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

Abstract

Abstract. Dissolved Fe (dFe) concentrations were measured in the upper 1300 m of the water column in the vicinity of the Kerguelen Islands as part of the second KErguelen Ocean Plateau compared Study (KEOPS2). Concentrations ranged from 0.06 nmol L −1 in offshore, Southern Ocean waters to 3.82 nmol L −1 within Hillsborough Bay, on the north-eastern coast of the Kerguelen Islands. Direct island runoff, glacial melting and resuspended sediments were identified as important inputs of dFe that could potentially fertilise the northern part of the plateau. A significant deep dFe enrichment was observed over the plateau with dFe concentrations increasing up to 1.30 nmol L −1 close to the seafloor, probably due to sediment resuspension and pore water release. Biological uptake was shown to induce a significant decrease in dFe concentrations between two visits (28 days apart) at a station above the plateau. Our work also considered other processes and sources, such as lateral advection of enriched seawater, remineralisation processes, and the influence of the polar front (PF) as a vector for Fe transport. Overall, heterogeneous sources of Fe over and off the Kerguelen Plateau, in addition to strong variability in Fe supply by vertical or horizontal transport, may explain the high variability in dFe concentrations observed during this study.

Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes

“…This area was fertilized by lateral Fe supply, coming from the Plateau or from glacial and fluvial sources (van der Bowie et al, 2015). A drawdown of dFe concentrations was observed (from 0.38 to 0.06 nmol L À 1 in surface waters; Quéroué et al, 2015) along with the bloom development (from 0.3 mg Chl-a L À 1 at E-1 to 0.9 mg Chl-a L À 1 at E-5; Lasbleiz et al, 2014). Similarly, the NPP increased from 578 to 1064 mgC m À 2 d À 1 from E-1 to E-5 stations respectively.…”

“…-a reference station in the HNLC area (R-2), upstream of the island, not impacted by lithogenic contributions, with low dissolved Fe concentrations (dFeo 0.1 nmol L À 1 ; Quéroué et al, 2015), characterized by a low phytoplankton abundance (0.2 mg Chl-a L À 1 ; Lasbleiz et al, 2014) and low net primary productivity (NPP ¼132 mgC m À 2 d À 1 ; Cavagna et al, 2015). -a station over the Plateau (A3) was sampled twice over a period of 27 days.…”

“…A highly productive station (E-4W) located closer to the Kerguelen Plateau margin was also sampled but excluded from the time series, since being nonrepresentative to trace the seasonal evolution in the less productive PF meander Dehairs et al, 2015;Planchon et al, 2015). -a station north of the PF (F-L) was sampled at mid-cruise and displayed moderate dFe concentrations in surface waters (0.26 nmol L À 1 ; Quéroué et al, 2015), the highest chlorophyll-a (3 mg L À 1 ; Lasbleiz et al, 2014) and NPP (3380 mgC m À 2 d À 1 ; Cavagna et al, 2015) levels.…”

“…PN export fluxes are multiplied by 6.6 to examine directly the variations of the C:N Redfield ratio. a primary production data (mmol C m À 2 d À 1 ) from Cavagna et al (2015); b dFe concentrations (nmol L À 1 ) from Quéroué et al (2015); c N uptake data (mmol N m À 2 d À 1 ) from Cavagna et al (2015); d Si uptake data (mmol Si m À 2 d À 1 ) from Closset et al (2014); e total mesozooplankton biomass data (g C m À 2 ) from Carlotti et al (2015); f bacterial respiration rates (mmol C L À 1 d À 1 ) from Christaki et al (2014); g taxonomic data from Lasbleiz et al, 2016; h POC export fluxes (mmol m À 2 d À 1 ) from Planchon et al (2015). 0.66 mmol C L À 1 d À 1 at other stations (Christaki et al, 2014).…”

Sustained Upwelling of Subsurface Iron Supplies Seasonally Persistent Phytoplankton Blooms Around the Southern Kerguelen Plateau, Southern Ocean

“…Both definitions yield similar depths for the ferricline in all regions except the ACC, where dFe concentrations were extremely low and the threshold ferricline was thus considerably deeper than the gradient ferricline (Figure d). For all other regions, the ferricline depth was 100–150 m, which is slightly shallower than the 150‐ to 200‐m ferricline observed over the northern Kerguelen plateau (Blain et al, ) and the ~200‐m ferricline downstream of the northern Kerguelen plateau (Quéroué et al, ). Notably, it is considerably shallower than the mean Southern Ocean ferricline of 333 m estimated by Tagliabue et al () based on 140 dFe profiles from the open ocean.…”