Tidal pumping and nutrient fluxes on Georges Bank: A process-oriented modeling study Song Hu a, ⁎, David W. Townsend b , Changsheng Chen a , Geoffrey Cowles a , Robert C. Beardsley c , Rubao Ji d , Robert W. Houghton e a University of Massachusetts Dartmouth, Department of Fisheries Oceanography, School for Marine Science and Technology, 706 South Rodney French Boulevard, New Bedford, MA 02744, United States b University of Maine, School of Marine Sciences, 5706 Aubert Hall, Orono, ME 04469, United States c Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02548, United States d Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02548, United States e Columbia University, Lamont Doherty Earth Observatory, Palisades, NY 10964, United States article info abstract Article history: Received 8 October 2007 Received in revised form 3 April 2008 Accepted 10 April 2008 Available online 27 April 2008 Process-oriented studies with the unstructured-grid, three-dimensional Finite-Volume Coastal Ocean Model (FVCOM) of Georges Bank were used to examine the importance of physical processes on the cross-isobath transport of nutrients onto the Bank. Starting from idealized vertical profiles of NO 3 constructed from summertime climatologic fields, the nutrient field was integrated in time using a conservative tracer equation with both homogenous and stratified initial hydrography and both tide and wind forcing. The model results reveal that: a) nutrient fluxes are spatially inhomogeneous, with the greatest nutrient flux generated by tidal pumping into surface waters along the edge of the Bank's northern flank; b) a surface nutrient maximum occurs on the northeast flank as a result of advection along the northern edge and bifurcation of the flow as waters circulate clockwise and spread laterally around the eastern portion of the Bank; c) advection enriches nutrient concentrations downstream and around the Bank, generating a donut-shaped pattern of elevated nutrients; and d) waters on the top of the Bank, especially in the southwest portions, experience the lowest nutrient flux rates. The length of time required to reach a quasi-equilibrium state of nutrient distribution over the Bank is controlled primarily by tidal advection, with cross-frontal fluxes modulated by stratification, surface wind stress, and the initial nutrient concentration in the Gulf of Maine source waters. Published by Elsevier B.V. Keywords: Tidal pumping Georges Bank Gulf of Maine Nutrient flux FVCOM 1. Introduction Georges Bank (GB) is a shallow submarine feature located at the opening of the Gulf of Maine, a semi-enclosed con- tinental shelf sea in the northwest Atlantic Ocean (Fig. 1). The Bank is well known for its high biological productivity, which for centuries has supported important commercial fisheries (Backus, 1987); rates of primary production reported for the Bank are thought to be among the highest of any continental shelf sea, exceeding 400 gC m - 2 y - 1 in the central, shallowest portions of the Bank (O'Reilly et al., 1987). An important driver of the biological productivity of the Bank is its physical oceanographic processes, which are closely tied to its geomorphology. The Bank is a relatively large feature, measuring about 150 km by 200 km, with an area of about 34,000 km 2 shallower than 100 m. Tidal currents, which are most pronounced over the top of the Bank with the dominant constituent being the semi-diurnal M2 with the maximum currents of ~100 cm/s (Brown and Moody, 1987), are the dominant physical process. Mixing generated by such strong currents produces a vertically well- mixed water column in the crest area bounded by the 40– 50 m isobath throughout the year. The general pattern of residual currents in the GB region has been known since Bigelow (1927), who first described it as a general clockwise Journal of Marine Systems 74 (2008) 528–544 ⁎ Corresponding author. E-mail address: g_shu@umassd.edu (S. Hu). 0924-7963/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jmarsys.2008.04.007 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys