| SABSOON : Cross-Shelf Exchange |
The towers network is well situated to study cross-shelf exchange and its impact on primary and secondary production in the SAB. The region they occupy has been identified as a likely site for enhanced production on the shelf (Lee et al. 1991), and an area where freshwater may preferentially escape from the inner shelf(Blanton et al . 1994). The outer shelf of the SAB is strongly influenced by the Gulf Stream. Exchange of material between the shelf waters and the Gulf Stream is primarily the result of instabilities along the western edge of the Stream. Cyclonic cold-core eddies and meanders propagate northward along the shelf, bringing cold, nutrient-rich waters onto the outer shelf of the SAB. The extent to which the eddies intrude onto the shelf is dependent on atmospheric conditions and seasonal variations in the shelf hydrography.
The inner shelf of the SAB is occupied by low-density water of riverine origin that forms a coastal frontal zone. Flow in cross-shelf transport has been characterized as episodic. A persistent pool of reduced salinity water over the mid-shelf off of Brunswick, Ga, suggests a preferred path for removal of fresh water from the nearshore environment near the southern boundary of the towers array.
Cross-shelf and along-shelf currents determined with the bottom-mounted ADCP near M2 reveal a polarization of tidal frequency motion in the cross-shelf direction and sub-tidal (wind-forced) motion along-shelf (See figure below, top two panels). ADCP echo intensity (bottom panel) suggested diurnal migration of organisms between the sandy bottom and water column.
Stratification at R2 has been intermittent as shown in the figure below. In summer only brief periods of thermal stratification have been observed, while there was strong haline stratification during winter 2000. These observations are contrary to prior general descriptions of a well-mixed mid-shelf region in winter and thermal stratification in summer.
Large weekly to monthly period variability in salinity during winter 1999-2000 suggests enhanced cross-shelf exchange. Due to a prolonged drought, mid-shelf salinity has been high since the summer of 2000.