SOURCES AND CYCLING OF NITROGEN IN THE GULF-OF-MAINE

An analysis of water mass flows and nitrogen fluxes in the Gulf of Mai ne region shows that deep Slope Water that enters the Gulf through the Northeast Channel, and Scotian Shelf Water that enters at the surface , dominate the flux of nitrogen into the Gulf. A box model is develope d that examines internal vertical nitrogen fluxes, and reveals that th e flux of nitrogen into surface waters is sufficient to explain only a bout 59 gC m(-2) yr(-1) of new primary production, which is 20% of the total estimated Gulf of Maine primary production of 290 gC m(-2) yr(- 1). This means that the Gulf-wide f ratio (of ''new'' NO3-based produc tion to the total production based on both new NO3 and recycled NH4) i s 0.20, which is more typical of oligotrophic oceans than a productive continental shelf sea like the Gulf of Maine. The expected f ratio is nearer to 0.4, which would require an additional flux of new NO3 into the Gulf equal to about 40% of the total flux already accounted for b y all sources: Slope Water, Scotian Shelf Water, rivers and atmospheri c deposition. This additional supply of ''new'' nitrogen is argued to be the result of water column nitrification. The box model also shows, surprisingly, that nutrients delivered to surface waters of the Gulf by Scotian Shelf Water are roughly equal to that of Slope Water. It is concluded that better estimates are needed of water flows into and ou t of the Gulf, along with more measurements of their nutrient loads, a nd that measurements should be made of water column nitrification rate s. An overall conclusion is that the energetics of vertical mixing pro cesses that deliver nutrients to the productive surface waters set the upper limit to biological production in the Gulf of Maine, and that c onstruction of carbon and nitrogen budgets that consider only fluxes i nto and out of the Gulf, and not internal recycling, will be in error. (C) 1998 Elsevier Science B.V. All rights reserved.