Woods Hole Oceanographic Institution : 2003 Annual Report : Research : Science Highlights : The Picoplankton and the Whale

Great fleas have little fleas upon their backs to bite ’em, /And little fleas have lesser fleas, and ad infinitum. /And the great fleas themselves, in turn, have greater fleas to go on; /While these again have greater still, and greater still, and so on.

So wrote the nineteenth century mathematician Augustus de Morgan, noting expanding scales of life and their connections.

Today, biologists at the Oceanographic use miniaturization of technology and clever engineering to illuminate formerly invisible worlds at both ends of the size spectrum, from single-cell photosynthetic plankton to sperm whales.

At the smallest scale, life on Earth is made possible by humble photosynthetic organisms called phytoplankton, which lie at the base of the ocean food chain and produce most of Earth’s oxygen. The smallest among them, called picoplankton, make up in number what they lack in size: picoplankton, including bacteria, photosynthetic, and nonphotosynthetic cells, are the most abundant organisms on Earth. Studying them is a challenge, however, since they are too small to see, like dark matter making up most of the ocean universe.

Microscopic phytoplankton are detectable by proxy. In the laboratory, water samples are exposed to light, and the cells counted and characterized based on fluorescence of their chlorophyll. But no one has been able to study picoplankton in their environment, so their diversity and physiology are poorly understood.

Now biologists Heidi Sosik and Rob Olson have developed the Flow Cytobot (right), an automated submersible flow cytometer that detects small phytoplankton in situ. The cytometer (from Greek, kytos = hollow vessel, and Latin, meter = measure) continuously samples water from its immediate environment, counting cells as they stream through. It can be programmed to operate underwater, unattended for months, transmitting a continuous record of the phytoplankton community to shore.

With this instrument, Sosik and Olson identify cells and measure cycles of growth and division. In 2004, it will be installed at the Martha’s Vineyard Coastal Observatory, opening a window into seasonal, annual, and longer-term changes in numbers of phytoplankton of all sizes. Sosik and Olson will monitor ocean productivity on a small spatial scale in relation to changing local conditions. But their results may have implications for the entire ocean food chain.

“This is the beginning of a long-term effort to look at how phytoplankton communities respond over decades,” Sosik said, “and to understand the consequences of human-caused and natural environmental changes at the lowest level of the food chain.”

At the other end of the spectrum, Peter Tyack studies the diving behavior of sperm whales, carnivores at the top of the food chain. Like picoplankton sperm whales are hard to study because of their size, and because they are unseen most of the time. They typically surface for only five minutes, then dive for up to an hour, to depths of 1,000 meters (4,600 feet) or more (above).

New tools, called digital acoustic recording tags, are attached to whales with suction cups and track them throughout dives. Tag measurements are the result of collaboration: Engineer Mark Johnson designed and built the tags; Biologist Michael Moore developed the method of using a cantilevered carbon-fiber pole to touch an instrument to a whale; Johnson and Tyack adapted the pole for attaching tags (below). It’s an adventure steering a boat close to an unsuspecting whale and with a 40-foot pole attaching a tag and hoping the whale won’t notice. Sperm whales are not noted for friendly behavior toward boats. “Before I started doing this,” Tyack said, “I spent a day at the New Bedford Whaling Museum, reading records of sperm whale attacks. ”Luckily, whales don’t react to the tags. They are small—like a flea on a Great Dane—but huge in storage capacity. Time, depth, fluke beats, body orientation, sounds the whale makes, and ambient sounds are all stored. After an hour or two, the suction cup releases and the tags, and their data, are retrieved.

Every tagging increases Tyack’s understanding of sperm whale diving (right) and individual and population behavior. In 2003, tagging became routine. He followed whales, listened to them, and monitored external sounds, including whale calls, seismic, and sonar sounds. From this he reconstructed normal behavior and behavior in response to sounds. Using the tag as a dosimeter is an important advance in determining the effects of sound on whales. “This will help provide crucial data that decision makers need for the wise regulation of human-induced noise in the oceans,” Tyack said.

—Kate Madin (kmadin@whoi.edu)