PHYTOPLANKTON GROWTH AT THE SHELF-BREAK FRONT IN THE MIDDLE ATLANTIC BIGHT

The summertime front near the shelf break in the Middle Atlantic Bight is both thermohaline and baroclinic. Near the surface, large gradients of temperature (T) and salinity (S) exist with little cross-frontal variation in density. At depths > 50 m, an isopycnal boundary separates Slope Water from colder, fresher shelf water. Higher concentrations of chlorophyll are found in the upper part of the front, between water types of shelf and Slope Water origin. Calculations show also that the front is a region of enhanced phytoplankton growth. It is proposed that the relative fertility of the front is the result of large-scale deformations of the T/S boundary between shelf and Slope Water. The entrainment of deep shelf water along the shallowing, seaward-sloping, isopycnals in the deeper part of the front by these large-scale perturbations bring turbid, nutrient-rich water into clearer water that is also nutrient poor. The combination of this nutrient enrichment and a well-lighted water column makes the front more productive than elsewhere.