Volume 6, Issue 45 Summer 2009
One Ocean, Shaken and Stirred
Thomas "Zack" Powell is a professor of integrative biology and a former chair of the Scientific Steering Committee of the U.S. GLOBEC program researching how climate change affects marine ecosystems and fishery production. photo credit: Angela Cheer
Whether they be whelks or whales, perch or penguins, the vast majority of life forms in the ocean owe their existence to phytoplankton. Though microscopic in size, these floating algae provide calories for nearly everything that lives in the sea.
Like all green plants, phytoplankton require a combination of light and mineral nutrients. But plankton are weak compared to the great power of the ocean and can't travel to richer waters under their own power.
"Plankton are basically at the mercy of the water body in which they're found," says Thomas "Zack" Powell, a Berkeley professor of integrative biology.
Instead, plankton rely on wind, waves and currents to deliver nutrients to their doorsteps. Powell uses computer models to study how physical transport processes affect plankton and the marine food web they support. In the 1970s, Powell became one the first scientists to study the impacts of transport-and ultimately climate-on marine organisms. Today, he is a researcher and a former chair of U.S. GLOBEC, a program that aims to understand how climate variability affects the abundance and distribution of fish and other marine species.
To demonstrate how ocean movements affect plankton, Powell picks up a children's toy. A sealed Lucite container about the size of a butter dish, it's filled to the top with two layers of liquids-the lower one tinted royal blue and the upper one clear. Powell gives the contraption a shake. The two strata disappear in a chaotic swirl of blue dye and bubbles, only to separate moments later as the turbulence subsides.
Surface chlorophyll concentration, bottom bathymetry and surface temperature in Autumn 1998, off Oregon. A southward meandering jet flows along the offshore edge of the cold, upwelled water (blues and greens in the right panel), which is also rich in chlorophyll (reds and yellows in the left panel). The bottom topography around Heceta Bank (44 - 45.5° N) and coastal geometry around Cape Blanco (42.8° N) deflect the jet offshore, creating myriad jets and eddies. image credit: courtesy Ted Strub
As in this miniature ocean, the ingredients required for plankton growth tend to be compartmentalized in different ocean layers. The light, of course, is found near the surface. But nutrients such as nitrogen, phosphorous, silicon and iron tend to be concentrated in the dense, cold waters of the deep sea.
"The places where mixing is occurring and breaking down that barrier are the most productive parts of the oceans," Powell says. To achieve these conditions, strong winds must push warm surface waters offshore. Then colder, nutrient-rich water layers get pulled up to take their place.
This phenomenon, called coastal upwelling, occurs in just a handful of places on the globe. Not coincidentally, these are among the most productive fisheries on the planet. Upwelling centers, such as those off the coast of Chile and Peru and along northern and central California, are responsible for roughly half of all marine production.
"There are no areas in the world ocean where the physics is more important to biological processes," Powell says. "The whole rest of the ocean is more of a desert than the deserts we think of in California."
The North Pacific Gyre Oscillation (NPGO) is an ocean mode linked to marine productivity. The NPGO features strong north to south winds (lines, top panel) that favor upwelling and strong sea level pressures northeast of Hawaii but weaker pressures in the North Pacific (colors, top panel). The severity of the NPGO closely tracks the depth of upwelling mixing off the coast of northern and central California (bottom panel). image credit: courtesy Di Lorenzo et al. 2008
As critical as upwelling centers are, climate models do a shoddy job of calculating plankton productivity in these waters. Part of that is a problem of scale. Major global climate models divide the ocean into boxes 100 km on a side or larger. The smaller currents and eddies that drive coastal mixing don't even register.
Powell has been working to bridge this gap. With support from Berkeley's Miller Institute, the National Center for Atmospheric Research, and Enrique Curchitser of Rutgers University, Powell was able to focus on integrating global ocean models with a finer scale regional model for Northern California waters. "The regional model catches the squirts, jets, and filaments that are characteristic of our coast but are lost in the larger model," Powell says.
Along with Emanuele Di Lorenzo of the Georgia Institute of Technology, Powell is now studying how one of the Pacific's two major cycles, called the North Pacific Gyre Oscillation (NPGO), affects the ocean along coastal California. Its strong north-to-south winds appear to bring a heavy dose of salt and nutrients to the surface, fertilizing big phytoplankton blooms. At the same time, nearly opposite conditions of lower salinity and weaker upwelling appear in the coastal Gulf of Alaska.
This roughly ten-year cycle closely matches previously unexplained fluctuations of salinity, nutrients and chlorophyll in the North Pacific. Though research into the NPGO is still in its infancy, the cycle already appears to be a primary influence on many fish populations living along the Northern California coast. Powell relishes the prospect of finding what the newly discovered NPGO does to fish and plankton alike. "We've just started to scratch the surface," he says.