Iron availability in the Southern Ocean controls phytoplankton growth,
community composition and the uptake of atmospheric CO
The concentration of carbon dioxide in earth's atmosphere, and therefore
earth's climate, is highly sensitive to modification in the marine carbon (C)
cycle due to the growth of phytoplankton in the Southern Ocean (Sarmiento and
Gruber, 2006). These single-cell plants remove inorganic carbon from surface
seawater during photosynthesis, and this inorganic carbon can be directly
transferred into the deep sea when the plants die and sink, or indirectly
through the food web. The Southern Ocean is responsible for 30 % of
global ocean carbon export (Schlitzer, 2002). As first demonstrated over 20
years ago, phytoplankton growth in the Southern Ocean is limited by the
availability of the micronutrient trace element iron (Fe; Martin, 1990). Low
dissolved iron (dFe) availability limits the annual uptake of atmospheric
carbon dioxide (CO
Artificial mesoscale ocean iron fertilisation experiments have unequivocally
demonstrated the role of Fe in setting phytoplankton productivity, biomass
and community structure in high-nutrient low-chlorophyll (HNLC) regions (de
Baar et al., 2005; Boyd et al., 2007). However, the “carbon sequestration
efficiency” of ocean fertilisation as a means to sequester atmospheric
CO
The natural resupply of iron to Fe-depleted waters is a more efficient process (Blain et al., 2007), although in part this depends on the mode of Fe delivery (e.g. from above, laterally or from below) and on the ability of organic ligands to keep the supplied Fe in solution (Gerringa et al., 2008), and for continued ocean fertilisation, it is in part reliant on the concurrent supply of other major nutrients. In the Indian sector of the subantarctic Southern Ocean, natural Fe supply from the Kerguelen Plateau (Blain et al., 2007) and Crozet Islands (Pollard et al., 2009) results in increased phytoplankton biomass during summer, with chlorophyll levels increasing to more than 1 order of magnitude above the background, as revealed by NASA MODIS satellite chlorophyll climatology for January (2003–2010) (Westberry et al., 2013). Previous research on blooms in these localised “natural laboratories” has provided invaluable insights into mechanisms linking iron fertilisation and carbon cycling in the Southern Ocean, especially since studies of natural systems can address the effects of persistent, varying and multiple Fe sources that are not accessible through deliberate artificial mesoscale fertilisation experiments.
The KEOPS-1 (KErguelen: Ocean and Plateau compared Study 1) project, which took place in the late austral summer of January–February 2005, demonstrated that this natural fertilisation of the Southern Ocean resulted in dramatic changes in the functioning of the ecosystem with large impacts on marine biogeochemical cycles (Blain et al., 2007, 2008a). These observations of the bloom were largely confined to the plateau region, where vertical upwelled supply from the plateau sediments (Blain et al., 2008b; Zhou et al., 2014) and lateral advection of water that had been in contact with the continental shelf of Heard Island to the south (Chever et al., 2010) were the dominant sources of dissolved and particulate Fe (as confirmed using rare earth element (REE) and radium (Ra) isotope tracers; van Beek et al., 2008; Zhang et al., 2008). The interaction of waters, islands and plateau of the Kerguelen Archipelago with several circumpolar fronts of the Southern Ocean allowed us to make a first attempt at placing our regional KEOPS-1 observations within a broader basin-scale context (Blain et al., 2007).
The KEOPS-2 project was designed to improve the spatial and temporal coverage
of the Kerguelen region. During KEOPS-2, which was approved as a GEOTRACES
process study
Since Fe is actively taken up into phytoplankton and transferred throughout the food web, including removal by particle settling and remineralisation in deep waters, the assessment of its availability is quite complex and cannot be judged from dFe levels in surface waters alone (Breitbarth et al., 2010). Advances in chemical oceanographic techniques for trace elements through the GEOTRACES program (SCOR Working Group, 2007) now allow the measurement of Fe associated with different phases (dissolved and particulate), internal biological recycling and Fe export from surface waters. The results from earlier iron biogeochemical budgets for FeCycle-I (Boyd et al., 2005; Frew et al., 2006), KEOPS-1 (Blain et al., 2007; Chever et al., 2010), CROZEX (CROZet natural iron bloom and EXport experiment; Planquette et al., 2007, 2009) and SAZ-Sense (Sensitivity of the subantarctic zone to environmental change; Bowie et al., 2009) have highlighted that the dominant “new” Fe fluxes are associated with the particulate phase. Particles thus represent an important transport vector for trace metals in the marine ecosystem, although their bioavailability or transfer into a bioavailable fraction remains uncertain. Suspended particles have also been shown to be important aspects of sedimentary, boundary layer Fe sources and export processes (Tagliabue et al. 2009; Homoky et al., 2013; Marsay et al., 2014; Wadley et al., 2014), with particles being transported laterally over hundreds of kilometres in the ocean (Lam et al., 2006; Lam and Bishop, 2008). The biological cycling of particulate Fe may therefore be the most important aspect of the complete Fe biogeochemical cycle, especially since earlier budgets have demonstrated that biological Fe “demand” cannot be satisfied by the new Fe supply (Boyd et al., 2005; Blain et al., 2007; Sarthou et al., 2008; Bowie et al., 2009; de Jong et al., 2012). A simple one-dimensional vertical model that correctly represented the input of dFe to surface waters during KEOPS-1 did not accurately represent the supply of other geochemical tracers or particulate Fe (Blain et al., 2007; van Beek et al., 2008; Zhang et al., 2008), and the role of dissolved and particulate Fe earlier in the season (winter stock) in the Kerguelen region has yet to be quantified.
