We are now at Station # 41 (-67 11.640°S; 74 29.762°W about 1500 local) and are finally able to resume our normal station operations in spite of continued strong winds of 33 to 38 kts out of North Northeast (020). This is an improvement over the conditions that have prevailed the past 24 to 36 hours. Skies are darkly overcast with the clouds coming down to the sea surface such that visibility was only a few hundred meters. There was a lot of sea spray as the ship quartered into the sea during the transect out across the shelf on survey transect #6, but very little precipitation.
Yesterday was by far the roughest day of the cruise thus far. Shortly after the deployment of BIOMAPER-II around 0100, the wind and seas began to build and continued building the entire day. By early evening sustained winds were over 40 kts with frequent gusts in the 50 knot range and occasional gusts over 60 kts. It was not until the early morning of 9 May (today) that the winds began to drop into the low 30 knot range and the seas started to lessen. BIOMAPER-II, however, was left in the water and continued to towyo between stations as we steamed along the survey line at speeds of 4 to 5 kts.
Eileen Hofmann reports that as of 8 May the CTD group has completed casts at 35 of the survey stations. Due to the strong gale (Beaufort Scale 9-10) on 8 May, we had to suspended CTD operations due to dangerous working conditions. As a result we did not do CTD casts at survey stations 37, 38, 39, and 40. At these stations we dropped XCTDs, which do not require the ship to stop. However, these instruments still require that someone hold a launcher over the side of the ship. After the second XCTD drop it was decided to move the launcher from the main deck to the 01 deck. A large wave over the side and winds gusting in excess of 50 knots prompted this decision.
Good vertical profiles of temperature and salinity were obtained from all the XCTD drops. These good data are due in large part to the efforts of the Raytheon MTs Matt Burke and Dave Green. Additional sampling at the XCTD stations consisted of taking a surface water with a bucket. This water was used for primary production and nutrient measurements and to obtain protozoan samples.
The horizontal temperature distribution below 200 m shows that the southern boundary of the ACC is located along the outer edge of the sixth survey transect. Modified Upper Circumpolar Deep water, determined by the location of the 1.4°C isotherm, overlies the central portion of the shelf. The isotherm pattern suggests that there may a meander in the southern ACC boundary. The existence of this feature will be determined as we now begin to move inshore along the seventh survey transect.
Bob Beardsley has provided a description of the CTD in use on the R/V Palmer. Seawater is a mixture of pure water and dissolved materials (e.g., sodium chloride or NaCL) that are collectively called "salt". The amount of "salt" present in a sample of seawater changes the density and electrical conductivity of the sample. Over the years, oceanographers have developed a quantitative scale for the amount of salt in seawater (called "salinity") and methods to measure the "salinity". Since the density of seawater depends directly on its temperature and salinity, in-situ measurements of temperature and salinity as a function of water depth are essential to characterize the 3-dimensional structure of the ocean and its currents. Also, since there are no internal sources or sinks of salt within the interior of the ocean, the salinity of a water parcel is conserved (i.e, remains constant) as the parcel moves in the ocean, thus providing an excellent tracer of water movement through the ocean.
The CTD is the primary instrument used today to measure the temperature and conductivity of seawater as a function of depth (hence the name Conductivity/Temperature/Depth). It is lowered on a conducting wire from the ship to near the bottom and sends up the wire to the CTD deck unit the measured values of pressure, temperature, conductivity and data from any other sensors mounted on the CTD. The deck unit both records the data during the CTD cast and displays the data on a computer screen for the operator to monitor. After the cast, the pressure, temperature and conductivity data are used to compute salinity and density. The CTD being used on this cruise also carries sensors for dissolved oxygen (another excellent tracer for water), light transmission (a measure of water clarity), fluorescence (a measure of phytoplankton concentration), and photo synthetic available radiation (PAR) (a measure of the amount of sunlight available for plants to use in photosynthesis).
