Date:20 June 2004
Author: Tracey Sutton (HARBOR BRANCH Oceanographic Institution) and Annelies Pierrot-Bults (Zoological Museum, University of Amsterdam)
Having reached the midpoint of our journey, we have begun a preliminary synthesis of the some of the data collected thus far. One of the unique aspects of this cruise is the manner of the on-board trawl sample processing. It is standard procedure on most deep-pelagic trawling surveys to preserve catches whole, then analyze the material later in lab.
A deep-sea smelt, with sample data tag.
A sample processing system has been devised for this cruise in which taxonomic specialists on board sort trawl samples immediately, in most cases (e.g. fishes, cephalopods, gelatinous organisms) to species. Each species or taxonomic group is counted, individually weighed, and given a barcode tag (see picture) for data organization. Each species or taxonomic group is then packaged separately according to its later usage. Voucher specimens, rare specimens, and specimens for diet study are being preserved in formalin. Specimens for molecular and biochemical analyses are individually frozen, and tissue samples for genetic analyses are preserved in ethanol. All data are then recorded in a database management system designed to facilitate data extraction and material archiving for later use by scientists all over the world. So far, thousands of samples have been archived, ranging from tiny zooplankton (microscopic animals) to larger fishes and squids.
Using trawling gear that is larger than can be employed on most oceanographic research vessels, we have concentrated our efforts in the deeper reaches around the Mid-Atlantic Ridge (MAR). This gear has revealed abundances of larger organisms, such as large sawtooth eels (Serrivomeridae), daggertooths (Anotopteridae), and tubeshoulders (Platytroctidae), which normally are not sampled by smaller gear. These organisms represent a substantial fraction of animal biomass in the deep North Atlantic, and therefore would be expected to be important components of the ecosystems associated with the MAR.
Trawl sample being sorted and processed.
Results of trawling are preliminary, but some findings may help shed light on data obtained during the ongoing, large-scale acoustic survey. This survey has detected an unexpectedly persistent deep acoustic scattering layer at around 2000 m depth on the flanks of the MAR. Trawl sampling suggests that the major contributors of this layer are deep-sea smelts (Bathylagidae) and large concentrations of tubeshoulders (Platytroctidae). Over the crest of the ridge, the abundances of these fishes remain relatively constant, but a very large increase in the biomass of large lanternfishes (Lampanyctus macdonaldi) and eels (Serrivomer beani) was detected. Another acoustic layer found in the mesopelagic zone (200-1000 m) has been dominated by lanternfishes, particularly species of Lampanctus and Notoscopelus. The shallowest acoustic layer detected (0-200 m) have been dominated by the small lanternfish Benthosema glaciale, the small pearlside Maurolicus muelleri, as well as by large jellyfishes (Periphylla).
Not only are fish caught on board the G.O.Sars, but zooplankton research is also being conducted. One of the groups studied is the Chaetognatha, or “arrow worms.” They are called arrow worms because they move very swiftly, like an arrow, to catch their prey, which are copepods. They can only move over short distances because plankton, by definition, cannot move against the water currents. Arrow worms are not really worms, although they are long and thin and have very transparent fins on the sides and on the tail (see picture). They are a separate phylum in the animal kingdom and we currently do not know their closest relatives. They live only in the sea in large numbers and are an important part of the zooplankton biomass. They are very transparent in the upper layers (to be invisible to predators) but living at depths below 1000 m they are red due to their diets.
Sagitta maxima (transparent) and Eukrohnia fowleri (with red pigments). Copyright: Zoological Museum, Amsterdam
On this cruise we use a plankton net with a small mesh size to catch chaetognaths. When the net comes on board the content is quickly brought into the zooplankton lab and analyzed under the dissecting microscope. All species are identified as far as possible and counted. The size we get here is from about 12 cm to 10 mm, depending on the species. The biggest species here in the North Atlantic is called Sagitta maxima. Some specimens are put in acetone to be used for genetic analyses. These must be as fresh as possible. The bulk of the sample is kept in formalin so that we can do further analyses when back ashore.
Overall, a diverse array of biological samples have been, and will continue to be collected on this cruise for future scientific research.
Tomorrows expected highlights
After finishing an all-night sample, we will be steaming south, and we anticipate warmer weather, a higher diversity of fauna, and maybe, just maybe, we will see the sun.