This paper presents a short-term (days to weeks) Fe budget for the period of KEOPS-2 for each of three process sites: (i) a “plateau” bloom site
(A3) on the central Kerguelen Plateau studied during late summer on KEOPS-1
and reoccupied during spring on KEOPS-2; (ii) a “plume” bloom site (E) east
of the Kerguelen Islands, which was located within a quasi-stationary,
bathymetrically trapped recirculation feature near the polar front (PF); and (iii)
a “reference” site (R-2) south of the PF and upstream
(southwest) of the Kerguelen Islands in HNLC waters. We focus on mixed-layer integrated
pools of dissolved Fe and particulate Fe (which we further separate into
biogenic and lithogenic fractions using elemental normalisers), estimate the
fluxes of Fe associated with new and recycled Fe sources, and compare
Fe supply and demand with implications for bloom duration and magnitude. Our
observations also include particulate measurements in both suspended-water-column (in situ pump; ISP) and sinking-export (free-floating sediment
trap; “P-trap”) particles below the mixed layer, with linkage to food web
processes via a discussion of Fe
MODIS ocean-colour satellite images showing the development of the
plateau and plume blooms during KEOPS-2. Surface chlorophyll
(
The KEOPS-2 (KErguelen Ocean and Plateau in compared Study 2) expedition was
carried out in the Indian sector of the Southern Ocean in the vicinity of the
Kerguelen Plateau between 7 October and 30 November 2011 on the RV
The Kerguelen bloom has two main features: a northern branch that extends
northeast of the island into waters both south and north of the PF and a
larger bloom covering
The hydrology and circulation around and above the Kerguelen Plateau have been described by Park et al. (2008a, b, 2014a), van Beek et al. (2008), Zhang et al. (2008) and Zhou et al. (2014). The mean circulation is shown in Fig. 1b. Briefly, the Kerguelen Plateau constitutes a barrier to the eastward flowing Antarctic Circumpolar Current (ACC), the main jets of which are the Subantarctic Front (SAF) and PF. Most of the ACC is deflected north of the Kerguelen Islands as Subantarctic Surface Water (SASW) but some filaments pass between the Kerguelen Islands and Heard Island (as the PF) and further south between Heard Island and Antarctica (Roquet et al., 2009). Above the plateau, the remainder of the ACC comes from the western part of the plateau. Currents of AASW travelling along the western flank of the plateau are deflected south and east of Heard Island as a branch of the Fawn Trough Current (FTC) (Sokolov and Rintoul, 2009) before travelling in a broadly northwest direction up along the eastern shelf break. The water flow is then deflected toward the east of the Kerguelen Islands, where there is an intense mixing zone consisting of mesoscale eddies which travel many thousands of kilometres in the ACC towards the Australian sector of the Southern Ocean.
All trace metal sampling and analytical procedures followed recommended
protocols in the cookbook
Water column samples were collected using 10 L externally closing,
Teflon-lined Niskin-1010X bottles deployed on an autonomous 1018 intelligent
rosette system (TMR – trace metal rosette, specially adapted for trace metal work; General
Oceanics Inc.). The polyurethane-powder-coated aluminium rosette frame was
suspended on Kevlar rope which passed through a clean block with a plastic
sheave (General Oceanics) and was lowered to a maximum depth of 1300 m.
