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

Abstract

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 NOx 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 NOx characteristically enriched in 18O was present.