Elizaveta Ershova, Cornelia Jaspers, and Russ Hopcroft steady the bongo plankton net package, while Russ motions to the winch operator to raise the net. The bongo nets are the primary way zooplankton are captured during the 2009 RUSALCA expedition. Click image for larger view and image credit.
September 28, 2009
It's cold and windy, a typical night in the Chukchi Sea. We are on the back corner of the deck, hauling our elaborate “butterfly net” in by hand, and anxious to see what tiny creatures — known as zooplankton — have been swept from the ocean’s depths. Every sample is a little bit different, with only the largest “bugs” identifiable to the naked eye, as we pour our catch gently into a five-gallon pail for sorting. On this cruise, the “big” stuff is usually comb-jellies (the most fragile group within the zooplankton) plus an assortment of other jellyfish, krill, and other small crustaceans. Our task now is to rush into the lab and sift through this fresh collection, making notes on what we find and removing specimens for various purposes.
Under the microscope, the formerly indistinct contents of the pail spring to life: some animals hang motionless, suspended in the small dish and "hoping" no one will notice them, while others glide or hop across our view field. Like other samples, the most abundant things are copepods — small shrimp-like animals ranging in size from 0.1 to 10 millimeters (1/250 to 2/5 of an inch). They are generally translucent, with bright red highlights in many polar species (click here for more information). Many of the older animals are bulging with oil sacs stocked up to carry them through the long arctic winter soon to come. I’m briefly distracted by an amphipod (sand-flea) that is racing through the sample like a madman (or my kids on too much sugar). Elsewhere, a krill trapped against the side of the dish is creating turmoil as the strong current created by its swimming legs entrains copepods and tumbles them across the dish. On the positive side, this sample is largely free of the filamentous algae that has clogged other samples and made sorting slow.
After this cursory scan, our team sets to the more systematic task of searching for females of our target species group; they are within the samples and among similar-looking juveniles and the mass of other species. Our target is a copepod genus called Pseudocalanus, which is generally the most abundant species complex on the Chukchi shelf, and the females we seek range in total length from about 1 to 2 mm (1/25 to 2/25 of an inch). A total of five species exist in the regions we have sampled. They are difficult to separate alive, so we will sort that detail out after the cruise.
Depending on the composition of the sample, it will take as little as an hour for one of us, or up to three hours for three of us, to pick out the 150 females we need for experimentation. The experiment involves incubating each of these females in a small flask for the next two days to determine how many eggs each will lay — an index of how “happy” or “healthy” they are in the location from which they were collected. Many eggs indicates a good feeding environment and acceptable water temperature; few eggs means some combination of poor food or a suboptimal temperature. So far this cruise, we have set up 25 such experiments, with hopes to complete another 9 to 12 experiments by the end of the cruise. In 2004, we set up 21 experiments from many of the same locations and are curious to compare the “happiness index” between years.
While we "hunt for our prey," we also remove representatives of other species plus anything unusual, for digital photography and/or to archive for genetic analysis. Both approaches are contributing to databases aimed at describing the full diversity of life in the Arctic Ocean (click here for more information). The more quantitative side of our work comes from another series of larger nets that were thankfully deployed with the ship’s winches. These other nets employ several different sizes of mesh to capture the smallest and slowest of the zooplankton, as well as towed nets to capture the larger and faster zooplankton, such as krill. In general, these other samples are preserved immediately for later analysis back at the lab at University of Alaska – Fairbanks, but for one of nets, samples are poured out as thin layers on a light-table so we can scan for what’s present, in terms of larger stuff.
A major task at this point is counting and measuring the “gelatinous” zooplankton, particularly the comb-jellies (also known as ctenophores or sea-gooseberries — click here for more information). The existence of most comb-jellies in the samples can be quantified only in a live sample, because they dissolve in the preservatives we typically use for long-term storage of samples. During this cruise, two species of comb-jellies have been present at literally every station, and we would be unaware of their presence without such extra effort.
Once the cruise is finally over, and all the preserved samples are analyzed, it will be time to try and make sense of it all. To a large extent, differences in the zooplankton communities observed will be related to the physical properties of the distinctly different water masses we have encountered during the expedition. Overall, we’re seeing similar species to what we observed in 2004, but perhaps in different proportions.
The ultimate goal will be try to tease out the relative influence of location, seasonality, between-year variability, and long-term changes currently occurring in the region (knowing that summer water temperature has increased and timing of ice retreat and advance are changing). We know that major changes in the zooplankton would have a cascade effect, impacting the wide range of fish, sea birds and marine mammals directly or indirectly dependent upon them. Monitoring this overall structure of the complex Chukchi ecosystem this is the central purpose the RUSALCA program.
The zooplankton group is jointly led by Drs. Russ Hopcroft (University of Alaska – Fairbanks) and Ksenia Kosobokova (Shirshov Institute, Russian Academy of Sciences). The pace of sampling relies on critical assistance provided by graduate students Liza Ershova (Shirshov) and Conny Jaspers (Technical University of Denmark).