Livingston Island, Antarctica, viewed from the LM Gould (November 20, 2011).
Photo Peter H. Wiebe

WHAT’S IN SEAWATER?

Figure 1. Deploying the CTD and Niskin bottles in order to study what exactly is in this seawater the ship is sailing through.

“What’s in seawater?” I asked some folks in the lounge this evening (while watching our 3rd episode of the television series “Weeds” – I’m TOTALLY hooked!)The answers I got ranged from “dinosaur pee” and ”the world’s toilet” to ”pollution from our ship” and ”whale poo.” It has become apparent to me that perhaps it is time to have a quick lesson in what exactly IS in seawater and why are we out here studying it.

One aspect of the water we are interested in is its physical properties (such as its temperature, salinity and density). We study these properties with an instrument called a CTD (Conductivity Temperature Depth sensor). Both temperature and salinity (i.e. the salt content of the water) affect the water’s density. In most parts of the world’s ocean, temperature is the most important factor controlling the density of the water. However, the Southern Ocean is different. Here, the temperature doesn’t change much…its cold…often. The salinity, however, changes much more because of the melting of the seasonal sea ice. Therefore, the changes in how much freshwater is melting into the ocean from the ice change the density of the water here much more than the small seasonal changes in temperature.

Another thing we study when looking at the water is its biological and chemical properties. In order to do this, we collect water from bottles (called Niskin bottles, named after Shale Niskin, who patented the bottle design in

Figure 2. A look at the vertical profile of the water column at one of our stations. This information is being sent live over a data wire from the CTD, so we can see in real time what the water looks like!

1966) located around the CTD; we shut these bottles at various depths to collect water from the bottom of the ocean all the way to the surface. We then filter this water to look at these properties. The Warren lab is filtering water for chlorophyll. This is a pigment found in the phytoplankton (the ‘plant plankton’ of the ocean). By measuring the amount of chlorophyll, we can look at approximately how much phytoplankton (i.e. salp and krill food) is in the water. This is a useful piece of information for us!

Both the Warren and Bucklin science teams on our cruise are filtering water for clues about the environment here. I’ll hand this over now to Paola to talk about what her group is interested in finding out about the seawater.

Figure 3. Paola and Chelsea pause for a photo while collecting water from the Niskin bottles around the CTD. They will bring this water inside and filter it to look at the seawater’s various biological and chemical properties.

-- Melissa Patrician (Stony Brook University)


In our hunt for salps, we are trying to understand their distribution patterns and the chemistry of the ocean holds lots of clues as to where we might find these gelatinous critters! The Bucklin lab is looking at both the nutrients and particulates in the seawater.

In the area of Antarctic waters where we are working, concentrations of nutrients (nitrate and phosphate) are much higher than those found in other oceanic waters. They tend to be lowest at the surface and greatest in the warm deep waters.

Particulates, alternatively referred to as particulate organic matter (POM), are tiny particles of solid material present in the water column. Particulates within the water column come in a multiplicity of sizes and from a great variety of sources: dead phytoplankton cells, fragments from attached macroalgae, dead bacteria, dead protozoa, dead micro- and macro-zooplankton, crustacean exuvia, and fecal pellets, especially those from copepods, euphausiids, and salps.

Figure 4. Paola concentrating on filtering the seawater for nutrients and particulates.

Salps are indiscriminate filter feeders, utilizing an internal mucus net to capture particles as water is pumped through the body. Therefore, they are assumed to ingest all particulate matter (both living and dead) small enough to fit through their oral opening and large enough to be retained by the mucus net. This may include particles as small as 1 to 2 microm.

So, if we want to look at the big picture and understand the behavior or occurrence of salps in the Southern Ocean, the characterization of the chlorophyll by the Warren team will help with understanding the prey (phytoplankton) distribution. The Bucklin side of the analysis will help with understanding under what nutrient and particulate concentrations we tend to find salps more frequently.

Well, that’s it for today’s lesson: “What’s in Seawater?” Hope you learned something!

