Declines in Phytoplankton Cell Size in the Subtropical Oceans during 1998-2007 Estimated from Satellite Remotely-sensed Temperature and Chlorophyll

PIFSC scientists in the Ecosystems and Oceanography Division have made further strides in developing tools for monitoring the effects of climate change on ocean ecosystems. Jeffrey Polovina and Phoebe Woodworth used satellite remotely-sensed measurements of sea surface temperature (SST) and surface chlorophyll to estimate the median cell size of phytoplankton over the global ocean from 1998-2007. The estimates had a temporal-spatial resolution of month and 11 km2.

The median cell diameter of phytoplankton, as estimated using remotely sensed variables over the North Pacific, varies 
               spatially and seasonally. Results are shown for February (top) and August (bottom), 2002.
The median cell diameter of phytoplankton, as estimated using remotely sensed variables over the North Pacific, varies spatially and seasonally. Results are shown for February (top) and August (bottom), 2002.

The temporal dynamics of median phytoplankton cell size were examined for the Pacific equatorial upwelling region and the subtropical oceans. For the equatorial upwelling region, cell size varied coherently with the El Niño Southern Oscillation with smaller (larger) median cell size during El Niños (La Niñas). Specifically, estimated median cell diameter increased by 34% between the 1998 El Niño and the 1999-2001 La Niña. In the subtropical oceans, over the period 1998-2007, median cell diameter exhibited statistically significant linear declines of about 2% in the North and South Pacific, and 4% in the North Atlantic. Pooling the data over all subtropical oceans, over the period 1998-2007, global median cell diameter declined about 2%.

These results suggest that phytoplankton cell size may vary on interannual and decadal scales, resulting in changes in the length of food chains and hence energy transfer to higher trophic levels. Further, a shift to smaller phytoplankton has been hypothesized as a response to ocean warming. Thus, this approach, estimating phytoplankton cell size from remotely-sensed temperature and chlorophyll, has the potential to provide global monitoring of an aspect of phytoplankton community structure likely to be responsive to future climate change.