External N inputs and internal N cycling traced by isotope ratios of nitrate, dissolved reduced nitrogen, and particulate nitrogen in the eastern Mediterranean Sea

The eastern Mediterranean Sea is an unusually nutrient-poor ocean basin where the ¹⁵N/¹⁴N isotope ratios in many compartments of reactive nitrogen are lower than in comparable oceanic settings. To elucidate possible reasons, we determined stable isotope ratios in nitrate, suspended particulate nitrogen (PN), and total dissolved reduced nitrogen for stations across the eastern Mediterranean Sea occupied in January and February 2007; sinking PN was collected at one of the stations in the period from February to September 2007. The δ¹⁵N levels of all reactive N compartments in waters of the basin are very low (grand average 2.6‰) compared to other oceanic environments. Deep water nitrate below 500 m water depth (δ¹⁵N = 2.2 ± 0.3‰) was more depleted in ¹⁵N than nitrate generally found in deep water nitrate pools of other oceans (δ¹⁵N ranges from 4.7 to 5.4‰), whereas ¹⁵N was enriched in suspended particulate N (δ¹⁵N = 7.3 ± 0.8‰) and reduced dissolved N (δ¹⁵N = 5.7 3.8‰) compared to nitrate and sinking particulate N intercepted in sediment traps (δ¹⁵N = 0.9 ± 0.8‰). We infer that extensive mineralization is the cause of the isotopic makeup of reactive N in deep water, in concert with the lack of water column denitrification. Nitrogen and oxygen isotope ratios in nitrate of the mixed layer suggest an external source of nitrate depleted in ¹⁵N, probably anthropogenic NO _x rather than fixed nitrogen. To explain the observed isotope anomaly in the mixed layer, either the ammonium formed by the breakdown of organic matter must be predominantly nitrified, or atmospheric NO_x characteristically enriched in ¹⁸O was present.