『Abstract
　The Eastern Mediterranean Sea (EMS) is a relatively small ocean basin with an unusually high nitrate to phosphate ratio in the deep waters (〜28:1). As a consequence, the typical winter phytoplankton bloom is P rather than N limited. Based on a critical review of the existing evidence, we conclude that the unusual nutrient ratio is due to high N:P values in all the external nutrient inputs to the EMS, coupled to low denitrification rates within the ultra-oligotrophic basin. Thus, we rule out the alternative hypothesis that P limitation in the EMS is due to regionally high rates of diazotrophic N2 fixation. The first line of evidence is a basin-wide nutrient budget, which demonstrates that N removal from the EMS is balanced by N inputs by rivers and atmospheric deposition without the need to invoke additional N2 fixation. The budget further indicates that riverine and atmospheric inputs all have N:P ratios that significantly exceed the Redfield ratio (16:1), and that atmospheric deposition is the major external source of bioavailable N to the EMS. The second line of evidence is a series of recent δ¹⁵N measurements showing depleted values in both wet and dry N deposition in the EMS. Hence, the depleted δ¹⁵N values of nitrate measured in deep waters of the EMS (2.4±0.1‰) do not reflect N2 fixation, but rather a signal inherited from the nitrate deposited from the atmosphere. The few direct rate determinations of N2 fixation provide the third line of evidence: they show very low activities in both pelagic and coastal areas (＜3 μmol m^-2 d^-1). A single extremely high N2 fixation rate (15 mmol m^-2 d^-1) reported for the Cyprus warm-core eddy is likely an artifact, as there is no evidence for unusually high numbers of diazitrophs at the time of the rate determination. A nutrient budget for the same warm-core eddy also implies the absence of significant N2 fixation. Overall, the extreme P limitation of the EMS seems to efficiently preclude N2 fixation. Thus, normal N and P cycling processes are operating in the EMS, albeit in a peculiar oceanographic setting that allows deep waters to build-up high N:P ratios, prior to export via the Straits of Sicily. The present-day situation is very different from that prevailing during times in the recent geological past when organic-rich sediments (sapropels) were being deposited. The inferred high rates of N2 fixation during sapropel episodes were probably the result of high rates of denitrification and enhanced P recycling as the oxic-anoxic boundary migrated up into the water column.』

1. Introduction
2. Oceanography of the Eastern Mediterranean
3. Nutrient budgets
4. Nitrogen stable isotopes
5. Nitrogen fixation rate measurements
6. Nitrogen fixing organisms in the EMS
7. Nutrient uptake by phytoplankton in the EMS
8. How do EMS deep waters accumulate high N:P ratios?
9. Implications of this study
Acknowledgements
References