Water leaving the Mediterranean Sea through the Strait of Gibraltar is richer in nitrogen than the water entering from the Atlantic. What is the source of the budget-balancing nitrogen of the Mediterranean? Two possibilities are nitrogen input from rivers and fixation of atmospheric N2. Pantoja et al. used analyses of the stable isotopic composition of nitrogen in particles and in chlorins (derivatives of chlorophyll) to evaluate the importance of biological N2 fixation as well as past changes in the nitrogen cycle. They calculate that up to 20% of nitrogen in the western basin and up to 90% in the eastern basin may derive from biological N2 fixation. These data add to a growing body of evidence that N2 fixation supplies a significant fraction of the bioavailable nitrogen in subtropical surface waters and that a large percentage of subsurface nitrate is generated from the oxidation of newly fixed nitrogen. -- HJS
Deep-Sea Res. I 49, 1609 (2002).
