Experimental work during a cruise along a W‐E transect in the Mediterranean Sea suggests that (1) orthophosphate concentrations in the upper photic zone show a decreasing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6–2ߝµm size fraction, probably Synechococcus , have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic...