What was done
The authors investigated "the potential relationship between Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-a (Chl-a) measurements in the Central Northeast Atlantic and North Sea (1997-2002) and simultaneous in situ measurements of the Phytoplankton Color Index (PCI) ... collected by the Continuous Plankton Recorder (CPR) survey, which is an upper-layer plankton monitoring program that has operated in the North Sea and North Atlantic Ocean since 1931 (Reid et al., 2003)."  By developing a relationship between the two data bases over their five years of overlap, they were able to produce a retrospective calculation of Chl-a averaged for the Central Northeast Atlantic and the North Sea for the period 1948-2002, over which time span the methodology of sampling and measurement of the PCI remained constant.

What was learned
Raitsos et al. report that "an increasing trend is apparent in mean Chl-a for the area of study over the period 1948-2002."  They also say "there is clear evidence for a stepwise increase after the mid-1980s, with a minimum of 1.3mg m^-3 in 1950 and a peak annual mean of 2.1 mg m^-3 in 1989 (62% increase)."  Alternatively, it is possible that the data represent a more steady long-term upward trend upon which is superimposed a decadal-scale oscillation with still finer-scale variability.  In a final comment on their findings, they also note that "changes through time in the PCI are significantly correlated with both sea surface temperature and Northern Hemisphere Temperature (Beaugrand and Reid, 2003)."

What it means
In the introduction to their study, the five scientists state that "phytoplankton produce >45% of the primary production of plants on Earth (Falkowski et al., 2004), absorb the greenhouse gas carbon dioxide (CO2) from the atmosphere, and contribute to the biological pump, which ensures that the climate of the world is much cooler than would otherwise be the case (Reid and Edwards, 2001)."  In an ominous comment in the final paragraph of their discussion, however, they say that Sabine et al. (2004) "showed that this oceanic sink of the key greenhouse gas CO2 may well be declining," and that, "if true, this result implies that concentrations of atmospheric CO2 are likely to increase at a more rapid rate over the next 100 years than currently predicted."

In reality, Sabine et al. only say there is an indication of a recent decrease in the oceanic carbon sink, and that it is "not statistically significant."  In addition, the much more direct data of Raitsos et al. depict a dramatic increase in phytoplankton over the past half-century, which if true for the majority of the world's oceans implies that concentrations of atmospheric CO2 could well increase at a less rapid rate over the next 100 years.  Also, the finding that the increase in phytoplankton parallels the increase in hemispheric temperature suggests the existence of a negative feedback loop that helps to further support the latter hypothesis.

References
Beaugrand, G. and Reid, P.C.  2003.  Long-term changes in phytoplankton, zooplankton and salmon related to climate.  Global Change Biology 9: 801-817.

Falkowski, P.G., Katz, M.E., Knoll, A.H., Quigg, A., Raven, J.A., Schofield, O. and Taylor, F.J.R.  2004.  The evolution of modern eukaryotic phytoplankton.  Science 305: 354-360.

Reid, P.C. and Edwards, M.  2001.  Plankton and climate.  In: Steele, J.H., Thorpe, S.A. and Turekian, K.K. (Eds.), Encyclopaedia of Ocean Sciences.  Elsevier, New York, NY, USA, pp. 2194-2200.

Reid, P.C., Matthews, J.B.L. and Smith, M.A., Eds.  2003.  Achievements of the Continuous Plankton Recorder survey and a vision for its future.  Progress in Oceanography 58: 115-358.

Sabine, C.L., Feely, R.A., Gruber, N., Key, R.M., Lee, K., Bullister, J.L., Wanninkhof, R., Wong, C.S., Wallace, D.W.R., Tilbrook, B., Millero, F.J., Peng, T.-H., Kozyr, A., Ono, T. and Rios, A.F.  2004.  The oceanic sink for anthropogenic CO2.  Science 305: 367-371.