Maine Perspective Front Page
UMaine Biogeochemist Studies Marine Snow Beneath the Depths of the North Pacific
During her most recent research trip to the central North Pacific, Cindy Pilskaln spent a lot of time watching TV. It wasn't that the work was boring. In fact, the screen was filled with snow.
Pilskaln, an associate professor of oceanography in the University of Maine's School of Marine Sciences, has developed an underwater video system to collect information on what oceanographers call marine snow, the small particles that drift slowly but constantly down from the surface into the deep sea.
Marine snow is the ocean's dust balls: fecal pellets, parts of dead zooplankton and their feeding structures, algal remains and living micro-organisms.
Along with oceanography master's student Christina Darkangelo and research associate Charlotte Lehmann, Pilskaln has been analyzing those particles to help solve a mystery: how life processes are fueled in the vast open reaches of the oceans - areas which are generally so unproductive that they are described as "deserts."
Emerging information already led scientists to revise views on how quickly nutrients are transported and consumed in these areas. Ultimately, the findings of Pilskaln and her colleagues will improve the accuracy of models used to estimate how much carbon is absorbed by the world's oceans - a critical factor in global warming scenarios.
Pilskaln's research is part of an international effort funded by the National Science Foundation (NSF). It involves researchers from institutions such as the University of Texas, the Bermuda Biological Station, the University of California - Santa Barbara and the University of Massachusetts at Boston and Dartmouth. For her part, Pilskaln has garnered more than $166,000 in support over the last three years from the NSF.
"Strictly speaking, my academic background in sedimentary geology defines me as a geologist," she says. "Although I started out in the mud, I jumped up into the water column because I became very interested in the interconnected biological and chemical processes which ultimately determine sediment composition as well as nutrient input to the deep sea. Today, this type of research defines me and a significant number of my colleagues as biogeochemists."
In turbulent coastal waters, nutrients are brought to the surface by seasonal wind-mixing and periodic upwelling. However, in large areas of the open ocean, such mixing rarely occurs. Dissolved nutrients tend to concentrate in a layer about 100 or so meters deep called the nutricline. The nutricline is isolated from the ocean surface by water temperature and density differences, but some compelling new evidence suggests that somehow, compounds from the deep nutricline are finding their way back to the surface. Thus the problem: how does that happen?
In each of the last two years, Pilskaln has traveled to the central North Pacific on the R/V Moana Wave and the R/V New Horizon, oceanographic research vessels operated out of the University of Hawaii and Scripps Institute of Oceanography. She uses a device called a structured light and high resolution video system which she developed at the Monterey Bay Aquarium Research Institute (MBARI) in California in the late 1980s. Her device is based on a much larger instrument engineered by colleagues at the Woods Hole Oceanographic Institution. It consists of an underwater video camera and a paired light and mirror system, all mounted on a self-propelled remotely operated vehicle (ROV). An ROV operator controls the system via a cable from a ship on the surface. The cable transmits power to the vehicle, and data and video imagery back to the ship.
As it moves through the water during a research cruise, the system's camera focuses on an area with a known volume. Illuminated marine snow particles look somewhat like stars streaming past the Star Trek Voyager at slow speed.
Back in Orono, Pilskaln and Lehmann subject the video images to computerized analysis. Particles are counted and sized.
Their abundances are determined, and their shapes are categorized. The results of their calculations are estimates of how much marine snow exists and is sinking through the ocean on a daily or annual basis.
Using other devices called sediment traps, Pilskaln and her colleagues are also collecting large sinking particles and analyzing their chemical composition. Their estimates are thus producing a sharper picture of particulate nutrients such as carbon, nitrogen, phosphorus and silica that are moving through the water.
While typical marine snow drifts downward transporting nutrients to greater depths, scientists have discovered that in certain open ocean regions, there are aggregates of living algae which migrate back and forth between the surface and the deep nutricline. These so-called algal mats are generally several centimeters across. They appear to be particularly numerous in certain areas of the subtropical open Pacific.
Very few algae exhibit this deep migrating behavior. However, where nutrient levels are quite low, such as in the surface waters of the subtropical ocean, such behavior would provide an advantage. It would give these algae access to nutrients that would normally be out of their reach.
Work on these algal mats was pioneered by one of Pilskaln's colleagues, Tracy Villareal of the University of Texas. In a 1993 article in the journal Nature, he suggested that they might bring significant amounts of nitrogen and carbon to the ocean surface. They would thus affect the cycling of nitrogen and, in turn, carbon in large open areas of the oceans.
As part of the multi-institutional team studying these mats, Pilskaln and Darkangelo used Pilskaln's ROV-mounted video system in combination with another device, a video plankton recorder from Woods Hole. Their goal was to document the occurrence of these migrating algal mats at depths near the nutricline. They also wanted to quantify the amount of marine snow in the water between the surface down to a depth of 250 meters.
"Our numbers indicate that, first of all, there appears to be a lot more marine snow than scientists expected," says Pilskaln. "Secondly, near the nutricline, we have documented migrating algal mats which are presumably carrying much-needed nutrients to the surface where the algae can grow efficiently. The result of all this growth and feeding activity is the production of marine snow that sinks down from the surface carrying nutrients with it. Only a small percentage of the organic-rich snow makes it to the greatest ocean depths. Most of it is remineralized or consumed in the upper couple of hundred meters. Nutrients are at such a premium that they are very rapidly consumed in these open ocean areas.
"These areas are still typified by very low annual rates of primary production. That hasn't changed. But the amount of particulate material that we're seeing in the upper water column is a lot greater than we hypothesized based on low productivity rates."
It's too early to say what the numbers of Pilskaln and her colleagues mean for the global nitrogen and carbon cycles. Scientists working on the ocean flux project are still analyzing their data. Pilskaln and her colleagues will present their initial findings at the biannual Ocean Sciences Meeting in San Diego, sponsored by the American Geophysical Union and the American Society of Limnology and Oceanography.
Pilskaln's master's student, Christina Darkangelo, is completing her thesis on estimates of the nutrient flux via sinking marine snow and is helping to quantify the transport of nitrogen by migrating algal mats in the North Pacific.