-- Paola Batta-Lona (University of Connecticut)

Picture of the Day - November 21, 2011

Cape Petrels ride the waves in Bransfield Strait, Antarctica.
Photo Peter H. Wiebe

THREE WEEKS LATER

We have now completed work at 13 stations (Fig.1) and completed 14 CTD casts, 15 MOCNESS tows, and 14 IKMT tows. We are on track – in terms of progress through our cruise plan – to complete work at nearly all of our planned stations. At this writing, we are heading back into the open waters of Drake Passage with the goal of working at four deep-water locations (Stns #6, #5, #4, and #24), two shelf stations (#23 and #3), and a station in protected waters of Gerlache Strait.

Figure 1. Station locations for LMG11-10. As of 20 Nov 2011, work has been completed at stations shown with circled station numbers.  The cruise track and order of stations were changed to accommodate weather. The star shows the ship’s location at 8:00 pm on Nov. 20th.

Our cruise has already been very successful. Our samples from vertically-stratified MOCNESS tows at each station are yielding a useful view of the pelagic community of the Drake Passage and Bransfield Strait.  Taxonomic analysis of these quantitative samples will help us characterize the early spring assemblage of the Western Antarctic Peninsula region.  We will compare our findings with an earlier study, the comprehensive US GLOBEC Southern Ocean Program carried out during Fall and Winter, 2001 and 2002. Peter Wiebe led four US GLOBEC cruises as chief scientist; he collected zooplankton samples using a MOCNESS just like the one we are using now and equipped with strobe lights to reduce net avoidance by krill.

Our nets have brought up huge catches of krill, usually in surface nets sampling during the dark, have contrasted with generally sparse MOCNESS and IKMT samples. We have caught only a small number of salps.  Our team has processed 65 aggregates and 10 solitaries – some of them exceptionally large and packed with embryos, which can quickly generate a population “bloom” of chain-forming aggregates.  Other specimens collected, identified, and flash-frozen for genomic and transcriptomic analysis  (please look up the definitions of those terms yourself) include ~300 individuals of Euphausia superba (including larval, juvenile, and adult stages), with the telson (tail) preserved separately in alcohol, so individuals can used for analysis of cohort (life stage) structure.  We have also flash-frozen various zooplankton that caught our interest, including copepods, gastropods, ctenophores, and amphipods, among others.

Our goal is to obtain material for genomic and transcriptomic analysis, especially for the Southern Ocean salp. The best source of material for these analyses is from our IKMT tows, which are shallower and usually yield living zooplankton. These specimens are identified and flash-frozen for analysis at UConn.

Why so few salps this year?  In December 2004, salp researchers caught tens of thousands of salps, which they caught live by SCUBA diving!  In December 2010, hundreds of salps were caught at many of the same locations we are visiting this year.  We speculate that the Spring population increase of salps may be delayed this year.  Several knowledgable people have remarked on the late ice cover this year of coastal waters of the Western Antarctic Peninsula region.

So our salp hunt continues, and we are looking forward to seeing what the open shelf and offshore waters may hold. 

-- Ann Bucklin, University of Connecticut

PLAYING THE WAITING GAME IN THE ANTARCTIC SPRING

The Drake Passage, the narrow passage that extends from the tip of South America to the tip of the Western Antarctic Peninsula is well known for treacherous sea conditions. Many sailing ship’s of past centuries ended their careers while sailing around “the Horn” (Patagonia) and into the Drake. The past 24 hours has seen the kind of wind and seas for a sustained period of time that must have caused sailors grave concern. It has certainly cost us valuable “ship time” (the number of cruise days alloted to our project by the NSF). We have been standing by near our Stn #19, which is at the western end of Bransfield Strait, waiting for weather and sea conditions to moderate to a level where we can carry out our planned work. Similar to most stations, we want to do a series of observations and sample collections: CTD cast to 1000 m, MOCNESS tow to a 1000 m, IKMT to shallower depths, and an acoustic “Towfish” as we steam toward the next station (see ours blog s for Nov. 10th, 17th, and 18th).

