Influence of anticyclonic eddies on the Biogeochemistry from theOligotrophic to the Ultraoligotrophic Mediterranean (BOUM cruise)

[摘要] We studied a longitudinal transect in the Mediterranean Sea (MS) and alongthis transect, the influence of anticyclonic eddies at three long duration (LD)stations. The deep chlorophyll maximum depth, the euphotic layer depth andthe top of the nitracline depth are clearly correlated outside of theeddies, and deepen from the oligotrophic western to the ultraoligotrophiceastern MS. We provide evidence that the locations of the three LD stationsstudied were near the axis of the eddies. Their diameters were close to 100km and the studied areas were less than 10 km from the centre of the eddies.The positions of the LD stations are marked by an increase in the fluxfunction and a decrease in apparent oxygen utilization (AOU) and in excessdensity σ), as expected for anticyclonic eddies. Integrated meanprimary production measured in situ inside the three studied eddies confirms theprevious conclusion that integrated primary production (IPP) about 150 mgC m⁻² d⁻¹ may appear as a lower limit for IPP during strongoligotrophic conditions. The mesoscale activity is strong enough to locallymodify the very well-documented western-to-eastern gradient of trophicconditions in the MS. We proposed a new calculation for mixed layer depths(MLDs) enabling the determination of MLD to take into consideration processesoccurring with time scales ranging from a few hours to several days, andalso the winter MLD. Studying the main physical, chemical and dynamicalcharacteristics of the three eddies enables us to consider that the vorticitybarrier prevents any strong mixing and advection of outer water inside theeddy and explains why the depth range of eddies starts from the surface. Asa first approximation, the anticyclonic eddies could be considered as closedsystems dating back to the previous winter, making possible to draw first-order budgets.The daily new N-input in the photic zone is virtuallyidentical to the N-export measured at 230 m by drifting traps. This meansthat the eddies are close to an equilibrium state where input is equal toloss. The annual N-input by winter convection, which is a fundamentalcriterion for new nutrient availability, may be extremely variable insideeddies, with W-MLD varying from 90.5 m at the western station to 396.5 m atthe eastern station. W-MLDs are always deeper inside the eddies thanoutside where they are in keeping with climatological averages. AOU was lowinside the eddies; this together with the near-identical export measured at230 and 460 m seems to indicate that eddy cores are areas where lowmineralisation of particulate organic matter occurs. "In" and "out" AOUcomparisons indicate lower mineralisation inside the eddies suggesting ahigher efficiency for CO₂ sequestration via sedimentation ofparticulate organic matter. The three eddies are enriched in dissolvedorganic carbon (DOC). Sequestration of CO₂ by vertical export ofaccumulated DOC therefore seems to be higher inside eddies. The relativeimportance of DOC transport in the biological pump is probably one of themain characteristics of low-P low chlorophyll (LPLC) areas, and it is likelyto be reinforced inside anticyclonic eddies. The numerous anticycloniceddies in the MS are likely to influence the water masses and theirdispersion, and therefore have a strong impact on the biogeochemicalproperties at the scale of the MS.

[学科分类] 地球化学与岩石