Evidence for Biologically Driven Seasonal Topographic Changes on the Inner Shelf of New Jersey

High-frequency multibeam sonar surveys conducted in August 2001 and May 2002 as part of a study of essential fish habitat at the LEO-15 research site (a sand ridge offshore New Jersey) have provided new insight into seasonal variability on the inner shelf. LEO-15 lies in 8 to 15 m of water approximately 4 km off Little Egg Inlet. The data were acquired aboard the R/V Seawolf using a hull-mounted Simrad EM 3000 multibeam system operating at 300 kHz. The swath produced by the EM 3000 is approximately four times water depth, and the resulting data contains bathymetry as well as backscatter information. A few grab samples were taken in May 2002 to ground truth the backscatter data. The topography and backscatter patterns in August and May are quite similar, although the May survey revealed that numerous mounds developed over the winter of 2001-2002. Individual mounds are up to one meter high and from 10 to 100 m across, and at least 500 mounds were identified in a 12.5 sq. km area. The sediment of the mounds is distinctively different from the sand-size sediment of the ridge. The mounds also have distinctive low backscatter. A grab sample from a mound recovered fine-grained sediments with numerous tube-building polychaetes (Asabellides oculata) at abundances in of well over 1,000 individuals/sq. m. Settlement of A. oculata apparently occurs in the fall, perhaps as a result of increased sediment output from local estuaries, and lower finfish predation in the winter months allows the population to bloom. The dense population of A. oculata tubes trap fine-grained sediments creating a topographic feature. It is not known how long these mounds remain distinctive features, or whether they are created every year. Occasional tube mats of A. oculata have been identified during studies of existing and potential sand borrow pits in the inner shelf, although the factors that control the distribution of the tube mats have not been well understood. Comparison between the topographic and backscatter patterns of the two surveys suggests that the mounds at LEO-15 tend to have developed in areas where low backscatter sediments were at or near the surface in August 2001, although perhaps covered by a thin layer of sandy sediment. These pre-existing fine-grained sediments are most likely old estuarine sediments that are now exposed at the sediment-water interface as a result of beach retreat and erosion. This analysis suggests that the distribution of these tube worm mounds is not random but depends on the nature of the underlying strata.