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Ocean’s least productive waters are expanding |
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Jeffrey J. Polovina, Evan A. Howell, and Melanie Abecassis |
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| Abstract:
A 9-year time series of SeaWiFS remotely-sensed
ocean color data is used to examine temporal trends in
the ocean's most oligotrophic waters, those with surface
chlorophyll not exceeding 0.07 mg chl/m3. In the North
and South Pacific, North and South Atlantic, outside the
equatorial zone, the areas of low surface chlorophyll
waters have expanded at average annual rates from 0.8
to 4.3%/yr and replaced about 0.8 million km2/yr of
higher surface chlorophyll habitat with low surface
chlorophyll water. It is estimated that the low surface
chlorophyll areas in these oceans combined have expanded
by 6.6 million km2 or by about 15.0% from 1998 through
2006. In both hemispheres, evidence shows a more rapid
expansion of the low surface chlorophyll waters during the
winter. The North Atlantic, which has the smallest
oligotrophic gyre is expanding most rapidly, both annually
at 4.3%/yr and seasonally, in the first quarter at 8.5%/yr.
Mean sea surface temperature in each of these 4 subtropical
gyres also increased over the 9-year period. The expansion of
the low chlorophyll waters is consistent with global
warming scenarios based on increased vertical stratification
in the mid-latitudes, but the rates of expansion we observe
already greatly exceed recent model predictions.
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