Report of Activities on the RVIB N.B. Palmer Cruise 02-04

13 September 2002

For the second day in a row, a bright sun came up over Arthur Harbor creating a picturesque sunrise. During the morning of 13 September, light winds and a calm harbor aided Kendra Daly in completing the calibration work with the HTI acoustic system. At the same time, MT Stian Alesandrini did a dive on the seawater intake system at Palmer Station to investigate a problem. The N.B. Palmer raised anchor about noon and headed east down the Bismark Strait along the inland passage route. This route was taken for three reasons. First, the waters were protected for most of the 20 hour steam and during that time much of the work outside on the deck breaking down equipment and packing it away prior to arrival in Punta Arenas could be done effectively. Second, the new CMiPS turbulence sensors on the CTD provided a unique opportunity to conduct a hydrographic survey of the zone within the Gerlache straits where water of very different origin come together (as described in the CTD Group Report below). Third, the transit along the inland passage provides spectacular views of the rugged snow and ice covered mountains of the Western Peninsula. On this day, we turned off the normal route along the passage and went through the Neumayer Channel. This narrow passageway is lined with steep sided mountains on both sides and ice cliffs running along channel edge, the end points for the glaciers on the mountains flanks. We passed by Port Lockroy, a favorite stopping point for cruise ships during the summer months.

Around 1600, we arrived at the first of seven scheduled CTD station locations in the Gerlache Strait spaced at 13 nm intervals. The casts at each station took one to two hours and by midnight, work had begun at the fourth station.

The morning sun gave way to increased cloudiness as we steamed along the inland passage. There were multiple cloud layers and along the route there were low cloud patches with intense flurry activity. The sun occasionally shone on the peaks of some of the mountains as we moved through the inland passage in the early afternoon, but the clouds thickened and a light snow began falling about 1700. The wind picked up about this time from less than 10 kts to around 25 kts and became predominately northeasterly. Air temperatures ranged from -4ºC to 0ºC throughout the day, while the barometric pressure made a slow decline from 1016 to 1012 mb.

CTD Group report (Eileen Hofmann, Bob Beardsley, Baris Salihoglu, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

About mid-day on 13 September we departed Palmer Station for the last part of cruise that will take us back to Punta Arenas, Chile. About three hours after departure, we entered the southern reaches of Gerlache Strait. At this point we began CTD/CMiPS casts at a series of stations located along the axis of the Strait. In total we intended to do 7 stations that were spaced at 13 nm intervals. During the afternoon and evening of 13 September we completed 4 of these stations.

Gerlache Strait provides a deep connection between Bransfield Strait and the west Antarctic Peninsula continental shelf. The cold, fresher Bransfield Strait water flows southward through the Strait and meets the warm, salty Circumpolar Deep Water (CDW) at the southern end. CDW also enters the Strait via deep channels on either side of Brabant Island. As a result there are well developed fronts at several locations along the Strait. Thus, the series of casts along the axis of the Strait was designed to characterize the inflow of modified CDW into Gerlache Strait, to characterize the frontal boundaries between this water mass and the outflowing Bransfield Strait water, and to provide repeat casts with CMiPS in areas of active mixing and exchange.

At each station CMiPS casts are done from the surface to 350 m. The number of casts is dependent on the observed temperature and salinity structure, but is usually two to three casts per station. Following the CMiPS casts, one additional cast is done which extends to within a few meters of the bottom. On this cast, water samples were taken at three depths for microzooplankton studies by Gallager and Thompson.

The stations occupied during 13 September showed the front at the southern end of the Strait between CDW and Bransfield Strait water and the inflow of CDW around Brabant Island, which appeared as temperature and salinity maxima at 200 m to 300 m. There was considerable fine scale structure apparent in the vertical temperature and salinity profiles, which suggests active mixing.

At the start of our cruise, almost seven weeks ago, we did an XBT transect along the axis of Gerlache Strait during the transit to the study region. Comparison of the temperatures observed then with those observed now show that considerable warming has occurred in the surface waters. The surface temperature was about 1.0 C warmer (-0.5ºC versus -1.80ºC) and was well above freezing. The Winter Water layer was eroded and there was the start of the development of the Antarctic Surface Water layer. These water properties indicate that spring is starting in this part of the Antarctic Peninsula.

ROV report (Scott Gallager, Phil Alatalo, Alec Scott)

ROV deployment 23 was performed at Station 17 (-66º 58.335′; -69º 29.787′) on 8 September. From the surface, the ice was 60/40 coverage with medium sized (10 to 100 m), broken first year floes, 50 cm thick, rafted into deep piles and interspersed with open water leads. After checking all systems the ROV was deployed at 2036 only to find that the port thruster was producing a current overload. When this happens, it means that water has entered the motor most likely through the lip seal of the main shaft. One of two things can occur: the thruster will continue to function, but eventually burn itself out because of the excess current draw, or the thruster motor controller board in the main body of the ROV will overload and burn out a power transistor. The procedure when this happens is to shut off the controller for the bad thruster inside the topside power pack, which we did. This left the ROV unable to follow a straight course and turn only in the direction of port. Knowing there would not be time to retrieve the ROV, repair the thruster and get back into the water at this station, we positioned the ROV facing aft and began a transect in a slow arc out away from the ship and then back in towards the starboard side about the position of the A frame. This worked well, so we repeated the operation with a longer tether lead and followed another arc shaped transect. The ice was rafted deeply so it was difficult to remain at one depth. However, even with these shortcomings, we were able to get two 100 m long transects. Unfortunately, the furcilia abundance was low with scattered animals below even the more rugged surfaces. The concentration was roughly 1 to 2 per cubic meter. A few ctenophores were noted but they were not in high abundance. The ROV was retrieved and the port thruster repaired by draining the oil-filled housing, rinsing with fresh oil, and refilling. This operation required about 45 minutes before we were back in action and ready for the next station.

