Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column |
| |
| Institution: | 1. Marine Apex Predator Research Unit (MAPRU), Institute for Coastal and Marine Research, Nelson Mandela University, Port Elizabeth 6031, South Africa;2. DST-NRF Centre of Excellence at the FitzPatrick Institute for African Ornithology, Department of Zoology, Nelson Mandela University, Port Elizabeth 6031, South Africa;3. Centre d''Etudes Biologiques de Chizé, Station d''Écologie de Chizé-Université de La Rochelle, CNRS UMR7372, 79360 Villiers-en-Bois, France;4. CESAB-FRB, Bâtiment Henri Poincaré, Domain du Petit Arbois, Ave Louis Philibert, 13100 Aix-en-Provence, France;5. Geology and Geophysics, University of Utah, Salt Lake City, United States;6. Ocean and Earth Science, University of Southampton Waterfront Campus, Southampton, United Kingdom;1. CSIR-National Institute of Oceanography, Dona Paula, Goa 403004, India;2. Council of Scientific and Industrial Research, Ministry of Science and Technology, Government of India, Anusandhan Bhawan, Rafi Marg, New Delhi 110001, India;1. CSIR National Institute of Oceanography, Biological Oceanography Division, Dona Paula, Goa 403004, India;2. Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India;3. School of Oceanographic Studies, Jadavpur University, 188 Raja S.C. Mallick Rd, Kolkata 700032, India;4. Bharthidasan University, Tiruchirappalli, TN 620024, India;5. Laboratoire d''Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN-IPSL, Sorbonne Université, IRD, CNRS, MNHN), 4 Place Jussieu, 75005 Paris, France;1. Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand;2. Department of Marine Science, University of Otago, 310 Castle Street, Dunedin, New Zealand;3. Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK;4. Department of Geosciences, Princeton University, Princeton, NJ, USA |
| |
| Abstract: | We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5–74 m) was more depleted in 15N in the eastern basin (?0.3±0.5‰) than in the western basin (+2.4±1.4‰), suggesting that nitrogen supplied by biological N2 fixation may be an important source of new nitrogen in the eastern basin, where preformed nitrate from the Atlantic Ocean could have been depleted during its transit eastward. The δ15N of nitrate in the deep Mediterranean (～3‰ in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8–5‰), also suggesting an important role for N2 fixation. The eastward decrease in the δ15N of surface PON is greater than the eastward decrease in the δ15N of the subsurface nitrate, implying that the amount of N2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the δ15N of phytoplankton biomass from the δ15N of the nitrate upwelled from below. Variations in productivity associated with frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the δ15N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) and nitrate concentration in the Alboran Sea. 15N enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.4±0.7‰. The δ15N in sinking particles caught at 100 m depth (4.9–5.6‰) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Δδ15NPON-chlorin]=+6.4±1.4‰) in the surface, similar to the offset reported earlier in cultures for cellular N and chl-a. This indicates that δ15N of phytoplankton biomass was retained in surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
