Seasonal variability of nitrate supply and potential new production in the Gulf of Maine and Georges Bank regions

Abstract

The mean annual cycle of new primary production is determined for each of five hydrographic provinces contained within the combined Gulf of Maine and Georges Bank region using potential new production (PNP) as a proxy for new primary production. PNP is computed using in situ data and defined as the difference between the time rate‐of‐change of vertically integrated nitrate (NO3) in the euphotic zone and vertical flux of NO3 into the euphotic zone assuming Fickian diffusion, after conversion of nitrogen to carbon using the Redfield ratio. Maximum recharge rate of NO3 in the euphotic zone occurs primarily during fall, between yeardays 300 and 330 (late October to late November), in agreement with the timing of fall turnover of the water column. Maximum utilization rate of NO3 in the euphotic zone occurs between yeardays 60 and 120 (March to April), in agreement with the timing of the spring bloom, with little phase difference between provinces. Peak‐to‐peak amplitude (maximum recharge rate to maximum utilization rate) is largest for Jordan Basin, Northeast Channel, and the western Gulf of Maine province. Wintertime NO3 recharge into the euphotic zone within the Gulf of Maine is largely the result of vertical NO3 flux, except for eastern Gulf of Maine waters where advective NO3 flux into surface waters is important. However, there still exists a significant deficit for fall NO3 recharge for eastern Gulf of Maine which cannot be accounted for by either vertical diffusive or horizontal advective NO3 fluxes. Annual totals of integrated total primary production (ITPP) and PNP summed over the full study region (including the southern New England shelf) are 44.33 tg C yr−1 and 9.96 tg C yr−1, respectively, for an annual mean potential f‐ratio of 0.22. Exclusion of the southern New England shelf increases the annual mean potential f‐ratio value for the remaining Gulf of Maine and Georges Bank region to 0.25. Inclusion of a time‐varying horizontal advective flux of NO3 into the eastern Gulf of Maine south of Cape Sable, Nova Scotia, increases the annual total of PNP for the full study region to 10.66 tg C yr−1, for an annual mean potential f‐ratio of 0.24. Annual mean potential f‐ratio for only the Gulf of Maine (excluding Georges Bank and southern New England shelf) using model‐derived Kz values and horizontal advective flux of NO3 into eastern Gulf of Maine waters is 0.17, above oligotrophic ocean values of ∼0.1, but well below the value of 0.40 used to calculate nitrification within the Gulf of Maine. Greater than one half (53.3%) of total regional PNP occurs within the hydrographic province (annual mean potential f‐ratio = 0.58) bounded by the tidal mixing front on Georges Bank but comprises less than one fifth (17.3%) of the region's total area. PNP indicates primary production on the crest of Georges Bank is largely recycled, with an annual mean potential f‐ratio of much less than 0.1, suggesting nitrogen limitation. However, error estimates for PNP and potential f‐ratio on the crest of Georges Bank are large due to the paucity of data collected in this shallow province.