TY - JOUR
T1 - Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column
AU - Pantoja, Silvio
AU - Repeta, Daniel J.
AU - Sachs, Julian P.
AU - Sigman, Daniel Mikhail
N1 - Funding Information:
We thank Hervé Claustre and Louis Prieur for the invitation to participate in the cruises Minos, ALMOFRONT-2, and Prosope in the Mediterranean Sea. H. Claustre and P. Morin graciously provided access to unpublished ambient chlorophyll and nitrate concentrations measured during the ALMOFRONT-2 cruise, respectively. N. Leblond provided us subsamples from sediment traps. This research was supported by NSF OCE-9711702 to D. Repeta and by NSF OCE-9981479 to D. Sigman. S. Pantoja was supported at WHOI by funds provided by the Johnson Foundation.
PY - 2002/9/1
Y1 - 2002/9/1
N2 - 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 (△δ15N[PON-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.
AB - 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 (△δ15N[PON-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.
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U2 - 10.1016/S0967-0637(02)00066-3
DO - 10.1016/S0967-0637(02)00066-3
M3 - Article
AN - SCOPUS:0036710604
SN - 0967-0637
VL - 49
SP - 1609
EP - 1621
JO - Deep-Sea Research Part I: Oceanographic Research Papers
JF - Deep-Sea Research Part I: Oceanographic Research Papers
IS - 9
ER -