Figure 1. Wind speeds between 30 – 50 kts, barometric pressure dropping, and other weather data shown on the ship’s DAS (Data Acquisition System) screen for the previous 24 hrs from 11:00 am (1300 GMT) Nov. 19th.

For the past 24 hours, we have had sustained winds between 30 and 50 kts out of the East (Fig. 1). The barometric pressure had been up around 1004 mb 2 days ago, when we had flat seas, clear skies, and excellent working conditions. It had dropped to 969.2 mb by this morning and seems to be bottoming out. This may be the longest stretch of high winds and seas we have had this cruise. 

In planning our cruise, selecting our station locations, and laying out the cruise track, we assumed Bransfield Strait would be protected from the prevailing winds, which are usually out of the N-NW. We expected the early Austral Spring would be stormy, but we did expect that stations in the lee of the islands would be workable despite high winds because of the short fetch (distance the wind blows across the water to whip up the waves).  Instead, there is an intense low pressure to our North, and the winds are coming from the East and blowing directly down the Strait. The long fetch has built the large swells that we are experiencing now.

So where did this severe weather originate? The isobar images that we get daily on the ship provide an answer. The low that settled in over the Drake Passage formed as a relatively weak low in the South Pacific Ocean off the west coast of South America on Nov 15th. It began moving SE towards the Drake Passage on Nov. 16th (Fig. 2A). The system intensified while still West of the southern tip of South America on Nov. 17th (Fig. 2B); it moved into the Drake Passage and pushed our good weather off to the East during Nov. 18th (Fig. 2C). The latest image shows the low pressure intensified, with barometer reading of 976 mb at the center (Fig. 2D).

Figure 2. Isobar images of the pressure fields around the Antarctic Continent for: A) 16 November, B) 17 November, C) 18 November, and D) 19 November 2011. The small yellow dot marks the position of our Stn #19 at the western end of Bransfield Strait; the low pressure system that is causing our high winds and seas is marked by the arrow.

The current pressure isobar image underestimates the intensity of the low, since at the ship we had a minimum barometer reading of 969 mb. With the barometer beginning to rise and from the wind chart (Fig. 3), which has light winds forecast for later today and tomorrow, work here at this station may resume in another 6 to 12 hours.

Figure 3. Forecast wind speeds and directions for the period 19-20 November 2011. Our area (Stn 19 is marked by the yellow dot) is forecast to have light winds for the next 24 hours or so.  Note the red LMG dot on the chart is an old LMG position.  

And so we will continue to stand by at our Stn #19. Work in Antarctic waters adds new meaning to the phrase: “Hurry up and wait!”

-- Peter Wiebe, Woods Hole Oceanographic Institution

Picture of the Day - November 18, 2011

View from Bransfield Strait during a lovely Spring day in Antarctica.

(Photo by Peter H. Wiebe, 17 November 2011)

USING SOUND TO SEE INTO THE OCEAN

Land creatures – including us - are used to using our vision to detect the world around us. Ecologists walking in forests, meadows, grasslands, or deserts can immediately pick out the patterns of the life forms inhabiting the space and easily design sampling protocols to see the relationships to each other and their environment. Not so in the ocean environment. As we stand on the deck of a ship peering into the darkness of the sea surface, we can rarely visualize the animals and plants living just below the surface – much less those living in the depths of the ocean. Divers swimming in the shallow reaches of the ocean have limited visibility (only a few meters in coastal regions, up to 20-30 meters in very clear water), because seawater is a very poor medium for transmitting visible light.  Light is absorbed, scattered, and reflected more in seawater than in air by orders of magnitude.  This limitation affects even the remotely operated and autonomous vehicles with cameras and video systems that can roam the ocean depths, although this technology has given us images of the organisms living deep in the ocean and are leading to new insights about their spatial patterns and behavior on small spatial scales. So how is it possible to view the fascinating 3-dimensional ocean habitat and visualize the spatial arrangement and behaviors of marine organisms on larger spatial and temporal scales?

