The character and motion of suspended particulate matter over the shelf edge and upper slope off Cape Cod

Abstract

Time series of suspended particulate matter (SPM) concentration made by transmissometers deployed on the shelf edge and upper slope off New England are examined together with data from companion sensors. At the shelf-edge instrument cluster (on the 125-m isobath) the highest SPM concentrations were apparently due to sediment resuspended further onshore and advected to the transmissometer by offshore motion of the shelf-edge density front. This was due to a tendency for more frequent episodes of high bottom stress, and thus more sediment resuspention, progressing onshore from the outer shelf. The decrease of surface wave current amplitude with depth was primarily responsible for this cross-shelf gradient in bottom stress. On one occasion very turbid water was observed at the shelf edge when the bottom stress was low and the frontal motion was small. Data obtained from the U.S. National Marine Fisheries Service showed unusually intensive bottom trawling activity in the vicinity of the transmissometer during this event. This data further indicates that resuspension by trawlers may be an important source of suspended material on the shelf edge. The record of the transmissometer deployed over the upper slope indicates that transport of particulate material in suspension from the near bottom on the shelf into the slope region occurred primarily along density surfaces, and was not continuous but intermittent.