EPISODIC STORM CURRENTS SORT THE STELLWAGEN BANK SEABED INTO MAPPABLE SUBSTRATES, ONE OF WHICH IS THE PREFERRED HABITAT FOR SAND LANCE (AMMODYTES), A MAJOR FORAGE FISH

The seabed of Stellwagen Bank (25-70 m water depth) located off eastern Massachusetts is sand and gravel of glacial origin. A multibeam sonar survey of the central part of the Stellwagen Bank region (360 km²) revealed the presence of sand sheets, whose morphology suggests sediment movement is principally westward from the bank’s gently-sloping east flank (8 km wide) onto its more steeply-sloping west flank (3 km wide). Published sediment transport modeling studies predict that currents generated by major storms from the northeast quadrant cause episodic erosion of the bank. Bathymetric imagery was groundtruthed using 881 grain size analyses of surficial sediment (0-2 cm interval). Plots of individual grain sizes (phi classes) along east-west transects across the bank indicate that three major substrate types have formed due to segregation of sediment transported by currents from the northeast. Mud (11 to 5 phi) is present at the foot of the west flank (>70 m) but is almost absent on shallower parts of the bank. Fine-grained sand (4 and 3 phi) is present on the lower west flank (50-70 m) but not on the crest or east flank. A major transition in seabed composition occurs at 40 to 50 m on the west flank, at the leading edge of a large sheet of coarse-grained sand (2, 1, and 0 phi) that extends eastward across the bank crest and covers the east flank to 50 m.

Sand lance (Ammodytes) are small (10-20 cm long) schooling fish that are a major food source for marine mammals, fish, and seabirds on Stellwagen Bank as well as in other areas of New England and northern Europe. Coarse-grained sand substrate with very low mud content is the preferred habitat for sand lance who feed on zooplankton in the water column by day and hide in the sandy seabed by night to avoid predators and to hibernate during the winter. In the study area, the coarse-grained sand substrate covers the bank crest and the east and west flanks to a water depth of 50 m, an area of ~180 km². It is likely that substrate types produced and maintained by storm-driven sediment transport are habitats for recognizable communities of invertebrates and fish.