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Abstract

The eastern Mediterranean Sea is an unusually nutrient-poor oceanbasin where the 15N/14N isotope ratios in many compartments of reactivenitrogen are lower than in comparable oceanic settings. To elucidatepossible reasons, we determined stable isotope ratios in nitrate,suspended particulate nitrogen (PN), and total dissolved reducednitrogen for stations across the eastern Mediterranean Sea occupiedin January and February 2007; sinking PN was collected at one ofthe stations in the period from February to September 2007. The δ15Nlevels of all reactive N compartments in waters of the basin arevery low (grand average 2.6‰) compared to other oceanic environments.Deep water nitrate below 500 m water depth (δ15N = 2.2 \pm 0.3‰)was more depleted in 15N than nitrate generally found in deep waternitrate pools of other oceans (δ15N ranges from 4.7 to 5.4‰), whereas15N was enriched in suspended particulate N (δ15N = 7.3 \pm 0.8‰)and reduced dissolved N (δ15N = 5.7 \pm 3.8‰) compared to nitrateand sinking particulate N intercepted in sediment traps (δ15N = 0.9\pm 0.8‰). We infer that extensive mineralization is the causeof the isotopic makeup of reactive N in deep water, in concert withthe lack of water column denitrification. Nitrogen and oxygen isotoperatios in nitrate of the mixed layer suggest an external source ofnitrate depleted in 15N, probably anthropogenic NOx rather than fixednitrogen. To explain the observed isotope anomaly in the mixed layer,either the ammonium formed by the breakdown of organic matter mustbe predominantly nitrified, or atmospheric NOx characteristicallyenriched in 18O was present.