Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column

We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5-74 m) was more depleted in N-15 in the eastern basin (-0.3 +/- 0.5%.) than in the western basin (+ 2.4 +/- 1.4parts per thousand), suggesting that nitrogen supplied by biological N-2 fixation may be an important source of new nitrogen in the eastern basin, where preformed nitrate from the Atlantic Ocean could have been depleted during its transit eastward. The delta(15)N of nitrate in the deep Mediterranean (similar to 3parts per thousand in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8-5parts per thousand), also suggesting an important role for N-2 fixation. The eastward decrease in the delta(15)N of surface PON is greater than the eastward decrease in the delta(15)N of the subsurface nitrate, implying that the amount of N-2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the delta(15)N of phytoplankton biomass from the delta(15)N of the nitrate upwelled from below. Variations in productivity associated with frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the delta(15)N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) and nitrate concentration in the Alboran Sea. N-15 enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of + 7.4 +/- 0.7parts per thousand. The delta(15)N in sinking particles caught at 100m depth (4.9-5.6parts per thousand) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Deltadelta(15)N[PON-chlorin] = + 6.4 +/- 1.4parts per thousand) in the surface, similar to the offset reported earlier in cultures for cellular N and chl-a. This indicates that delta(15)N of phytoplankton biomass was retained in surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity. (C) 2002 Elsevier Science Ltd. All rights reserved.