Editor's Note: Woods Hole Oceanographic Institution oceanographer and photographer Chris Linder and science writer Helen Fields are taking part in a six-week cruise of the Bering Sea, a scientific expedition to study the effects of climate change on this polar ecosystem. This is the fifth blog post. To see all their posts, see "60 Seconds in the Bering Sea."

ON THE ICEBREAKER HEALY IN THE BERING SEA (at latitude 59.8189 N and longitude 171.7072 W), May 7, 2009—Crustaceans come in all sizes. At the top end of the scale are crabs with foot-long legs and tasty lobsters. Down near the bottom are copepods, buglike critters the size of a pencil point. Copepods live in fresh water and in the ocean. They get in your hair when you go swimming at the beach. And if you put a plankton net over the side of a boat in the Bering Sea, you’re going to get a ton of them.

This female Calanus copepod is about 5 millimeters long.

“They’re an important link in the food chain,” says Carin Ashjian, a biological oceanographer at Woods Hole Oceanographic Institution and the head of the zooplankton team on this cruise. Copepods eat single-celled organisms in the water—stuff like diatoms and dinoflagellates. In turn, copepods are food for chaetognaths (arrow-worms), larval fish, bigger fish, seabirds such as auklets, and whales. So they’ve got a big job to do, ecosystem-wise.

A plankton net returns from the water at the back of the ship.

Every morning, the zooplankton team puts a net into the water off the back of the ship and pulls it up again, filled with copepods of several species. Then they start experiments with the animals from the nets.

To find out what the copepods are eating, scientists put a few copepods into a bottle, filled with water from where they were caught, for 24 hours. It looks like plain old seawater to us, but to the copepods, it’s food. The water is full of single-celled organisms. Scientists take samples of the water before the copepods go in and after they’ve been in for 24 hours; later, someone will look at the samples under a microscope to find out what the copepods ate.

“The ultimate goal here is to try to understand how much they can eat,” says Ashjian. “It’s like cows grazing down a pasture. Can the zooplankton eat all the plant material that is produced?” The team also puts some female copepods in a “bug hotel”—more like a bug maternity ward—and after 24 hours they collect eggs to see how fast the copepods are reproducing.

The biggest animals in this picture are about 5 millimeters long.

Here are some of the animals that came up in a vertical plankton net on Tuesday, including the three main kinds that the zooplankton team has been seeing. The biggest copepods in this picture are Calanus. The smallest are Pseudocalanus. There are a few Metridia in here, too—one is the clear, smallish copepod near the top of the picture, a bit left of center. Just think, an animal as big as a 60-foot bowhead whale can support itself on animals this teeny. But then, copepods are pulling off a similar trick, eating single-celled organisms 1/1000th their size.

The water in the microscope picture above has tricaine, a fish anesthetic, to calm the animals down; most are lying on their backs. Not Metridia. They keep on swimming. “They’re cute,” says Ashjian. “I like Metridia because they’re just so nutter. They eat everything.” They also glow, producing light by a chemical reaction.

The camera’s shutter stayed open for four seconds as we poured water through the sieve; the Metridia swam around and left trails of light.

“The thought is that they emit this blue light to scare off a predator,” says Ashjian. It’s a beautiful sight, but unfortunately, they don’t make good pets—“Culturing copepods can be quite time-consuming.”

Copepods are only one part of the Bering Sea ecosystem; as the Healy has wound back and forth across the sea over the last five weeks, scientists have made dozens of stops and run dozens of experiments. This cruise ends this Tuesday, May 12, when we pull back into Dutch Harbor, Alaska, but the project goes on. Many of the scientists are going on another Bering Sea cruise this summer, and this cruise will be repeated next spring. Eventually these results will be combined with other measurements and other cruises to make one giant model of the Bering Sea ecosystem.

Images: Photos by Chris Linder, WHOI