EVIDENCE OF CHANGE IN THE WINTER MIXED-LAYER IN THE NORTHEAST PACIFIC-OCEAN

Sea-surface temperatures in the Northeast Pacific Ocean show a warming trend, and salinities show a declining trend, in data collected over the last 60 years. These changes combine to reduce the density of the surface layer over a large area of the Northeast Pacific. The declinin g surface density changes the energetic requirements for the formation of a surface mixed layer, and observations at Ocean Station Papa indi cate that mid-winter mixed layer depths are showing a marked decline. The reduction in the depth of penetration of the winter-time mixed lay er should reduce the nutrients entrained into the upper ocean each win ter. Observations suggest that near surface nutrient levels are declin ing at Papa but remain well above levels that might inhibit productivi ty. However, at present the productivity of large phytoplankton appear s to be limited by iron supply which is thought to be mainly from the atmosphere. A shallower mixed layer depth could increase the concentra tion of iron in this layer. The increase in iron would increase the ut ilization of nitrate, mainly by diatoms, and new production and the f ratio would increase. Copyright (C) 1998 Elsevier Science Ltd. All rig hts reserved.