The capacity of marine ecosystems to sustain fish and other animal populations depends on the growth of phytoplankton, tiny drifting plants that convert carbon dioxide into living organic matter. In ocean systems, nutrient availability often sets limits on this production. Therefore, changes in upwelling circulation, increasing or decreasing mixing of ocean waters, or changes in freshwater runoff patterns could reduce or shift nutrient inputs, in turn causing changes in phytoplankton productivity at both regional and larger scales. Fluctuations in this productivity would ultimately affect larger marine animals-such as fish, whales, and seabirds-throughout the oceanŐs food web, starting with the tiny zooplankton upon which they directly or indirectly feed.

Changes in food availability may result in changes in species abundance and shifts in their distribution. Such changes may cascade throughout the food web, ultimately altering population stability in economically-important fish species. U.S. GLOBEC researchers are investigating questions like why various populations of zooplankton-feeding sardines in the Pacific basin fluctuate in apparent synchrony. Fluctuations in catches of these fisheries may be dominated by environmental changes: if ocean temperatures and circulation patterns are changing, distribution and abundance of sardines-and the plankton upon which they rely for food-may be responding to these changes.