On the front lines
Carolina oceanographers study little phytoplankton while on a big expedition.
Some of the planet’s tiniest organisms are visible from space. Phytoplankton — single-celled, water-dwelling algae — are one-millionth of a meter in size and produce about 50 percent of the oxygen we breathe. These microscopic oxygen factories inhabit the sunlit portion of the world’s oceans, supplying nourishment for much larger organisms and valuable information for scientists.
Millions of phytoplankton swirl around in jugs of seawater, collected just moments before from the Equatorial Pacific Ocean. Adrian Marchetti filters the water in his makeshift lab — an old garden shed on the Sierra Negra. This Galápagos National Park Service vessel is typically used to monitor the waters surrounding the islands. “These vessels don’t have the equipment we usually use to do our work, so we had to improvise,” Marchetti, a UNC-Chapel Hill oceanographer, says with a laugh.
Meanwhile, his partner on the expedition, Scott Gifford — also an oceanographer from UNC — examines heterotrophic bacteria. When describing these organisms, Gifford compares the ocean ecosystem to a farm. “The phytoplankton are the crop,” he says. “Ultimately, the organic matter produced by those crops support us and other animals. In the ocean, heterotrophic bacteria are like the insects; they consume that organic matter. If they consume a lot of it, that means less food available to support animals higher in the food web such as fish and marine mammals.”
Scientists call this process the “microbial loop,” and Marchetti and Gifford are particularly interested in this one because the waters surrounding the Galápagos are warmer than normal. El Niño has arrived. Every five to eight years, this climate anomaly throws the islands off balance for 12 to 15 months. Phytoplankton decrease, causing a disruption in the food supply. Small fish that eat the phytoplankton die off, and that cascades up the food chain. A surplus of rain boosts the terrestrial environment; land plants grow like wildfire and the bird populations become overwhelming.
Alive and upwell
On the ship deck, Marchetti squints as the Galápagos sun sets over the Pacific. He lifts a round, plastic sampling device — called a Niskin bottle — and connects it to a long steel cable attached to the ship. He flashes a thumbs down at Gifford, signaling the device is ready to go into the water. Using a simple cable-and-weight system, the bottle drops to various depths, collecting slightly different samples of water to compare the diversity, growth characteristics, and interactions of the phytoplankton and bacterial communities (called the marine microbiome) at those depths.
Phytoplankton growth depends on available sunlight, temperature, and nutrient levels — the Galápagos Archipelago houses some of the most nutrient-rich waters on the planet.
When strong winds blow across the ocean, surface water gets pushed away, causing cold nutrient-rich water to flow up from the deep ocean through a process called upwelling. “Those nutrients fuel a lot of phytoplankton growth in that area,” Marchetti says. “And that cascades up the food web to support these very rich, charismatic organisms that live in the Galápagos — the fish, the sharks, the iguanas, the sea lions, the sea turtles, and of course, the penguins.”
When El Niño strikes, upwelling slows, so fewer nutrients make it to the surface, and the waters heat up. “El Niño can vary in severity,” Marchetti says. “Sometimes it’s mild, meaning a less drastic decrease in upwelling. But then you have ones that are incredibly strong — and they can last a lot longer and have a serious impact on the Galápagos.”