ROV deployment 24 began at station 5 (-66º 24.316′; -68º 22.772′) on 9 September, the last station on the northern sector of the grid. Surface ice conditions were 40/60 coverage with flooded floes and open leads. The ROV was deployed at 2332 local time in a 25 kt northeast wind, 19ºC wind chill, and blowing snow. The deck crew was prepared with goggles and face covers to guard against the harsh conditions. All the tether was paid out as the ROV ran directly away from the starboard quarter and dove to 8 m where a good view of the under ice surface could be attained. The under ice surface was surprisingly rough given the smooth conditions observed from the surface. During the first transect, a few small swarms of 50 to 100 furcilia were noted embedded in the upper ice crevasses. The ROV turned 90º towards the bow of the ship and proceeded on a bearing of 30º. A strong current took the ROV to the north and eventually under the ships hull. This would not have been a problem with the exception that the tether had, in the meantime, wrapped around the starboard rudder and through the opening between the two main screws. The ROV tether had become lodged between the ships hull and a large block of ice up under the hull just forward of amidships. The ensuing one and a half hours were a bit tense as the ROV was jockeyed around to get a better look at the snag. Unfortunately, the scope between the snag and the ROV was short enough to prevent turning a full 180º.

First Mate Mike Watson, Ice Pilot Vladimir Repin, Second Mate Paul Jarkiewkz, and Third Mate Rachelle Pagalunan, all were observing the conditions and suggesting potential moves. In a final attempt to dislodge the ROV, Captain Joe began gently swinging the forward thruster back and forth towards the starboard side and aft. Within a few seconds, we could see water moving past the ROV. The ROV dove hard and suddenly we could feel some freedom in the tether. With the ROV at safe depth of 40 m, the deck crew began pulling the tether free from the rudder. Soon, and with a sigh of relief, the deck crew reported the ROV lights in sight and retrieval operations commenced.

On 11 September, the first (#25) of two ROV deployments took place at the 24 hour time-series station just south of Renaud Island (-65º 10.382′; -65º 37.560′) during daylight hours. The purpose of this last set of deployments was to determine if furcilia undergo diel vertical migration. Some forms of plankton swim up into the water column at night and down to depth during daylight hours. There are many potential reasons for this behavior, but the most likely one appears to be avoidance of visual predators during the day. Adult and juvenile krill have been observed to migrate hundreds of meters a day, but vertical migration in their furcilia stage larvae has not been observed. The Renaud Island deployments began at 0909 local time in full light. The surface ice conditions were 90/10 flooded floes with 30 to 50 cm snow cover. The ice team proceeded cautiously on what could have been dangerous conditions. The 1-m MOCNESS had just collected large quantities of juvenile krill in the upper 25 m and seals and penguins were sighted in the area. The ROV immediately came across large swarms of furcilia rafted up into crevasses between floes. After completing the first transect arc out to the starboard side and then aft, we came across one swarm that was in direct association with the ice surface. A small pocket about 30 cm across appeared to contain thousands of furcilia. Larvae were swarming in the water and crawling on the ice surface like ants on a dirt mound. The ROV kept the 3D VPR cameras fixed on this swarm for more than 12 minutes allowing concentration estimates and swimming behavior observations to be conducted. Three more transect lines were completed and the ROV was retrieved after one hour 30 minutes of observations. By far this station had the greatest number of furcilia, and they were present in bright sunlight. Estimated concentrations range from 5,000 to 100,000 larvae per cubic meter in swarms and 200 to 1000 per cubic meter on average. At the end of the deployment, the ROV dove to a depth of 50 m and turned upwards to view the tapestry pattern produced by sunlight projecting through the snow covered ridges and open leads.

The second ROV deployment (#26) was performed at the 24 h time-series station just south of Renaud Island (-65º 07.709′; -65º 30.728′) close to where ROV 25 took place, but during night time hours. Surface ice conditions were the same as in ROV 25 with loosely packed floes which were flooded and snow covered. The ROV was deployed at 0046 on 12 September in a flurry of adult krill swarming at the surface. This phenomenon had not been observed before on this cruise so it attracted a number of people to the sides of the vessel. The ROV observed many adults, juveniles, and stage 6 furcilia, but the furcilia were not swarming as seen earlier in the day. On average about 10 to 100 furcilia per cubic meter were observed. As a side note, divers observed very dense furcilia concentrations during the daylight hours. The conclusions based on this and all our observations of day vs. night time deployments is that there does not appear to be any diel vertical migration in response to time of day or daylight in young furcilia stages. In fact, more intense swarming behavior was observed during day time deployments than at night. One reason for this may be that larvae are attracted to light because sunlight is needed for algal growth and ice algae and microplankton are their major prey. Predator avoidance does not seem to play a major role in their behavior. The under ice surface may provide sufficient refugia from predation and serve as a critical nursery with an abundance of food where sunlight can penetrate as a source of energy for photosynthesis. Even if the ice algae at this time of the year are not actively photosynthesizing, they could be releasing organics that act as an attractant to the hungry furcilia.

Current Position and Conditions

The N.B. Palmer has left the inland passage and is now proceeding across the continental shelf towards the Drake Passage. Along the route XBTs and XCTDs are being taken. Our current position at 2155 on 14 September is -62º 15.9950′S; -62º 31.077′W. It is cloudy and very foggy. The air temperature is -0.9ºC and the barometric pressure is 1007.1 mb. Winds are 9-10 kts out of the north-northwest (352). There are occasional patches of pack ice, but for the most part we are steaming in open water.

Cheers, Peter