The transmission of sound at low and moderately high frequencies (1 Hz to 100 kHz) is much more efficient in the ocean than in air. Above 100 kHz, sound is more rapidly attenuated, largely because of absorption due to the salt (principally magnesium sulfate) in seawater.  Despite this limitation, high-frequency sound in the 38 kHz to 500 kHz range is proving exceedingly useful for studies of zooplankton (our target organisms), because it can be used to detect the presence of animals 10's to 100's of meters away from the transducer producing the sound.

Figure 1. Echograms of 38 and 150 kHz acoustic data (Nov. 18, 2011). The vertical axis is depth (m); the horizontal axis is time. Intensity is shown by color (see color bar). The intense scattering shown on both echograms is probably krill patches. Image J.D. Warren

On this cruise, there are several acoustic systems being deployed to provide information about the distribution of zooplankton and larger organisms (such as fish) in the water column. The ship has a hull-mounted Acoustic Doppler Current Profiler (ADCP) with 38 and 150 kHz transducers, which is principally used for measuring current speed and direction with depth under the ship. This system depends on organisms in the water column to reflect sound and produce backscattering (i.e., the portion of the transmitted sound that is reflected off organisms back to the transducer receiver). This can be interpreted as current flow from Doppler shifts (i.e., shifts in the frequency of sound emitted by the transduers) in the returning echoes. Also recorded is the intensity of the sound returned as echoes off the organisms. As the ship steams along, the ADCP provides echograms of the backscattering intensity at two frequencies (Figure 1), providing a continuous indication of high and low concentrations of organisms below the surface.

Figure 2. A) The BioSonics towfish equipped with 38 and 120 kHz transducers being launched from the LM Gould (15 November 2011).  B)The echogram display of the BioSonics frequencies. Photo and Image P.H. Wiebe

A second system is a dual frequency BioSonics echosounder with 38 and 120 kHz transducers mounted in a towed body (Figure 2). This system is being towed off the starboard quarter of the ship for two hours at the end of a station while heading for the next station when sea conditions are good.

Figure 3. A) Joe Warren and MPC Jullie Jackson discuss the zodiac setup; note the orange transducer module on the end of the stainless steel square tubing next to the engine – it will be moved to a down position underwater during the survey. B) The Zodiac is launched. C)  Joe Warren climbs down into the zodiac. D) Kelley Watson, Krista Tyburski, and Joe Warren during a small-boat survey. Photos P.H. Wiebe

The third system is a Simrad echosounder that is battery powered and has transducers operating at 38 and 200 kHz.  It is being used from a Zodiac small boat (Figure 3) to conduct surveys of krill distribution over small spatial scales in areas of interest, where our large vessel is unable to go due to water depth. At our Stn #16, all three echosounders were operated for the first time during this cruise. Conditions were ideal, with low winds and seas – except for a long-period swell running through the survey area.

To help interpret the acoustics data, the small boat survey was conducted along the towing path of the MOCNESS, which provided depth-specific collections of animals and environmental measurements (especially temperature and salinity) in the water column at the station.  The combination of the MOCNESS and IKMT zooplankton samples and the acoustic data will provide a comprehensive picture of the vertical and horizontal distribution of zooplankton living in this Antarctic ecosystem and will allow evaluation of their status in the face of the rapid environmental changes now taking place here.

-- Peter H. Wiebe (Woods Hole Oceanographic Institution) and Joseph D. Warren (Stony Brook University)

P.S. This is Ann Bucklin with a postscript for today’s blog. Bioacoustics is an aesthetically-pleasing (echograms in deep shades of blue and red) and computationally-challenging (huuuuuge data files) field.  The simplicity of the underlying concept – bouncing sound off bugs – is captured in slogans used on Peter and Joe’s T-shirts: “We only measure voltage and time”.  It is also apparent in Peter’s haiku poetry on the subject, including this one:

Echoes

A loud ping goes out
A whisper echo returns
From deep-sea creatures

-- Peter Wiebe