CZCS chlorophyll patterns in the South Atlantic Bight during low vertical stratification conditions

Abstract

We examine all available high-resolution Coastal Zone Colour Scanner (CZCS) data for the South Atlantic Bight (SAB), and adjacent Gulf Stream, during low vertical stratification months (October–May, from 1978 to 1986) to investigate the seasonal/long-time variations and the dominant spatial scales and patterns of near surface pigment concentrations. Each CZCS image is divided into four regions and 21 areas, as well as into 24 across- and 21 along-shelf transects. A distinct seasonal cycle is observed everywhere, superimposed to a linear time decay that reached almost 50% by the end of the mission. Mean winter SAB pigment concentrations (1.3 mg m−3) are about 50% greater than spring values (0.8 mg m−3), confirming that winter chlorophyll production represents a significant fraction of the total year's production. Mean pigment concentration (and absolute variability) generally decreases seaward, in winter from 2.2 mg m−3 (1.3 mg m−3) over the inner shelf to about 0.3 mg m−3 (0.3 mg m−3) over the Gulf Stream. Local maxima take place in the inner and middle shelves off all capes, in the northern region separated by some 150 km. High relative pigment variability is observed both along and across-shelf in narrow bands (order 10 km) on the middle and outer shelves (except across-shelf off Charleston). Along-shelf dominant scales of pigment variability increase offshore, from some 50 km in the inner and middle shelves, possibly related to squirt type fingers, to some 90 km in the outer shelf, related to meander-streamer patterns, and up to 170 km in the Gulf Stream, associated to the meanders themselves. High pigment concentrations in the inner shelf appear to be overestimated due to high turbidity water, permanently off the capes and on a seasonal basis (fall/winter maxima) within the inner and middle shelves. There are instances of squirts that emanate from all along the coast, although the dominant feature in the northern inner and middle shelves are along-shelf filaments stretching from the capes. Cold air outbreaks are responsible for the frequent vertical redistribution of chlorophyll and nutrients, which may originally be associated to Gulf Stream intrusions, and cause that winter productivity is not nutrient-limited