Although NO,--is generally considered to limit primary productivity in most of the world's oceans, previous studies have suggested the Mediterranean Sea may be an exception. In this study of the southeastern Mediterranean, we found that all the POd3-was removed from the upper water column during the winter phytoplankton bloom in the core and boundary of a warm-core eddy, while measurable (0.3-0.6 PM) NO,-remained. The N: P (N0,-:P0,3-) ratio in the core and boundary of the Cyprus eddy was 27.4 and the slope of the linear portion of the N vs. P scattergram was 25.5 with a positive intercept of 0.5 PM on the NO,-axis. A similar N : P ratio (28-29), slope (21-23), and intercept (0.9-l. 1) was found for the water column across much of the southern Levantine basin. These data, taken together with the results of incubation experiments, lead us to conclude that the southeastern Mediterranean is strongly P limited. The degree of P limitation increases from west to east across the entire basin. We suggest that removal of PO,'-by adsorbtion on Fe-rich dust particles may bc an important process controlling the concentration of P in the water column.N03-is generally considered to be the master chemical variable limiting primary productivity throughout much of the ocean (Codispoti 1989). It is based principally on considerations of the N : P ratios of upwelling waters which are generally < 16 : 1 I At present: Department of Earth Science, Leeds University, Leeds L52 9JT, U.K. AcknowledgmentsThe chemical determinations of dissolved nutrients were carried out by L. Israilov. We thank U. Fiedler for technical help on-board ship; we also thank members of the Physics and Electronic department of IOLR. The data collected in September 1989 were sampled as part of a Group for Aquatic Productivity (GAP) cruise to the eddy organized by T. Berman. We thank the captain (A. Ben-Nun) and crew of the RV Shikmona. This manuscript was written while M.D.K. was on sabbatical leave at the University of Rhode Island. We thank M. E. Q. Pilson and D. Kester for the use of their facilities and for the many discussions which went into the preparation of this manuscript. We thank M. Wood for pointing out the importance of P limitation in this area and for critical reading of the manuscript by her and T. Berman. Finally we greatly appreciate the detailed comments of two reviewers, who helped us improve the quality of the final version of this manuscript.

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.

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The eastern Mediterranean Sea is one of the most extreme oligotrophic oceanic regions on earth in terms of nutrient concentrations and primary productivity. Nitrogen fixation has been suggested to contribute to the high N : P molar ratios of approximately 28:1 found in this region. Surprisingly, no molecular biological work has been performed in situ to assess whether N(2) fixation genes actually occur in the eastern Mediterranean Sea, or to determine which organisms are responsible for this process. In this study, we examined the presence and expression of nitrogenase genes (nifH) in the upper water layer of the eastern Mediterranean. Clone libraries constructed from both DNA and reverse-transcribed PCR-amplified mRNA were examined and compared. We observed different nifH genes from diverse microbial groups, such as Cyanobacteria, Proteobacteria and methanogenic Archaea. Interestingly, numerous phylotypes were observed in coastal stations at the DNA level but none were active. However, in far offshore stations, the phylotypes observed at the DNA level were the ones that were actually active. Our preliminary study revealed diverse diazotrophs that possess and express nifH genes, which may support N(2) fixation in the eastern Mediterranean Sea.

We report N(2) fixation rates measured from two stations monitored monthly off the Mediterranean coast of Israel during 2006 and 2007, and along a transect from Israel to Crete in September 2008. Analyses of time-series data revealed expression of nifH genes from diazotrophs in nifH clusters I and II, including cyanobacterial bloom-formers Trichodesmium and diatom-Richelia intracellularis associations. However, nifH gene abundance and rates of N(2) fixation were very low in all size fractions measured (> 0.7 µm). Volumetric (15) N uptake ranged from below detection (∼ 36% of > 300 samples) to a high of 0.3 nmol N l(-1) d(-1) and did not vary distinctly with depth or season. Areal N(2) fixation averaged ∼ 1 to 4 µmol N m(-2) d(-1) and contributed only ∼ 1% and 2% of new production and ∼ 0.25% and 0.5% of primary production for the mixed (winter) and stratified (spring-fall) periods respectively. N(2) fixation rates along the 2008 east-west transect were also extremely low (0-0.04 nmol N l(-1) d(-1), integrated average 2.6 µmol N m(-2) d(-1) ) with 37% of samples below detection and no discernable difference between stations. We demonstrate that diazotrophy and N(2) fixation contribute only a minor amount of new N to the P impoverished eastern Mediterranean Sea.

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