Estimates of global photosynthesis by NASA, the Intergovernmental Panel
on Climate Change and others are that roughly 50% occurs in the oceans
and 50% occurs on land. I have seen older textbooks with estimates
60/40 both ways so estimates vary. The current 50/50 estimates may
change as more accurate techniques to measure global photosynthesis are
employed.

http://www.nasa.gov/home/hqnews/2005...o_problem.html
http://www.newstarget.com/005139.html
http://www.noc.soton.ac.uk/JRD/SCHOO...phsynth01.html

The oceans are about twice the area of land but ocean photosynthesis is
often lower than on land as the color maps in the following NASA
website indicate:

http://earthobservatory.nasa.gov/Newsroom/NPP/npp.html

David R. Hershey