SOURCES AND CONTROLS OF SUSPENDED SEDIMENTS AND TURBIDITY IN THE GREAT BAY ESTUARY, NEW HAMPSHIRE

The Great Bay Estuary, New Hampshire is a mesotidal, tide-dominated embayment characterized by strong tidal currents, bedrock outcrops, and a relatively limited sediment supply. Coarse-grained tidal channels and muddy sand to sandy mud intertidal flats are the major depositional environments in the lower estuary. In contrast, the upper estuary has large fine-grained subtidal shoals, large muddy tidal flats, and relatively thick fine-grained sedimentary deposits. Extensive bedrock outcrops occur along the flanks and shallow subtidal portions of the upper and lower estuary. Previous studies using high-resolution observations of physical and sedimentological processes in Great Bay Estuary demonstrated over most of the year the major control of turbidity and suspended sediments in the water column was resuspension of bottom sediments from the large shoal areas in the upper estuary (Little Bay and Great Bay). In order to better understand the processes controlling resuspension of sediments from these shoal areas and subsequent transport and distribution throughout the estuary, detailed observations from closely spaced stations were made under varying physical conditions. Approximately 30 to 40 sampling stations were sampled during four cruises in 1999 and 2000 within the upper six kilometers of the estuary. The four cruises occurred during or immediately following strong winds (wave resuspension), maximum yearly spring tide (tidal resuspension), high river discharge (riverine inputs), and a phytoplankton bloom (biological impacts). Initial results indicate major resuspension occurred during the wind event increasing turbidity significantly (light transmission values as low as 5%). There were strong horizontal gradients in turbidity suggesting transport from the large shoal areas into the main channel and subsequently to the middle and lower estuary. It appears these shoal areas act as temporary storage sites for fine-grained sediments introduced into the estuary via the riverine tributaries.