Cruise Report

R/V OCEANUS Cruise 311
to Georges Bank

Acknowledgments

The array of moorings, set in January, was scheduled for recovery during this cruise. However a number of problems led to non-standard retrieval of gear and many hours of dragging. Our successes were only possible due to the maintenance of good humor during long hours of work by both the scientific party and shipboard personnel. The Boatswain, Horace Medeiros, deserves special recognition for his skill on deck.

This report was prepared by R. Schlitz with J. Manning.

This Research was sponsored by the NOAA Coastal Ocean Office under funding for the U.S. Global Ocean Ecosystems Dynamics Program Northwest Atlantic Field Studies, Phase 2

Table of Contents

List of Tables

List of Figures

1. Purpose of Cruise

2. Cruise Narrative

3. Cruise Results

4. Scientific Personnel on Cruise, 16-27 August 1997

List of Tables

Table 1. Summary of information on the moorings recovered during R/V OCEANUS (OC311)

Table 2. Event log for OC311

List of Figures

Figure 1. Stations occupied during R/V OCEANUS Cruise 311, 16-27 August, 1997 (OC311). The top panel shows the mooring sites and the bottom panel the location of CTD stations

Figure 2. CTD section across the southern flank of Georges Bank along the mooring line from data collected during OC311. The panels from top to bottom are temperature (C ), salinity (psu), density (sigma-t ), transmission (volts), and fluorescence (volts)

Figure 3. CTD section across Great South Channel along the mooring line from data collected during OC 311. The panels from top to bottom are temperature (C ), salinity (psu), density (sigma-t ), transmission (volts), and fluorescence (volts)

Figure 4. CTD section across Great South Channel south of the mooring line from data collected during OC 311. The panels from top to bottom are temperature (C ), salinity (psu), density (sigma-t ), transmission (volts), and fluorescence (volts)

Figure 5. CTD section across Great South Channel at the southern end from data collected during OC 311. The panels from top to bottom are temperature (C ), salinity (psu), density (sigma-t ), transmission (volts), and fluorescence (volts)

Cruise Report
R/V OCEANUS Cruise OC311
Woods Hole, MA to Woods Hole, MA
16-27 August 1997

1. Purpose of Cruise

The primary purpose of the cruise was to recover an array of nine moorings in the vicinity of Great South Channel as part of the U.S. GLOBEC Program Northwest Atlantic Field Studies, Phase 2. A secondary purpose was to describe the hydrographic regime around the moored array. The overall GLOBEC goal is to examine retention (in the form of recirculation around the inner part of Georges Bank) and losses from the southern flank of the Bank into Slope Water or westward into the Middle Atlantic Bight.

2. Cruise Narrative

The R/V OCEANUS departed Woods Hole at approximately 1400 EST on Saturday, 16 August 1997. The positions occupied are found on Figure 1. Upon leaving, the shipboard alongtrack sensors began recording, including meteorological (IMET) and sea surface parameters as well as 150 kHz narrow-band and 300 kHz broad-band ADCP systems. We arrived at mooring site 7 (Figure 1) on Sunday, 17 August in the morning. Scientific operations began with a CTD station nearby the surface marker. The mooring was released and recovered routinely. We then steamed to site 8 and did not find the surface buoy. Interrogation of the release was successful, indicating that it was in place and upright. The configuration of the mooring was unknown. A CTD cast was completed. We then moved to site 6, performed a CTD cast and recovered the mooring without any problem. At site 5 the surface float was missing as previously reported. Acoustic ranging to the tripod at the location indicated that it was in the same position as set. After steaming to site 4 the surface mooring was recovered routinely. There was one potential serious problem because a section of fishing net was draped around the orthogonal fans of the VMCM located at 5 m depth. Mooring operations stopped at about 19:30 local time.

During Sunday night CTD stations 4-10 (Figure 2) were completed across the southern flank, starting north of mooring 4 and ending south of mooring 7. On each station one salinity sample was collected at the bottom of the cast for calibration of the conductivity sensor on the instrument. Also water samples for O18 were collected in cooperation with R. Houghton of Lamont Doherty Earth Observatory

of Columbia University (also on other stations during the cruise). This sampling was done at each CTD cast during the cruise.

Mooring work resumed at site 9 on Monday morning. The surface mooring was recovered without problem. The VACM was missing the rotor, probably lost during setting. A CTD cast was then completed. The release on the tripod was then triggered but the buoy attached to the recovery line did not surface. Triangulation by using the transponder on the acoustic release showed that the tripod was in position. We then returned to site 5 and fired the acoustic release on the tripod. Again the marker for the recovery line did not appear. After triangulating to determine a best position we dragged unsuccessfully for three hours. Moving to site 3, the surface mooring was routinely recovered. The float on the tripod was released and again was not seen at the surface. We then returned to site 5 prepared to continue dragging but sunset came and operations on deck were suspended for the night.

A line of CTD stations (12-17) starting at site 4 and running westward along the mooring line across Great South Channel was done (Figure 3). The positions matched a section from the cruise in January (OC296). These CTD stations were completed during Monday night before steaming back to site 5. The watch reported that the surface buoy at site 1 was missing and the light at site 2 was extinguished.

