WHAT LIVES BENEATH?

The ocean pelagic habitat has been divided vertically into five zones: epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadopelagic (Table 1). Most sampling is done in the top 200 m, called the epipelagic zone. A great deal of sampling has been in the top 1,000 m or through the mesopelagic zone. Little sampling is typically done below 1,000 m, in the bathypelagic (1,000-4,000 m), and much less even deeper.

Table 1. Depth zones of the pelagic habitat, with total volume in the world oceans in millions of cubic km [Vol (106 km3)] and percent volume (% Vol) of the total. Note that most of the ocean volume is considered bathypelagic (from Hedgepeth, 1957).

The vertical partitioning of the ocean, including the Southern Ocean, is very important, since the different zones have quite different environmental and habitat charateristics – and sometimes very different zooplankton species composition. Although some planktonic species vertically migrate each day, visiting the surface waters during the night and hiding from visual predators in the deep dark waters during the day, other species live in the deepest layers of the ocean all the time. In general, the very deep sea is thought to be a region of low biomass, low abundances, and high diversity. Since the deep layers are rarely sampled, they are also a zone of discovery – of new species and unsampled populations of known species.

For our project, our particular interest is in investigating earlier reports of very deep populations (below 2,000 m) of our target salp species in the deep waters of the Western Antarctic Peninsula region. We wonder whether there are unsampled deep-living populations of this species, or whether perhaps the species migrates between the surface and such great depths. Our molecular genetic and genomic analyses are designed to help answer this question.

For our salp survey, we are sampling from surface to as the near the bottom as possible. For stations deeper than 1,000 m, we sample to 1,000m, except for stations deeper than 2,500 m, when we sample to that depth. We are sampling zooplankton with a complicated instrumented net system, called a MOCNESS (Multiple Opening-Closing Net and Environmental Sensing System; Fig. 1) that provides data in “real time” (immediately) through a conducting cable to a ship-board computer.

Figure 1. The MOCNESS is launched from the LM GOULD. You can see the electronic instrumentation in pressure cases at the top and cod ends (PVC buckets) at the end of the nine nets. Photo Peter H. Wiebe

Yesterday we steamed NE in the Drake Passage and arrived at our Station #11 during the afternoon. We are particularly interested in this sampling area, which is offshore and among the deepest of our stations (estimated depth 3,069 m), so we asked that the LMG remain near the location so we can wait for better weather. We (Ann, Joe, and Peter) headed for bed, with a request to be waked up when conditions were workable. Amazingly, we got our “weather window” - winds below 30 kts (see Fig. 2) – and got the wake-up call at 3:30 am. Work could begin!

Figure 2. The red line shows wind speeds over the previous 24 hr at 12:00 Noon GMT or 9:00 am local time on Nov. 13th.

After an initial CTD cast, we launched our first “deep tow” of the cruise. The MOCNESS went over the side about 6:30 am and was recovered at 11:45 am. The net system had traveled 5,000 m in about five hours. Throughout the tow, environmental data (temperature, salinity, particulates, chlorophyl) and net information (depth, angle of the net, volume of water filtered) are displayed on the ship-board data acquisition computer (Fig. 3). During the uphaul, eight nets are opened and closed successively, sampling discrete depth zones of the ocean.

Figure 3. MOCNESS data acquisition screen, showing net trajectory (with different nets in different colors), environmental parameters, and position during a 2,500 m tow. 

We were happy to have completed this deep tow successfully and felt fortunate to have a long enough patch of good weather. This came after a very bad two-day stretch of wind and sea that prevented work. During the tow, we kept a close eye on the wind speed and also the barometer, which was moving from low to higher pressure as the storm center moved away. During the tow, the barometer peaked and the winds dropped to below 20 kts for a short time and then the barometer began to fall again. Unfortunately, our target species, the salp, Salpa thompsoni, was not present. However, salps are colonial animals that can form large patches or blooms very quickly. When we do find salps – and we remain quite optimistic that we will – we will likely find LOTS of them!

-- Ann Bucklin (University of Connecticut) and Peter Wiebe (Woods Hole Oceanographic Institution)