Pigments, elemental composition (C, N, P, 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 HPLC pigment signatures, whereas the iron-limited reference area was associated to a low biomass dominated by a mixed (nanoflagellates and diatoms) phytoplankton assemblage. A major difference from previous artificial iron fertilization studies was the observation of much higher Si : C, Si : N, and Si : P ratios (respectively 0.31 ± 0.16, 1.6 ± 0.7 and 20.5 ± 7.9) in the iron-fertilized areas compared to the iron-limited reference station (respectively 0.13, 1.1, 5.8). 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 POC, 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.

Shifts in bacterial community composition associated with increased carbon cycling in a mosaic of phytoplankton blooms

“…Concurrently, 4.8 l of the o0.8 μm filtrate were collected on a 0.2-μm cartridge (Sterivex, Millipore) and treated, as described above, for further diversity analyses. Lasbleiz et al (2014). b Bacterial growth rate was determined by diving bacterial heterotrophic production by bacterial biomass.…”

High variability in dissolved iron concentrations in the vicinity of the Kerguelen Islands (Southern Ocean)

“…Homoky et al, 2013). At station A3, high LSi concentrations (1.34 ± 0.07 µmol L −1 ; Lasbleiz et al, 2014) were observed just above the seafloor in the benthic boundary layer (BBL), also suggesting sedimentary inputs. This is corroborated by high pFe values at A3-1 and A3-2 (30 and 15 nmol L −1 , respectively) and pFe : pAl ratios that resemble basalt over the Kerguelen Plateau (van der Merwe et al, 2015).…”

“…In our study, the biological activity was much lower at station R-2 (Antarctic zone) than in cluster 4 (subantarctic zone). Indeed the highest integrated concentrations over 200 m for chlorophyll a (Chl a) were observed in cluster 4 (223-35 mg m −2 , Lasbleiz et al, 2014). Therefore, the lower dFe value at R-2 compared to cluster 4 might not reflect differences in biological activity but, rather, in Fe inputs (see below).…”

“…Frenot, personal communication, 2014). Finally, TEW-1 showed the highest lithogenic silica (LSi) concentrations of the study area (1.31 ± 0.14 µmol L −1 ; Closset et al, 2014;Lasbleiz et al, 2014) and TEW-2 showed slightly lower LSi concentrations (0.54 ± 0.02 µmol L −1 ). Gradients in LSi and dFe are probably indicative of glacial melt inputs, Fe being leached from nanoparticulate iron (oxyhydr)oxides present in glacial rock flour (Raiswell et al, 2010;Raiswell, 2011) and LSi being weathered from silicate-rich minerals (SiO 2 ; Doucet et al, 2005).…”

Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the KEOPS2 survey

“…2 and Supplement), but the sampling on the first visit to station A3 (A3-1, on 20 October) revealed pre-bloom conditions on the plateau and some stations (TNS9, TNS4) of oceanic waters (Jouandet et al, 2014;Lasbleiz et al, 2014). The bloom started in earnest in early November, first massively on the plateau and in coastal waters, and secondly in spatially heterogeneous low biomass oceanic waters (during our TEW transect and stations E1-E3), with higher chlorophyll values at stations (TEW 7, TEW 8, F-L) downstream from the Polar-Front bloom (Lasbleisz et al, 2014;Trull et al, 2015).…”