Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean

[摘要] The particulate matter distribution and phytoplankton community structure ofthe iron-fertilized Kerguelen region were investigated in early australspring (October–November 2011) during the KEOPS2 cruise. The iron-fertilizedregion was characterized by a complex mesoscale circulation resulting in apatchy distribution of particulate matter. Integrated concentrations over200 m ranged from 72.2 to 317.7 mg m⁻² for chlorophyll a 314 to744 mmol m⁻² for biogenic silica (BSi), 1106 to 2268 mmol m⁻²for particulate organic carbon, 215 to 436 mmol m⁻² for particulateorganic nitrogen, and 29.3 to 39.0 mmol m⁻² for particulate organicphosphorus. Three distinct high biomass areas were identified: the coastalwaters of Kerguelen Islands, the easternmost part of the study area in thepolar front zone, and the southeastern Kerguelen Plateau. As expected fromprevious artificial and natural iron-fertilization experiments, theiron-fertilized areas were characterized by the development of large diatomsrevealed by BSi size–fractionation and high performance liquidchromatography (HPLC) pigment signatures, whereas the iron-limited referencearea was associated with a low biomass dominated by a mixed (nanoflagellatesand diatoms) phytoplankton assemblage. A major difference from most previousartificial iron fertilization studies was the observation of much higherSi : C, Si : N, and Si : P ratios (0.31 ± 0.16, 1.6 ± 0.7and 20.5 ± 7.9, respectively) in the iron-fertilized areas compared tothe iron-limited reference station (0.13, 1.1, and 5.8, respectively). Asecond difference is the patchy response of the elemental composition ofphytoplankton communities to large scale natural iron fertilization.Comparison to the previous KEOPS1 cruise also allowed to address the seasonaldynamics of phytoplankton bloom over the southeastern plateau. Fromparticulate organic carbon (POC), particulate organic nitrogen (PON), and BSievolutions, we showed that the elemental composition of the particulatematter also varies at the seasonal scale. This temporal evolution followedchanges of the phytoplankton community structure as well as major changes inthe nutrient stocks progressively leading to silicic acid exhaustion at theend of the productive season.

Our observations suggest that the specific response of phytoplanktoncommunities under natural iron fertilization is much more diverse than whathas been regularly observed in artificial iron fertilization experiments andthat the elemental composition of the bulk particulate matter reflectsphytoplankton taxonomic structure rather than being a direct consequence ofiron availability.

[学科分类] 地球化学与岩石