Sources and cycling of nitrogen in the Gulf of Maine

Abstract

An analysis of water mass flows and nitrogen fluxes in the Gulf of Maine 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 developed that examines internal vertical nitrogen fluxes, and reveals that the flux of nitrogen into surface waters is sufficient to explain only about 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” NO 3-based production to the total production based on both new NO 3 and recycled NH 4) is 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 NO 3 into the Gulf equal to about 40% of the total flux already accounted for by all sources: Slope Water, Scotian Shelf Water, rivers and atmospheric 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 out of the Gulf, along with more measurements of their nutrient loads, and that measurements should be made of water column nitrification rates. An overall conclusion is that the energetics of vertical mixing processes that deliver nutrients to the productive surface waters set the upper limit to biological production in the Gulf of Maine, and that construction of carbon and nitrogen budgets that consider only fluxes into and out of the Gulf, and not internal recycling, will be in error.