Posted on behalf of Wendee Holtcamp, blogging for Nature aboard the research vessel Thomas G. Thompson
At first thought, plankton seem boring. Why care about tiny, floating organisms when walrus, albatross, or salmon live in the sea too? But Hatfield Marine Science Center technician Tracy Shaw showed me photos of Bering Sea zooplankton taken under a microscope, many caught on this very cruise, and it ratcheted up plankton’s cool-factor in my book.
I saw an alien-like squid larva, cute crab “zoea,” shrimp-like krill, and tiny jellies. Plankton refers to organisms that drift with currents. Some remain planktonic their whole lives, while others are merely larval forms of bigger animals, like sea stars, octopus, or fish. Their sizes range from microscopic to a few centimetres long.
Every night, Shaw and her colleague Megan Schatz, a technician at the University of Washington, collect zooplankton using a bongo net towed alongside the boat as it moves at slow speed. Krill – shrimp-like zooplankton – collectively move towards the surface at night to feed, making it easier to catch them in a net. If the krill team capture at least 20 krill in their bongo tow, they set up a grazing experiment in incubators on deck. Using a setup similar to Diane Stoecker’s microzooplankton experiments, Shaw and Shatz determine phytoplankton and microzooplankton abundance before and after the experiment, which lasts 24 hours. This tells them what and how much the krill ate.
“Our big, overarching goal is to determine whether the available food supply is dependent on sea ice. If sea ice conditions change, what will that do to krill?” says Shaw.
“Krill are very important for birds, fish, seals, and other animals," says primary investigator Evelyn Lessard, a University of Washington oceanography professor. "And they’re the reason salmon are pink.” She shows me the reddish pigment on their exoskeleton.
The krill Thysanoessa raschii dominates the middle domain, where the cold pool acts as a ‘footprint’ of the past winter’s sea ice. In the deep water off the shelf, they find another species, T. longipes. “In experiments with T. longipes the chlorophyll values didn’t change, but you could see a reduction in the number of ciliates,” says Shaw. On this trip, these krill preferred the ice-cream cone microzooplankton ciliates, and not phytoplankton. Few scientists have studied plankton grazing in the Bering Sea, so all these on-board experiments during the past three years should unveil a wealth of new information on these key components of the food web.
Even before the Bering Sea Project began, Alexei Pinchuk studied zooplankton in the subarctic and Arctic. His colleague Ken Coyle and several other scientists have compiled data collected in warm and cold years to revise the Oscillating Control Hypothesis. They found small and large zooplankton react differently to warm versus cold years, whereas the original hypothesis treats zooplankton as a single “box.” In warm years, small copepods abound, but large zooplankton – including lipid-rich krill and large copepods – decline. Because of the loss of large zooplankton in warm years, the result is still an overall decline in biomass – which means less food for seabirds, marine mammals, and larger age-class pollock. In fact, when total zooplankton declines, pollock become increasingly cannibalistic; the larger ones eat their own kind. A warming trend does not bode well for many organisms important to people.
Despite three cold winters and springs in the Bering Sea, scientists have definitively documented that the Arctic and subarctic regions are warming. According to NOAA, scientists have observed a northward biogeographic shift of several species as the region has warmed. More to the point, in a review of the geologic history of the Arctic, Leonid Polyak of Byrd Polar Research Center and colleagues found that sea ice has existed in the Arctic for the past 47 million years, and has consistently covered at least part of the region for the last 13-14 million years. Beginning in the late 19th century and becoming more pronounced during the past 30 years, ice loss “appears to be unmatched over the last few thousand years and unexplainable by any of the known natural variabilities”.
Fuente: nature.com
