Plankton, the organisms at the base of the marine food chain, are sensitive to ocean surface and near-surface temperatures. Stokstad (2004) reviews a paper by Richardson and Shoeman (2004) that report measurements taken by collecting plankton off silk lines trailed behind commercial freighters in the North Atlantic. Measurements made over the last 70 years in the Atlantic Ocean reveal changes in plankton distribution in response to temperature changes associated with global warming. While higher temperatures promote metabolic function and, in principle, would increase plankton density, higher sea-surface temperatures suppress vertical mixing and thereby reduce upward nutrient flux to support plankton growth. The net result is that phytoplankton abundance is increasing in cooler regions and decreasing in warmer regions.
References
Stokstad, E., 2004: Changes in planktonic food web hint at major disruption in
Atlantic. Science, 305, 1548-1549.
Richardson, A.J. and D.S. Shoeman, 2004: Climate impact on plankton
ecosystems in the northeast Atlantic. Science, 305, 1609-1612.