On Tuesday morning the position of the tripod 5 was checked by triangulation before dragging again. On one pass the surface floats were briefly seen. When the ship returned to the location prepared to recover the tripod, the marker was no longer visible. Dragging resumed at site 5 for much of the day. We then steamed to site 4 and released the markers. As with the other tripods the markers were not seen at the surface even though the acoustic release reported no problems. Dragging began and continued into the evening. During one pass a peak tension of 19,500 pounds was registered. Upon recovery the depressor was missing and the stock of the grappling hook bent through a large angle. We decided to stop here and went back to site 3 to look for the surface float. Just before sunset the marker was seen very low in the water of a calm sea. The balls were captured but there was much difficulty transferring the load to the heavier haulback line. There was high tension during the entire process of haulback. At times the line was paying out in opposition to the hauling of the capstan. Only when at the surface could we see that an entire dragging net was draped over the tripod. The tripod finally ripped out a section of the net as it came out of the water. Inspection of the tripod after recovery revealed that the ADCP was missing. The welds broke where the ADCP frame attached to the tripod. The breaks looked fresh leading to the conclusion that the ADCP was lost on recovery. Mooring work halted for the night.

A series of CTD stations then began south of and parallel to the previous section across Great South Channel (Figure 4). Stations (18-25) were completed, starting about 15 km south of site 4 and moving westward.

At daybreak on Wednesday morning we searched for the surface float at site 1 without seeing anything. Moving to site 2 the surface mooring was recovered without incident. Since a gale was forecast for Wednesday night and Thursday and the seas were now slight we decided to search for the surface markers for the tripods before leaving for Woods Hole. Surface markers were not seen at sites 9 and 4. During dragging at site 5 the tripod was hooked and recovered with all instruments. The three legs were sheared off at the welds. The remaining time was spent at site 9 dragging until we left for Woods Hole at 20:00.

R/V OCEANUS arrived in Woods Hole at 09:00 on Thursday 21 August in bad weather. Equipment and people were exchanged. At 16:00 R/V OCEANUS departed Woods Hole into ESE gales and headed for site 9.

The work for Friday began at site 9 using the USGS side-scan system as an experimental tool. We expected to closely define the attitude and integrity of the tripod and the topography of the surrounding area. After towing the side-scan fish, dragging resumed for the remainder of the day. The first set released the floats by severing the line. There were instances of high tension along the line but the tripod was not recovered. A CTD cast was done before ending mooring recovery for the day.

During the night a CTD stations 26-30 were done across Great South Channel south of the prior sections (Figure 5).

Saturday morning dawned with foggy weather and was devoted to a side-scan survey at site 4. This is a region of complicated rough topography and boulder fields. In the afternoon the fog cleared in time to search for the surface markers before dragging again; they were not seen. Late in the afternoon the tension indicated that we had caught something. During haulback we saw that the surface floats were caught and wanted to stop below the surface to snag the line. Unfortunately the trawl wire continued hauling and the floats and recovery line came out of the water and slipped off thereby losing the tripod. Dragging continued late into the night, then was suspended.

Since the weather was good we decided to work at site 8 on Sunday. The intention was that the side-scan equipment would define the configuration of this mooring located in 325 m of water. A number of technical problems degraded the quality of returns (e.g. strong thermocline). After ending the side-scan we triangulated on the release and determined that the mooring had not moved and that the release was still upright. A depressor weight was placed on the trawl line at 200m above the hook. The mooring was snagged on the first pass. An Aanderaa current meter fouled on the depressor weight. There was good fortune since this meter was near the release and the mooring line parted just below the release. Recovery essentially was from the bottom up, without knowing how much of the mooring remained. In addition there were two tethers which could only be handled by fully extending the crane and taking bights. Remarkably the entire mooring was recovered including the surface marker which sunk compressing the foam.

The remainder of the cruise was a series of side-scan and dragging operations at different sites without recovering more instrumentation but cutting all remaining surface markers to the tripods and destroying a grapling hook at site 1. Final triangulations were done at each remaining tripod. After dragging for one of the missing towers (reported during the spring), R/V OCEANUS departed from site 2 at 21:30 on Tuesday and arrived in Woods Hole at about 09:00 on Wednesday 27 August 1997.

3. Cruise Results

The primary goal of OC296, recovery of an array of moorings to examine factors leading to retention or loss of water and plankton along the southern flank of Georges Bank, was partially successful. The disappearance of two surface moorings, sinking of a third, and "loss" of the tripods is serious. A combination of biological fouling from the duration of the moorings (resulting in lower buoyancy and higher drag) and spring tides during the cruise probably caused the surface floats to remain just at or below the surface.

A second related goal, characterization of the hydrography in the area surrounding the moored array was also only partially successful. Sections across the southern flank of Georges Bank and Great South Channel along the mooring lines were collected. However the time required for dragging precluded an extensive hydrographic study.

Noteworthy features in the section across the southern flank (Figure 2) are penetration of Slope Water (defined as > 34 psu) to the 60 m isobath, very warm near-surface Slope Water, > 24 C, and evidence for the loss of shelf waters at mid-depth in date from the transmissometer and fluorometer.

The sections across Great South Channel (Figures 3-5) have some interesting features. Within the channel along the mooring line structure is typical for summer. At both ends, Nantucket Shoals and Georges Bank, weakly stratified hydrographic conditions are found. Along the axis there is strong stratification showing temperatures and salinity values in the lower layer consistent with values in the cold band (Figure 1). A patch of high fluorescence is found over Nantucket Shoals. On the section south of the mooring line (Figure 4) warm fresh water is again at the surface.The presence of a layer of Slope Water along the axis is clear. The section at the intersection of Great South Channel and the continental shelf is less structured than the others.

The event log for R/V OCEANUS 311 is presented in Table 2.

Table 1. Summary of information on the moorings recovered during OC311. This is an array to study physical factors causing recirculation around Georges Bank and losses from the region.

THIS PAGE IS FOR THE EVENT LOG SO IT IS BLANK.

EVENT LOG SECOND PAGE

4. Scientific Personnel on Cruise, 16-27 August 1997