The deep, mud-covered floor of Andvord Bay harbors an amazing and diversity of “megafauna” – large invertebrates and fish visible in our photographs of the seafloor. These animals live far below the ocean’s surface at depths >500 m and rely a rain of food from phytoplankton (small algae) blooming in the overlying water column. We count the abundance of these animals using our seafloor photographic surveys, but to actually sample these animals to identify them to species, as well as study their diets and reproduction patterns, we use the Blake trawl. This is a net suspended in a steel frame that is pulled across the seafloor, collecting megafauna from a swathe 1.5 m wide. The entire trawl operation takes about 2.5 hours because we lower the trawl on large cable, running out three times as much wire as the water depth. A total of 1600 m, or about one mile, of cable is spooled out. The trawls touches the bottom for about half an hour and then is winched back onto the vessel containing a bolus of mud and megafauna. Once the trawl reaches the ship’s deck, we open the net over a big plywood bin, and then quickly wash the mud on large sieving tables to collect the animals while they are still alive. Representatives of the dozens of species collected are quickly sorted by out Benthic Team into buckets of cold water labeled with the animal group names (fish, anemomes, sea cucumbers, polychaete worms, crustaceans, mollusks, etc.). This is a cold task because it must be conducted on the ships’ back deck to allow the mud to wash over the stern, but in the process exposing our team to the elements – high winds, blowing snow and frigid temperatures, with wind chill falling to below – 30 C (-15 F). It is also a very exciting process because we can see close-up the bizarre and beautiful life forms that have evolved in the extreme, isolated conditions of coastal Antarctica. On the first trawl we collected large numbers of two species of shrimp, giant polychaete scale worms with inch-long bristles and luminescent scales, bulbous sea cucumbers, many species of anemones and sponges, and at least five species of fish. Perhaps the most intriguing are icefish, which have no hemoglobin, so their blood and gills are icy clear. They are able to survive without hemoglobin because cold Antarctic waters contain such high concentrations of oxygen that these fish do not need hemoglobin to transport oxygen to their tissues – oxygen simply dissolved in their blood provides an adequate supply. The size of individual Antarctic invertebrates is also remarkable – most of the polychaete worms collected in the fjord basins are much larger than their relatives collected on the open continental shelf of Antarctica, or at lower latitudes.
After sorting animals collected by the trawl, we photograph them all to allow later size measurements, and preserve and freeze a subset of the animals for identification by specialists, and to allow studies of reproductive responses to the summer bloom and the structure of the fjord food web. These samples are providing materials for the PhD research of UH graduate student Amanda Ziegler, and the senior thesis of Kelcey Chung from the UH GES program. Those animals not retained for our studies are returned to the fjord is a good condition as possible.
Written by Craig Smith.