Retention Processes - Moorings and Highly-Resolved Hydrography

Ronald Schlitz and Mark Berman

We make physical and jointly physical/biological measurments as part of the 1997 GLOBEC intensive period focussing on sources, retention and losses of zooplankton and ichthyoplankton over Georges Bank. We focus on issues involving retention (in the form of recirculation around the inner part of the Bank) and losses from the southern flank of the Bank into the Slope Water and westward into the Middle Atlantic Bight. Our component will be a seven month, nine-element moored array on the southwestern end of the Bank to investigate the local kinematics and dynamics of recirculation and to observe properties of the shelfbreak front (which is in effect the southern boundary of the Bank). Second, we will participate on the SeaSoar surveys (for details see Brink and Lee section) providing a Tracor Acoustic Profiling System (TAPS).

The array will consist of eight mooring sites across Great South Channel and the southern flank of Georges Bank. An additional site will be in the turning area from alongshelf flow to inflow. Each mooring site will have a slack surface buoy supporting a Vector Measuring Current Meter (VMCM) or Vector Averaging Current Meter (VACM) at about 5 m depth. A subsurface mooring will hold a VACM at about 15 m (sites 1, 2, 6, 7, 8), and additional Aanderra current meters will be placed deeper on moorings at offshore sites (6, 7, 8) to sample the vertical extent of entrainment. At sites 3, 4, 5, and 9 we will deploy a USGS tripod with an upward looking Acoustic Doppler Current Profiler (ADCP). This will allow a detailed (bin size 1-2 m) examination of the recirculation that appears critical for plankton. Finally a second slack surface buoy will be placed to bracket the subsurface mooring. Every buoy with a current meter will be equipped with an Argos transmitter to provide location during the deployment. Temperature and conductivity will be sampled at each current meter, and within the water column at ADCP sites using Seacats and USGS instrumentation associated with the tripods.

We will have two cruises during 1997. The first in January (12 days) will set the moorings and carry out a CTD survey around the array to determine the hydrographic conditions. The second cruise in August (12 days) will recover all instrumentation and do a final CTD survey. Continuous ADCP measurements will be taken as part of normal shipboard sampling to provide a snapshot of the currents in the area.