past decade, the upper ocean in the NE Pacific has undergone changes in physical and chemical properties which are similar to the recent El Nino/La Nina cycle. During the 1989 La Nifia, winter waters at Ocean Station Papa (OSP) were relatively cool, saline and nitrate rich. With the onset of the 1991 El Nino period, however, winter waters at OSP were more saline by 0.3 psu, warmer by over 2°C and nitrate depleted by 30 %. In 1994, oceanic winter temperatures were the warmest ever observed in over 40 yr of sampling. The decrease in winter nutrient supply persisted eastward from OSP to the coast of Vancouver Island, and resulted in an expanded area of nitrate depletion in summer. Lower winter nitrate supply is estimated to have reduced new production through spring and summer by 40 % (2 million tonnes C) in a 290 000 km2 patch of ocean west of Vancouver Island. We suggest that declines in phytoplankton production of this magnitude affect both the productivity and composition of higher trophic levels.