Enhanced chlorophyll at the shelfbreak of the Mid-Atlantic Bight and Georges Bank during the spring transition

Ryan, John P., James A. Yoder, Peter C. Cornillon

Limnol. Oceanogr., 44(1), 1999, 1-11 | DOI: 10.4319/lo.1999.44.1.0001

ABSTRACT: In 8 yr (1979-1986) of Coastal Zone Color Scanner (CZCS) imagery, we find annual enhancement of chlorophyll at the shelfbreak of the Mid-Atlantic Bight (MAB) and Georges Bank during the spring transition from well-mixed to stratified conditions. Spatial and temporal extents of enhancement vary interannually, and expression is intermittent intraannually. This feature can span the entire MAB and southern flank of Georges Bank (~1,100 km) and can be expressed for as long as 10 weeks (mid-April to late June). Pigment concentrations within the feature average more than two times that of adjacent shelf and slope waters. Enhanced shelfbreak chlorophyll consistently coincided with the shelf-slope front and often extended inshore of the surface outcrop of the front a few to ~10 km. In all years except 1986, it coincided with seaward entrainment of shelf water by Gulf Stream warm-core rings (WCRs) or meanders. Shelfbreak chlorophyll enhancement was most pronounced during 1980. Using satellite and in situ observations, we found that during 1980, it coincided with the shelf-slope front for 10 weeks, and, unlike the spring bloom, it was dominated by the nanophytoplankton (<20 um) size fraction. During the peak of the 1980 occurrence, four WCRs simultaneously interacted with shelf water, and chlorophyll enhancement inshore of one WCR coincided with a slope-water intrusion onto the shelf. Empirical orthogonal function (EOF) decomposition of CZCS images for late March-June 1980 showed that shelfbreak enhancement was strongly pronounced in an EOF that accounted for >10% of the variance about the mean. This annual biological feature, brought to light in satellite ocean color imagery, is an important aspect of the shelf-slope ecology of the MAB and Georges Bank.

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