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

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 mm) 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. The continental shelf ecosystem of the MAB and Georges Bank (Fig. 1) is among the most productive in the world (O’Reilly and Bush 1984; O’Reilly et al. 1987). Understanding variability in oceanic primary production has been greatly augmented by synoptic distributions of phytoplankton pigment concentrations derived from satellite remote sensing. The first instrument for remote sensing of ocean color, the CZCS, yielded almost 8 yr of imagery between fall 1978 and summer 1986 (Hovis 1980; Feldman et al. 1989). CZCS imagery of this region has been used to study the spring bloom (Brown et al. 1985; Eslinger and Iverson 1986; Walsh et al. 1987a; Gregg and Walsh 1992), spatial and temporal 1 Present address: Monterey Bay Aquarium Research Institute, P.O. Box 628, Moss Landing, California 95039.