The CTD itself is mounted inside a pipe cage to protect the instrument and its sensors, and 24 water sample bottles are mounted on a "rosette" above the CTD. Before each cast, these bottles are opened, and cocked so that they will close on command using the CTD deck unit. The CTD operator watches the profile data being collected as the CTD is being lowered continuously to the bottom. Once the CTD has reached the bottom (actually, it is stopped about 5-10 m above the bottom), the operator decides at what depths to "fire" the bottles to collect water samples for chemical and biological analysis. On this cruise, bottles are used to sample at the bottom, and 50 m, 30 m, 20 m, 15 m, 1 0m, 5 m, and the surface (about 2 m) at all stations, and at other depths depending on the profiles and water depth. Once the CTD is brought back on board after the cast, different investigators draw water samples for nutrient, dissolved oxygen, salinity, chlorophyll, and phytoplankton and microzooplankton analysis.
The CTD is typically lowered over the top 50 m at a rate of 30 meters per minute, then the lowering rate is increased to 45 meters per minute. Thus, a cast to say 500 m would take only about 12 min for the CTD to reach the bottom, then about twice that long to raise the CTD back to the surface, stopping at selected depths to "fire" or "trip" the sample bottles. While most of the casts made on this cruise so far have been between 300 and 600 m, one deep cast to 3350 m has been made. This station took roughly 75 min for the CTD to reach the bottom, and then another 2 hours to bring the CTD back to the surface and onboard the ship with 21 full water samples. The CTD team, the scientist monitoring the CTD data being recorded on the computer screen, the winch operator controlling the CTD winch, and the marine tech monitoring the positioning of the wire relative to the side of the ship, all must be especially patient during the deeper casts. The CTD also carries a bottom sonar, which allows the distance the CTD is above the ocean bottom to be monitored on the ship. Using this approach, even in rough weather with large swell, the CTD can be safely lowered to within a few meters of the bottom in depths as deep as 5000 m (when there is more than 3 miles of wire out).
The CTD aboard the N B Palmer is a SeaBird 11 plus CTD. It is equipped with two separate temperature and conductivity pairs of sensors, a primary and secondary set. Water is drawn through each sensor set by a small pump, so that the temperature and conductivity sensors measure the same water at the same time. Temperature is measured with a very stable thermistor, while conductivity is measured with a 4-electrode cell. After 50 CTD casts, we have made a comparison of the temperature and conductivity data recorded at the same time when water samples were collected at the bottom and top of each cast (where the water tends to be well mixed, making it a good "bath" for these comparisons). The primary and secondary temperature sensors have a mean difference of 0.0013°C, while the conductivity cells differ in the mean by 0.0001 S/m. The mean difference in the primary and secondary salinity is only -0.0012 psu, and the mean difference between the primary and bottle salinity is only 0.0009 psu. This is excellent agreement, and means that the Palmer CTD is collecting very high quality data. In our study area on the west Antarctic Peninsula shelf, the water temperature varies from about -1.0 to 3°C, and the salinity from about 33.0 to 34.6 psu. Thus the temperature is being measured to one part in ten thousand, and the salinity to one part in a hundred thousand. We all hope the CTD continues to work as well during the rest of the cruise.
While the CTD and rosette are rugged instruments, they can be easily damaged if the CTD is dragged or bounced along the bottom or crashed against the side of the ship as the CTD is launched at the start of the cast or brought back onboard the ship at the end of the cast. Over the last 18 hours (May 8), the winds have increased from less than 10 kts to a steady 50 kts, meaning we are now in a strong gale. During this period, the seas continue to build, and the waves are now about 6-9 m high! During these conditions, it is too rough to make a CTD cast. The CTD could easily be smashed against the side of the ship by a large wave, and the crew handling the CTD during the launch and recovery could be seriously hurt by a swinging CTD or a large wave into the CTD room. The CTD, cage, and rosette with bottles weights over 500 pounds, so it would hurt to be hit by it as the ship moves around. Fortunately, we can still make a "CTD" cast using a XCTD, an eXpendable Conductivity Temperature Depth instrument. The XCTD is a small torpedo-shaped probe containing small temperature and conductivity sensors, which drops down as a fixed speed remaining connected to the ship by a very fine conducting wire. The temperature and conductivity data are recorded as a function of time from launch, which can be used to compute the probe depth since its descent rate is well known.
Normally the XCDT is launched from the main deck, but today it is too rough to even go out on the main deck. Fortunately, we can drop the XCDT from the next level above the main deck, and the instrument worked great down to a depth of 500 m. We will continue to stream along our survey line using XCDT's to collect our profile data until the gale has blown itself out and conditions improve to where the CTD can be safely used again.
