THE EKMAN TRANSFER OF NUTRIENTS AND MAINTENANCE OF NEW PRODUCTION OVER THE NORTH-ATLANTIC

The maintenance of new production requires a supply of nutrients to th e euphotic zone to offset the loss through biological export. The dyna mical supply of nutrients is usually discussed in terms of the vertica l transfer from nutrient-rich, deep waters. However, the horizontal tr ansfer is important in regions of downwelling over subtropical gyres, where nutrients may be transported across the intergyre boundaries by the surface Ekman drift or geostrophic eddies from the neighbouring nu trient-rich, upwelling regions. The Ekman transfer of nitrate to the e uphotic layer is diagnosed from climatology over the North Atlantic. T he vertical Ekman supply of nitrate is found to be significant over th e subpolar gyre, the tropics and eastern boundary, whereas the horizon tal transfer is found to be dominant at the intergyre boundaries. On t he northern flank of the subtropical gyre, the Ekman transfer provides a source of nitrate from 0.03 to 0.06 mol N m(-2) yr(-1), correspondi ng to a contribution to new production of between 0.4 and 0.8 mol C m( -2) yr(-1). This estimate represents a significant fraction of the tot al new production of typically 1 mol C m(-2) yr(-1) suggested by both remote chlorophyll and sediment trap observations. A simplified nitrog en cycle model is used to assess the role of the Ekman supply over the North Atlantic. In the model the Ekman supply of nitrate leads to a p lume of nitrate and enhanced productivity extending up to 1000 km into the subtropical gyre from the intergyre boundaries. This lateral scal e is controlled by the seasonal cycle of the mixed layer and the remin eralisation of the particulate organic fallout. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.