Why is the Eastern Mediterranean phosphorus limited?

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 diazotrophs 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.