Royal Norwegian Air Force 333 Squadron approaching RV G.O.Sars
Cannister holding ROV parts being dropped!!
Aeginura grimaldii A 4-cm wide narcomedusa collected at 900 m. This species ranges from 500 to greater than 1000 m. Feeding habits are unknown but members of this group consume other jellyfishes.
Date:21 July, 2004
Author: Franz Uiblein (IMR, Norway), Marsh Youngbluth (Harbour Branch Oceanographic Institution, USA)
As you might expect, diving into the deep sea is always exciting. Because this environment is so vast, we expect to observe something novel during each dive with a remotely operated vehicle (ROV). And we're not often disappointed. Usually, time passes much too quickly and reluctantly we must return to the surface of the sea. The ability to enter, explore and eventually conduct experiments in the water column and on the sea floor requires special technology and dedicated teamwork. Scientific and technical crews must work closely together to ensure success for each dive because such journeys into the dark, hyperbaric realm are generally expensive and relatively rare. On Leg II of this cruise we have used two ROVs - Aglantha (2000 m limit), named after a hydrozoan jewel, a 4 cm long medusoid jellyfish and Bathysaurus (5000 m limit), named for a deep-sea lizardfish.
Unfortunately for us, the ROVs have been temperamental and various components, e.g., cameras, thrusters, and the umbilical cable, have failed to perform consistently. These maladies are expected when such complex vehicles enter hostile deep-sea environments and extra parts are always brought aboard. But sometimes there is a little part that is overlooked or fails more frequently or has a design flaw. Such was the case for the thrusters of the ROV Bathysaurus. The vehicle has sat dormant this past week for lack of a special shaft seal for the thrusters. We expect the spare parts to fall from the sky at 1330 today. Why? Well, fortunately for us, the 333 Squadron of the Norwegian air force agreed to rendezvous with us, way out here - a high seas adventure and attempt to save the damsel in distress.
All hands were on deck for the once-in-a-lifetime event. We watched the plane zoom out from the horizon and roar straight for the ship, four black vapor trails streaming from the propeller-driven engines. At least 500 digital pictures must have been taken of the plane and a couple dozen videocams recorded the fly-overs! And, the drop was successful. Actually, five bright orange canisters were delivered, just off the stern, one by one. Awesome precision! Excellent job by the 333 Squadron pilot and crew! We are all very grateful!!
So what have we discovered in the upper 2000 m with ROV Aglantha? Zonation! We have observed ctenophores, siphonophores, appendicularians and medusae as the vehicle descends. The ctenophores, mainly Bathycyroe fosteri, often appear first, then colonies of physonect and calycophoran siphonophores are seen, primarily species of genera like Nanomia, Bargmannia and Forskalia, and further down appendicularians, presumably species of Oikopleura, become numerous. All of these animals are most conspicuous in the lower sector of the pycnocline (a layer of pronounced density that is 100-200 m thick at a depth ranging from 300-500 m below the surface). Further down (>800 m) when we enter water with a temperatures of 6 C, small (4 to 8 cm wide) medusae appear, especially common is Aeginura grimaldii. These observations are really encouraging because they confirm that populations of midwater planktonic fauna are usually sandwiched within relatively narrow, but predictable depth intervals. This layered phenomenon is difficult to substantiate by collections with plankton nets because it's hard to maintain nets in a given interval and, more important to note, collecting zooplankton with nets towed from the surface of the sea is just really a process of making blind grabs. But when we know from visual observations where the animals are concentrated, we can correlate their vertical distribution patterns with persistent physical and chemical factors, like temperature, salinity and oxygen. Also coincident with gelatinous zooplankton are their prey, principally calanoid copepods and large (2-5 mm long) aggregated particles. Our in situ information from the ROV dives are bolstered by similar, but finer-scale data recorded with the UVP.
We'll take the ROV Aglantha to a depth of 1700 m late this afternoon. After missing a day of diving because of gusty winds and unsettled seas, we are anxious to explore again. The pelagic group will work in the benthic boundary layer, a 50-100 m zone, just above the seafloor. Flocculent sediments, continuously resuspended by bottom-skimming currents, enrich this region with food particles. On earlier dives we have noticed that certain zooplankton, such as the omnivorous appendicularians, thrive within this particle-rich regime. We expect to collect unique species in this rarely visited environment.
The demersal trawl took a nose dive into a mound of clay sediment followed by a 25 min "jump" into the open water. This deepest trawl tow (3450 m) proved, however, to be quite successful by providing a representative sample from an abyssal plain area that partly covers the seafloor of the Charlie-Gibbs fracture zone. Among the fishes, several deep-sea grenadiers and eels, slickheads of the highly diverse family Alepocephalidae and a still to be identified species of the family Ophidiidae were collected. The latter is closely related to the giant brotula (Spectrunculus grandis) that was collected already by several trawling and longline operations and also observed by the ROV Bathysaurus. The ROV operations as well as a camera mounted on the trawl showed that ophidiids like rattails and slickheads often occur just above the bottom either in passive or active swimming positions. With their barbell-like ventral fins these fishes can maintain contact with the bottom like many deep-sea cods (family Moridae) and scan currents for odour plumes from food or mates. Slickheads are often encountered hanging "lazily" in the water and are easily collected by an approaching trawl. Together with lots of sediments giving an impression of the bottom surface of the wide plains adjacent to the ridge, today`s trawl also contained several invertebrates, among them rare cephalopods, deep-sea crinoids, sea urchins, and small bivalves.
LINK: Read more about the ROV Bathysaurus