Bottles were tripped at preprogrammed depths using a pressure sensor as the
TMR was being raised through the water column at approximately
0.5 m s
All sample processing was carried out under an ISO class-5 trace metal clean
laminar flow bench in a HEPA filtered-air clean container, with all materials
used for sample handling thoroughly acid-washed. Samples were drawn through
C-Flex tubing (Cole Parmer) and filtered in-line through 0.2
Suspended particles for trace elemental analysis were collected using 11
large-volume in situ pumps (McLane Research Laboratories WTS6-1-142LV and
Challenger Oceanics pumps), suspended simultaneously at prechosen depths
following methods reported in Bowie et al. (2009). Up to 2000 L of seawater
was filtered across a 142 mm diameter stack (134 mm diameter active area)
consisting of a 53
Sinking particles for trace elemental analysis were collected using PPS3/3
free-floating sediment traps (Technicap, France), specially adapted for trace
metals and deployed at 200 m. Traps were deployed for 5.3, 5.1, 1.9 and 1.5
days at stations E-1, E-3, A3-2 and E-5, respectively. The trap deployed at
station R-2 was lost and not recovered. Traps drifted between 10 and 43 km
over the course of the deployment. Full details of the trap deployments are
given in Laurenceau-Cornec et al. (2015) and Planchon et al. (2015). Samples
for trace elemental analysis were collected in three separate acid-washed
cups (specifically for trace metals) containing a low trace metal brine solution
(salinity
Dissolved Fe (dFe) was determined shipboard by flow injection analysis with chemiluminescence detection (FI-CL) using in-line preconcentration on an 8-hydroxyquinoline chelating resin (adapted from Obata et al., 1993, de Jong et al., 1998 and Sarthou et al., 2003). Dissolved Fe data were quality controlled against the SAFe (Sampling and Analysis of Fe) standard reference materials (Johnson et al., 2007). Full data including certification results and analytical figures of merit are reported in Quéroué et al. (2015).
Particulate Fe (pFe) was determined as follows. Sampled particles were acid
extracted in 1 mL concentrated HNO
For particulate organic carbon (POC) and particulate nitrogen (PN) analyses,
QMA quartz filters from the ISPs were subsampled in a flow bench using a
14 mm diameter plastic punch and transferred to silver foil cups (Sercon
brand
Trace metal clean seawater was collected from the mixed layer (20–40 m)
using the TMR, was transferred into acid-washed polycarbonate bottles and
0.2 nmol L
Since iron regeneration was not measured directly by experiment during
KEOPS-2, we used the following approach to calculate iron regeneration
fluxes. Bacterial Fe regeneration was estimated from bacterial turnover times
determined from bacterial production and biomass (Christaki et al., 2014),
assuming all loss of bacterial biomass through viral lysis and flagellate
grazing resulted in the regeneration of Fe (Strzepek et al., 2005) and using
a bacterial iron quota of 7.5
Full descriptions of the dFe and pFe distributions can be found in Quéroué et al. (2015) and van der Merwe et al. (2015), respectively, with further presentation of the distributions of other micronutrient trace elements (Mn, Co, Ni, Cu, Cd, Pb) from KEOPS-2 to be presented elsewhere. However, briefly our subset of stations used for the iron budgets can be described as follows.
In the upper 100 m, we observed a salinity minimum (33.8) and temperature
maximum (2.2
Dissolved Fe concentrations were very low at the surface (
The dFe profile at the KEOPS-2 reference station R-2 is similar to the
KEOPS-1 reference station C11 (with the exception of the R-2 enrichment in
the 200–700 m depth strata; Fig. 4a), but it should be noted that the location of C11 was
quite different – in HNLC waters to the southeast of the Kerguelen Plateau
(51
Stations A3-1 (Fig. 3b) and A3-2 (Fig. 3c) were in relatively shallow waters
on the central plateau, and were impacted by plateau sediments and possibly
fluvial and glacial runoff from the basaltic rocks of Heard Island
Surface chlorophyll images revealed that during the 28 days between the first
and second visits to A3, a large diatom spring bloom developed mostly
dominated by lightly silicified
The pFe profiles at A3 showed a similar structure to the dFe profile, with
lower values at the surface (
The spring (Oct-Nov) KEOPS-2 Fe profiles at station A3 showed a similar
structure to those from the late summer (Jan-Feb) during KEOPS-1, with surface
depletion, concentrations increasing with depth and enrichment just above the
plateau seafloor (Fig. 4b). Through the water column, dFe was between 2 and 5
times greater during KEOPS-2 than KEOPS-1 and pFe was
The E stations within the bathymetrically trapped complex recirculation
system showed similar hydrographic and nutrient distributions below the mixed
layer (Fig. 3d, e and f), which shoaled from 64 m at E-1 to 32 m at E-3 to
39 m at E-5, with some internal variability in water column structure at
mid-depths. Surface waters warmed from 2.7 to 3.4
Waters at the plume stations showed the largest spatial heterogeneity in
surface biomass as revealed by the evolution of a mosaic of complex blooms
seen in satellite images (see Supplement in Trull et al., 2015). We observed
moderate surface Chl
Due to operational constraints, no dFe data were available at station E-1. The
dFe vertical profiles at E-3 and E-5 were quite different, with a distinct
surface enrichment to 0.4 nmol L
Summary of iron standing stocks and fluxes for the upper mixed layer at KEOPS-2 process station sites R-2 (reference), A3 (plateau) and E (plume). For full details of the calculations, see text. Error bounds are provided where available. Due to logistical constraints resulting in missing data at some stations, we will focus on R-2, A3-2 and E-5 in the discussion. Data for stations A3-1, E-1 and E-3 are given to provide a context for spatial and temporal changes in the pools and fluxes during KEOPS-2.