Today, Chris Ribic and Erik Chapman surveyed for 4 hours 18 minutes during the day as we transited between stations 35 and 36. Winds were steady at 35 knots and spray coming off the bow was drenching the observation box outside on the bridge wing, so they made observations from inside the bridge. They saw a dramatic shift in species assemblage today as we traveled further from Marguerite Bay toward the shelf break. No longer were Kelp Gulls and Southern Fulmars seen in high numbers as they had in the bay and once again Antarctic and Blue Petrels appeared, two species that are typically associated with the open ocean. The Blue Petrel was the most frequently observed bird for the day. Kelp Gulls are not typically found further than 15 to 20 km from land, so their absence from today's surveys was not surprising. Again, they saw just one Southern Giant Petrel today. They continued to see Southern Giant in dark plumage, indicating that they are immature and juvenile birds. Southern Giant Petrels are long lived birds and do not develop their light gray and white adult plumage until they are five to seven years of old. They also observed Southern Fulmar and Cape Petrels feeding in a mixed species group, dipping their heads into the water to snatch zooplankton from just below the surface of the ocean.
Night surveys were continued using night-vision goggles during three half-hour surveys through the night. During these surveys, they observed Snow and Cape Petrels in low numbers.
Here are the day's final counts:
|4hr 18min||1 hr 30min|
|Antarctic Petrel (Thalassoica antarctica)||4||0|
|Cape Petrel (Daption capense)||6||2|
|Southern Fulmar (Fulmarus glacialoides)||8||0|
|Blue Petrel (Halobaena caerulea)||18||0|
|Southern Giant Petrel (Macronectes giganteus)||1||0|
|Snow Petrel (Pagrodoma nivea)||7||1|
|Kelp Gull (Larus dominicanus)||0||0|
|Unidentified Storm Petrel||1||0|
Ari Freidlander reported that no visual whale observations were made today due to poor weather conditions.
Catherine Berchok has not been getting the range out of the Sonabuoys
that she is accustomed to, so on May 7th, a new cable was strung to the
"stick" antenna on the science mast. It turned out that the cable was not
the problem. Later in the day, the antenna was replaced with the spare
antenna/preamp leaving the new cable in place. Now she seems to be getting
around 10 miles of range instead of a much shorter range. A difar Sonabuoy
was deployed to test the earlier cable replacement in the presence of a
minke whale (the smallest of the baleen whales). Possibly she recorded
two grunts from it, but it is really uncertain. Usually they will make
a train of pulsed grunts, so what was recorded might have been ship related
sound. However, by the time the new antennae was in place, She started
to pick up faint humpback moans that got louder as the ship was positioned
to do the MOCNESS tow. There seemed to be at least two humpbacks singing
at the same time. For May 7th, She recorded for approximately seven hours, with about two hours devoted to fixing the antenna.
On May 8th, Catherine spent most of the day working to get the sound files onto the public drive, which took longer than expected. As a result, no Sonabuoys were deployed on this day. Thus far this cruise, the sound files include recordings from orca, possibly minke, humpback, finback and possibly blue whales.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
As noted in the May 7th report, BIOMAPER-II was deployed about 0100 right at the end of Station #35 and began to be towyo'd between 0 and 250 m on the transit to Station #36. As described above, the weather deteriorated to the point where no CTD or net towing could be attempted; the scheduled MOCNESS tow did not take place at Station #37. BIOMAPER-II, however, remained in the water and was towyo'd along the transect in spite of the wind and seas. The reason this was possible was due largely to the Dynacon winch and slack tensioner system which was able to compensate for the motion of the ship when the towed body was near the sea surface. Despite the severe pitching as the ship steamed into the large seas, the slack tensioner took up most of the slack in the towing wire when the stern was moving down and paid out wire when the stern was lifting. Thus, the towed body experienced very little ship induced motion and cable tensions remained low.
In spite of the relatively short day length, the 120 kHz and 200 kHz echograms show evidence for diel vertical migration of some of the plankton living in the upper 100 m at night down to daytime depths of from 150 m to 300 m.