Underwater world gives up its secrets

Translucent cranchid squid larva, 10 mm in length
This cranchid squid larva is only 10 mm long but adults can grow up to 2 m. Their bodies remain translucent. (Photo: Russ Hopcroft)
Workers with Norpac net, consisting of 330 micron and 110 micron nets side by sideJapanese scientists measure the pelagic fish Pleuragramma antarcticum.A marine worm (Vanadis antarctica) from the macro-zooplankton.
Giant sea spiders, jellyfish and marine worms were among the many surprises encountered by scientists during the Collaborative East Antarctic Marine Census (CEAMARC), conducted recently as part of the International Polar Year (IPY) Census of Antarctic Marine Life. The census involved ships supported by four nations – Australia (Aurora Australis), France/Belgium (L'Astrolabe) and Japan (Umitaka Maru) – surveying transects in the Southern Ocean adjacent to Terre Adélie and George V Land in Antarctica.

While the Umitaka Maru and L'Astrolabe focussed on the pelagic (open ocean) realm, the Aurora Australis focussed on the sea-bed communities of the survey region. Common sites were sampled by all three vessels on the continental shelf. Umitaka Maru also surveyed sites north of the continental shelf to look at north-south distribution patterns, and the deep pelagic zone – a poorly sampled habitat known to have a high biodiversity of zooplankton and jellyfish.

Scientists aboard the L'Astrolabe sampled the inshore plankton of the area using a standard 'WP2' plankton net. This is a small net (0.25 m2 mouth area) with 200 micron (0.2 mm) mesh that is ideal for collecting small zooplankton and very young fish larvae. The team also conducted CTD (conductivity-temperature-depth) casts which measured the profiles of temperature, salinity and other physical properties in the water column. Bottles attached to the CTD frame collected water for studying phytoplankton and other micro-organisms that provide food for the zooplankton. While the L'Astrolabe survey was perhaps more routine, it provided continuity with past French surveys in the area.

The Umitaka Maru conducted a more detailed survey using a range of traditional sampling gear supplemented with a new, innovative Visual Plankton Recorder (VPR) developed by Dhugal Lindsay of the Japan Agency for Marine-Earth Science and Technology. The VPR has both high resolution still cameras and a high definition video camera to capture images of plankton swimming in their natural state.

In addition, CTD and bottle casts provided data on the physical and chemical properties of the water. Phytoplankton collected from the bottles were placed in incubation chambers to determine how quickly they grew and therefore how much food they would provide for zooplankton, which in turn support higher trophic groups such as fish, birds and mammals. A WP2 net and a 'Norpac' net, which consists of 330 micron and 110 micron nets side by side, were used to study all sizes of plankton.

The Umitaka Maru also used two large survey nets, cast to depths of up to 2000 m. A Rectangular Midwater Trawl net system was used to collect krill larvae and adults, small zooplankton (meso-zooplankton), larger macro-zooplankton and small fish. An International Young Gadoid Pelagic Trawl net was used to survey pelagic fish, such as the Antarctic herring (Pleuragramma antarcticum), as well as lantern fish, other small mid-water fish species, squid, and large jellyfish.

Both large survey nets produced many good quality specimens that were used by Canadian zooplankton biologist, Russ Hopcroft (Australian Antarctic Magazine 12: 28), to take high quality photos. The photos were taken with a high definition camera/microscope system and image capturing software. The whole system was mounted in such a way as to eliminate the vibration from the ship.

The still camera on the VPR allowed us to take quality images of plankton as small as a few millimetres. The VPR video system also provided 80 minutes of continuous footage, displaying both the natural colours and behaviour of plankton, as well as larger organisms, such as fish and huge jellyfish. Most plankton and gelatinous zooplankton captured in nets end up in poor condition or badly damaged. All usually change colour or die soon after reaching the deck, or become distorted after preservation. Seeing plankton alive in their natural state gave a completely different appreciation of the beauty and diversity of their shape and form.

Many of the jellyfish captured were new to the researchers and may be new records for the region. Tissue was taken for genetic analysis, to help determine if we have any previously undescribed species. A notable result of the voyage was the high abundance and diversity of jellyfish in the deeper waters down to 2000 m. Large specimens of Stygiomedusa were collected in near-perfect condition. These jellyfish have large bells and tentacles that extend 6-8 m in length. While we need to compare the results with past data, the collective memory of experienced Antarctic biologists on board suggests that the jellyfish are more abundant than before.

We've planned the survey and collected the samples and associated environmental data. The next task is to pull all the information together and address a range of questions such as: have the deep water zone communities changed and will this have an effect on the rest of the ecosystem? Meetings of key CEAMARC personnel are now in train, to begin collating results and building a picture of the biodiversity and ecosystem processes in the survey area.

GRAHAM HOSIE
CEAMARC leader, AAD