n.d.: no data
The pFe distributions at the three E stations were similar with a surface
(35–40 m) enrichment (1.6–1.9 nmol L
KEOPS-1 only occupied one station in the plume east of the
Kerguelen Islands (A11 at 49
The primary aim of this work was to use our observations of Fe pools and fluxes to understand the sources, sinks and biological Fe cycling and to evaluate whether Fe supply could meet demand in both the high-Fe and low-Fe environments in the vicinity of the Kerguelen Archipelago during KEOPS-2. Iron budgets have been constructed for previous studies in waters fertilised with Fe both naturally (Sarthou et al., 2008; Bowie et al., 2009; Chever et al., 2010; Ellwood et al., 2014) and artificially (Bowie et al., 2001) as well as low-Fe conditions (Price and Morel, 1998; Boyd et al., 2005). These budgets have combined geochemical and chemical components to demonstrate that the dominant long-term fluxes of Fe are associated with the particulate pool (dust supply and particle export), whilst studies on Fe uptake and microbial cycling have shown that short-term fluxes within the “ferrous wheel” are dominated by biological uptake and remineralisation (Strzepek et al., 2005). Here, we follow a similar approach to that used by Bowie et al. (2009) for the SAZ-Sense study south of Tasmania (Australia) at our three study sites. Since all parameters in our iron budget calculations were only measured at stations R-2, A3-2 and E-5, discussion will focus on these stations. Data for stations A3-1, E-1 and E-3 are given to provide a context for spatial and temporal changes in the Fe pools and fluxes during KEOPS-2, and they are collated in Table 1.
Iron and carbon pools were calculated by integrating the dissolved and
particulate profiles down to the base of the surface mixed layer, defined as
the depth where the potential density equalled the potential density at 10 m
Integrated pools of both dissolved (
Reference and plume waters contained roughly an equal fraction of biogenic and lithogenic Fe. The origin of this biogenic Fe pool will be a combination of biological uptake of dFe, physical adsorption onto suspended biological particles, and conversion from the lithogenic fraction (likely driven by microbes), with these processes operating on different timescales (Boyd et al., 2005; Frew et al., 2006; Planquette et al., 2011). By contrast, the plateau stations contain 19–69 times more lithogenic Fe than biogenic Fe, consistent with the supply from the nearby sediments of the plateau and Heard Island, as suspected by Zhang et al. (2008), van Beek et al. (2008) and discussed in Chever et al. (2010). The measurement of other geochemical “fingerprint” particulate tracers (such as Al, Mn) on the plateau confirmed the provenance of Fe supplied from the Kerguelen shelf sediments in the particulate phase (van der Merwe et al., 2015).
Vertical fluxes were calculated as follows. A vertical diffusivity
(
Upwelling was defined as the vertical velocity (
Entrainment of Fe by episodic (intraseasonal) deepening of the mixed layer has rarely been taken into account in field studies (Frants et al., 2013) due to the absence of data characterising the short-term variability of the mixed layer depth, yet a recent compilation of observations (Nishioka et al., 2011; Tagliabue et al., 2014) and modelling studies (Mongin et al., 2008) suggests that entrainment could be a major vertical supply mechanism fuelling surface biomass (Carranza et al., 2015). We used more than 6000 vertical profiles of salinity and temperature collected in the KEOPS-2 regions of interest to estimate the seasonality of the mixed layer depth and its variability (Supplement Fig. 3). We derived the vertical supply of Fe by entrainment via hypotheses regarding the relation between the size of mixed layer depth excursions and their frequency (see the Supplement). Entrainment data based on transient deepening of the mixed layer was not available for station R-2; therefore we calculated this by multiplying the dFe concentration in winter water (which reflects the dFe concentration of the winter mixed layer) by the winter mixed layer depth (MLD) and assume that this entrainment event happens once per year. “Detrainment” at R-2 was accounted for by multiplying this new entrainment flux by the summer-to-winter MLD ratio.
