ESTIMATING DENITRIFICATION IN NORTH-ATLANTIC CONTINENTAL-SHELF SEDIMENTS

A model of coupled nitrification/denitrification was developed for con tinental shelf sediments to estimate the spatial distribution of denit rification throughout shelf regions in the North Atlantic basin. Using data from a wide range of continental shelf regions, we found a linea r relationship between denitrification and sediment oxygen uptake. Thi s relationship was applied to specific continental shelf regions by co mbining it with a second regression relating sediment oxygen uptake to primary production in the overlying water. The combined equation was: denitrification (mmol N m(-2) d(-1)) = 0.019 phytoplankton productio n (mmol C m(-2) d(-1)). This relationship suggests that approximately 13% of the N incorporated into phytoplankton in shelf waters is eventu ally denitrified in the sediments via coupled nitrification/denitrific ation, assuming a C:N ratio of 6.625:1 for phytoplankton. The model ca lculated denitrification rates compare favorably with rates reported f or several shelf regions in the North Atlantic. The model-predicted av erage denitrification rate for continental shelf sediments in the Nort h Atlantic Basin is 0.69 mmol N m(-2) d(-1). Denitrification rates (pe r unit area) predicted by the model are highest for the continental sh elf region in the western North Atlantic between Cape Hatteras and Sou th Florida and lowest for Hudson Bay, the Baffin Island region, and Gr eenland. Within latitudinal belts, average denitrification rates were lowest in the high latitudes, intermediate in the tropics and highest in the mid-latitudes. Although denitrification rates per unit area are lowest in the high latitudes, the total N removal by denitrification (53 x 10(10) mol N y(-1)) is similar to that in the mid-latitudes (60 x 10(10) mol N y(-1)) due to the large area of continental shelf in th e high latitudes. The Gulf of St. Lawrence/Grand Banks area and the No rth Sea are responsible for seventy-five percent of the denitrificatio n in the high latitude region. N removal by denitrification in the wes tern North Atlantic (96 x 10(10) mol N y(-1)) is two times greater tha n in the eastern North Atlantic (47 x 10(10) mol N y(-1)). This is pri marily due to differences in the area of continental shelf in the two regions, as the average denitrification rate per unit area is similar in the western and eastern North Atlantic. We calculate that a total o f 143 x 10(10) mol N y(-1) is removed via coupled nitrification/ denit rification on the North Atlantic continental shelf. This estimate is e xpected to underestimate total sediment denitrification because it doe s not include direct denitrification of nitrate from the overlying wat er. The rate of coupled nitrification/denitrification calculated is gr eater than the nitrogen inputs from atmospheric deposition and river s ources combined, and suggests that onwelling of nutrient rich slope wa ter is a major source of N for denitrification in shelf regions. For t he two regions where N inputs to a shelf region from onwelling have be en measured, onwelling appears to be able to balance the denitrificati on loss.