The first study by Michael Behrenfeld, of Oregon State University, USA, one of the world's leading experts in the use of remote sensing technology to examine ocean productivity has appeared in the journal Ecology (1). His findings challenge years of conventional wisdom about the growth of phytoplankton. And they also raise concerns that global warming, rather than stimulating ocean productivity, may actually curtail it in some places.
This nine-year analysis of satellite records of chlorophyll and carbon data dismisses the long-held "critical depth hypothesis" - a theory first developed in 1953 - as a valid explanation for phytoplankton bloom initiation. That hypothesis, still commonly found in oceanography textbooks, states that phytoplankton bloom in temperate oceans in the spring because of improving light conditions -- longer and brighter days -- and warming of the surface layer. Warm water is less dense than cold water, so springtime warming creates a surface layer that essentially "floats" on top of the cold water below, slows wind-driven mixing and holds the phytoplankton in the sunlit upper layer more of the time, letting them grow faster.
But the rate of phytoplankton accumulation actually begins to surge during the middle of winter, the coldest, darkest time of year, which attracted growing criticism of the previous theory from various quarters in recent years. The fundamental flaw of critical depth hypothesis, Behrenfeld said, is that it did not adequately account for seasonal changes in the activity of the zooplankton, in particular their feeding rate on the phytoplankton. "To understand phytoplankton abundance, we have been paying way too much attention to phytoplankton growth and way too little attention to loss rates, particularly consumption by zooplankton. "When zooplankton are abundant and can find food, they eat phytoplankton almost as fast as it grows."
Recent models, supported by Behrenfled's study of satellite records, suggest that the spring bloom depends on processes occurring earlier in the fall and winter, as a response to 'uncoupled grazing'. As winter storms become more frequent and intense, the biologically-rich surface layer mixes with cold, almost clear and lifeless water from deeper levels. This dilutes the concentration of phytoplankton and zooplankton, making it more difficult for the zooplankton to find the phytoplankton and eat them -- so more phytoplankton survive and populations begin to increase during the dark, cold days of winter. In the spring, storms subside and the phytoplankton and zooplankton are no longer regularly diluted. Zooplankton find their prey more easily as the concentration of phytoplankton rises. So even though the phytoplankton get more light and their growth rate increases, the voracious feeding of the zooplankton keeps them largely in-check, and the overall rise in phytoplankton occurs at roughly the same rate from winter to late spring. Eventually in mid-summer, the phytoplankton run out of nutrients and the now abundant zooplankton easily overtake them, and the bloom ends with a rapid crash.
"What the satellite data appear to be telling us is that the physical mixing of water has as much or more to do with the success of the bloom as does the rate of phytoplankton photosynthesis," Behrenfeld said. "Big blooms appear to require deeper wintertime mixing." That is a concern, he noted, because with further global warming, many ocean regions are expected to become warmer and more stratified. In places where this process is operating -- which includes the North Atlantic, western North Pacific, and Southern Ocean around Antarctica -- that could lead to lower phytoplankton growth and less overall ocean productivity, less life in the oceans. Worth noting is that some of these regions with large seasonal phytoplankton blooms are among the world's most dynamic fisheries.
This is confirmed by a major study (2) just published in Nature (29 July issue) by Daniel Boyce, Marlon Lewis and Boris Worm of Dalhousie University, Halifax, Canada. Their thorough analysis of regional and global phytoplankton trends concludes that global phytoplankton biomass has declined from 1899 to 2009 in eight out of ten regions studied, with a global rate of decline of about 1% of the global median per year. Multiple lines of evidence provided by the authors suggest that these changes are generally related to climatic and oceanographic variability, and particularly to increasing sea surface temperature over the past century.
(1) Behrenfeld M. et al., 2010. Abandoning Sverdrup's Critical Depth Hypothesis on phytoplankton blooms. 2010. Ecology, 91(4): 977 DOI: 10.1890/09-1207.1
(2) Boyce D.G., Lewis M.R. and B. Worm, 2010. Global phytoplankton decline over the past century. Nature, 466: 591-596 DOI:10.1038/nature09268