SYNOPTIC STUDY OF VARIATIONS IN THE FLUORESCENCE-BASED MAXIMUM QUANTUM EFFICIENCY OF PHOTOSYNTHESIS ACROSS THE NORTH-ATLANTIC OCEAN

Using ship-based phytoplankton fluorescence techniques on a 9-d transe ct of the North Atlantic Ocean, we have produced the first synoptic vi ew of variations in the maximum quantum yield of photosynthetic energy conversion (phi(m)) on an ocean-wide basis. Based on our phi(m) measu rements, the North Atlantic can be divided into two basins. The southe astern basin (from the African coast to the midocean ridge) is charact erized by relatively little horizontal variation in phi(m) over vast s tretches of the ocean. In the northwestern basin (from the midocean ri dge to the Canadian coast), we measured lower values of phi(m) than in the eastern basin, particularly in the upper water column. Lowest sur face values were found in an area of strong horizontal flow. Our resul ts suggest that nutrient supply limits phytoplankton productivity over vast stretches of the North Atlantic in this period (early summer). H owever, surface values of phi(m) across the North Atlantic appear to b e independent of nutrient concentration and distance to nitrate supply . We propose that horizontal advection may play an important role in d etermining the local conditions to which phytoplankton productivity pa rameters respond. Our results also suggest that an apparent relationsh ip between remotely sensed variables (ocean color, sea surface height) and phytoplankton photosynthetic parameters exists that may provide t he basis for the development of new algorithms used to estimate phytop lankton productivity from satellites.