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

Abstract

Abstract. The particulate matter distribution and phytoplankton community structure of the iron-fertilized Kerguelen region were investigated in early austral spring (October-November 2011) during the KEOPS2 cruise. The iron-fertilized region was characterized by a complex mesoscale circulation resulting in a patchy distribution of particulate matter. Integrated concentrations over 200 m ranged from 72.2 to 317.7 mg m −2 for chlorophyll a 314 to 744 mmol m −2 for biogenic silica (BSi), 1106 to 2268 mmol m −2 for particulate organic carbon, 215 to 436 mmol m −2 for particulate organic nitrogen, and 29.3 to 39.0 mmol m −2 for particulate organic phosphorus. Three distinct high biomass areas were identified: the coastal waters of Kerguelen Islands, the easternmost part of the study area in the polar front zone, and the southeastern Kerguelen Plateau. As expected from previous artificial and natural iron-fertilization experiments, the iron-fertilized areas were characterized by the development of large diatoms revealed by BSi size-fractionation and high performance liquid chromatography (HPLC) pigment signatures, whereas the ironlimited reference area was associated with a low biomass dominated by a mixed (nanoflagellates and diatoms) phytoplankton assemblage. A major difference from most previous artificial iron fertilization studies was the observation of 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-fertilized areas compared to the iron-limited reference station (0.13, 1.1, and 5.8, respectively). A second difference is the patchy response of the elemental composition of phytoplankton communities to large scale natural iron fertilization. Comparison to the previous KEOPS1 cruise also allowed to address the seasonal dynamics of phytoplankton bloom over the southeastern plateau. From particulate organic carbon (POC), particulate organic nitrogen (PON), and BSi evolutions, we showed that the elemental composition of the particulate matter also varies at the seasonal scale. This temporal evolution followed changes of the phytoplankton community structure as well as major changes in the nutrient stocks progressively leading to silicic acid exhaustion at the end of the productive season.Our observations suggest that the specific response of phytoplankton communities under natural iron fertilization is much more diverse than what has been regularly observed in artificial iron fertilization experiments and that the elemental composition of the bulk particulate matter reflects phytoplankton taxonomic structure rather than being a direct consequence of iron availability.

The Biological Pump and Seasonal Variability of pCO2 in the Southern Ocean: Exploring the Role of Diatom Adaptation to Low Iron

“…It should be borne in mind that this does not constitute a typical model validation, but a partial and indirect verification of our model. For instance, the seasonal evolution of silicate is not only controlled by the abundance of diatoms but also by their level of silicification (their Si:C ratio), which can be highly variable (Lasbleiz et al, 2014;Martin-J ez equel et al, 2000;Sarthou et al, 2005) depending on the growing conditions this group experiences (Brzezinski, 1985;Bucciarelli et al, 2010;Hoffmann et al, 2007;Lasbleiz et al, 2014). We find that the seasonal evolution of surface silicate concentrations (Si) in the PHYSIO experiment is Journal of Geophysical Research: Oceans 10.1029/2018JC013775 improved when compared to climatological data in two hydrological provinces (Figures 11a and 11b).…”

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

“…It was hypothesized that the addition of silicic acid promoted the growth of diatoms, which increased export of POC (Engel et al, 2002;Kemp et al, 2006;Kemp and Villareal, 2013;Rynearson et al, 2013;Lasbleiz et al, 2014). However, later studies showed that artificial mixing of upper water layers also initiated aggregate formation even in the absence of diatoms, suggesting that diatoms (and indirectly silicic acid) were not the sole trigger of high POC fluxes (Svensen et al, 2001(Svensen et al, , 2002.…”

Mesoscale Variability of Conditions Favoring an Iron‐Induced Diatom Bloom Downstream of the Kerguelen Plateau

“…Satellite images of surface chlorophyll concentrations show that the Kerguelen plume is highly heterogeneous, displaying strong gradients even within the plume itself, in response to eddies and frontal structures (Figure 1c)). The community structure of the bloom's primary producers has been studied during ship-based oceanographic campaigns, which have focused mainly on differentiating diatoms types and on pigment and size analyses Lasbleiz et al, 2014Lasbleiz et al, , 2016Mosseri et al, 2008;Qu eguiner, 2013;Trull et al, 2015;Uitz et al, 2009). However, in-situ observations are limited in spatial extent by the large size of the plume and the remoteness of the area.…”