|Pigments, elemental composition (C, N, P, and Si), andstoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean |
(Article) Publié: Biogeosciences, vol. 11 p.5931 (2014)
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Abstract. The particulate matter distribution and phytoplanktoncommunity structure of the iron-fertilized Kerguelenregion were investigated in early austral spring(October–November 2011) during the KEOPS2 cruise.The iron-fertilized region was characterized by a complexmesoscale circulation resulting in a patchy distributionof particulate matter. Integrated concentrations over200m ranged from 72.2 to 317.7 mgm−2 for chlorophyll a314 to 744 mmolm−2 for biogenic silica (BSi), 1106 to2268 mmolm−2 for particulate organic carbon, 215 to436 mmolm−2 for particulate organic nitrogen, and 29.3 to39.0 mmolm−2 for particulate organic phosphorus. Threedistinct high biomass areas were identified: the coastal watersof Kerguelen Islands, the easternmost part of the studyarea in the polar front zone, and the southeastern KerguelenPlateau. As expected from previous artificial and naturaliron-fertilization experiments, the iron-fertilized areas werecharacterized by the development of large diatoms revealedby BSi size–fractionation and high performance liquid chromatography(HPLC) pigment signatures, whereas the ironlimitedreference area was associated with a low biomassdominated by a mixed (nanoflagellates and diatoms) phytoplanktonassemblage. A major difference from most previousartificial iron fertilization studies was the observationof much higher Si : C, Si : N, and Si : P ratios (0.31±0.16,1.6±0.7 and 20.5±7.9, respectively) in the iron-fertilizedareas compared to the iron-limited reference station (0.13,1.1, and 5.8, respectively). A second difference is the patchyresponse of the elemental composition of phytoplanktoncommunities to large scale natural iron fertilization. Comparisonto the previous KEOPS1 cruise also allowed to addressthe seasonal dynamics of phytoplankton bloom overthe southeastern plateau. From particulate organic carbon(POC), particulate organic nitrogen (PON), and BSi evolutions,we showed that the elemental composition of the particulatematter also varies at the seasonal scale. This temporalevolution followed changes of the phytoplankton communitystructure as well as major changes in the nutrient stocks progressivelyleading to silicic acid exhaustion at the end of theproductive season.Our observations suggest that the specific response of phytoplanktoncommunities under natural iron fertilization ismuch more diverse than what has been regularly observed inartificial iron fertilization experiments and that the elementalcomposition of the bulk particulate matter reflects phytoplanktontaxonomic structure rather than being a direct consequenceof iron availability.