Patterns and controls of the suspended matter distribution over the shelf and upper slope south of New England

Abstract

A well defined pattern of suspended particulate matter (SPM) distribution is seen in transmissometer transects taken on the shelf and upper slope south of New England during eight SEEP-I cruises carried out in different seasons of the year. Higher concentrations of SPM are located in the shallow part of the shelf (<60m), and along the foot of the shelf-slope front. Between these two areas SPM concentrations decrease to relatively low values. Seaward from the foot of the front, SPM concentrations decrease, but a significant increase is clearly detected near the shelf break where an intrusion of slope water, seen as a salinity maximum, intersects with the seabed. This distribution pattern is quite persistent throughout the year, although SPM concentrations change according to hydrodynamic and biological conditions. Persistent maxima in near-bottom SPM concentration, associated with the foot of the shelf-slope front and with the slope water intrusion, had not been observed before, either in this or in any other area. The SPM distribution in the study area is controlled by several different processes. An SPM maximum of primarily biological material is associated with the spring-summer thermocline, but not at other seasons. Surface waves generate persistent high SPM concentrations over the shelf shallower than 60 m. The high SPM values along the foot of the front and along the intersection of the slope water intrusion with the seabed are produced by the interaction of processes such as internal waves, and tides with these hydrographic frontal gradients. Sediment resuspended by these mechanisms is transported along the isobaths of the shallow shelf, along the foot of the shelf-slope front, and along the intersection of the slope water intrusion with the seabed, following the advective, mean flow. This distribution of near-bottom SPM is not related to the nature of the underlying sediment, but is controlled by the interaction of hydrographic and dynamic processes.