For the vertical fluxes, in spring on KEOPS-2, entrainment was the dominant
vertical Fe flux term on the plateau, delivering
For the lateral fluxes, the horizontal supply at reference station R-2 was
assumed to be zero since HNLC waters upstream and downstream of this station
contained similar dFe and pFe concentrations, and as phytoplankton growth and
biomass was low at this site, there would be little biogenic Fe exported
below the mixed layer. On the plateau, Fe supply at station A3 was taken from the
steady-state box model of Chever et al. (2010), which used the horizontal dFe
gradient and current velocities from Park et al. (2008a) to calculate the
lateral flux of 180 nmol m
By combining our in situ Fe measurements with estimated ages of the water
bodies in the plume, we calculate a first-order exponential scavenging
removal constant between 0.041 and 0.058 d
Considering only internal processes (diffusion, upwelling, entrainment, lateral transport) in supplying Fe to the surface mixed layer, the vertical terms dominated at the reference station; vertical terms were 6-fold greater than lateral terms on the plateau, whereas lateral advection was the dominant term in the plume (4–5-fold greater than the vertical terms). Since the particulate Fe stocks were abundant in surface waters (above the winter temperature minimum layer) and significantly higher than the dissolved pools (most notably on the plateau), it is likely that a fraction of the suspended lithogenic pFe from Heard Island or the Kerguelen Plateau sediments also contributed to the internal dFe supply and fuelled biological responses. This is discussed in more detail later.
Data on atmospheric Fe fluxes through dust deposition and the solubility of
Fe in the dust for all three study sites were taken from the nearby
land-based sampling site “Jacky” (49
Atmospheric fluxes were dominated by wet deposition (Heimburger et al.,
2012). Heimburger et al. (2013b) calculated the mean “soluble” Fe
deposition flux (defined as
Downward Fe and C fluxes were measured directly in free-floating sediment
P-traps at the plateau (A3-2) and plume (E-1, E-3, E-5) stations and
estimated using the
Aluminium was used as a normaliser to estimate the fraction of lithogenic Fe
in the exported material. The percentage lithogenic fraction of total pFe
exported at the E stations remained much the same at each deployment
(34–39 %), whereas the lithogenic fraction was a much larger component
at A3-2 (51 %), reflecting the close proximity to sources of particulate
material rich in Fe. The Fe
Iron export fluxes were greater during the spring study of KEOPS-2 compared
to the late summer study of KEOPS-1 (Table 2). This difference between the
KEOPS studies was also observed in Fe uptake rates (Fourquez et al., 2015).
Such observations may be simply related to the seasonal supply; in other
words, greater Fe supply in spring resulted in greater Fe uptake and export.
Determined pFe sinking fluxes were also greater than those observed during CROZEX study
(Planquette et al., 2011), the SAZ-Sense expedition south of Tasmania (Bowie
et al., 2009) and the FeCycle-I expedition east of New
Zealand (Frew et al., 2006) and of similar magnitude to those reported by Bowie
et al. (2001) during the Southern Ocean Iron Release Experiment (SOIREE )
(5.2
The export of particulate organic carbon (POC) into our P-traps followed the
same trend as that of pFe at the E stations, decreasing from
7.0
Fluxes of iron and carbon (mean and standard deviation, SD) exported in sinking particles (trap
deployed at 200 m) and ratio of Fe
n.d.: no data
Iron regeneration rates based on bacterivore and herbivore contributions.
Intracellular Fe uptake by phytoplankton and bacteria
The bacterial and mesozooplankton contributions to Fe regeneration were
calculated separately (Table 3). Volumetric values varied between 0.06 and
0.59 pmol Fe L
A similar Fe regeneration calculation was also performed based on the C
budget by using the percentage of gross community production (GCP) that is
remineralised for KEOPS-2 and results from Fe uptake experiments described
above. This yielded higher Fe regeneration estimates in the range of 1–11 pmol L
Iron regeneration fluxes can be compared with those from KEOPS-1
using the same first approach above. For station A3 on KEOPS-1, this resulted
in a Fe regeneration flux of 1 pmol L
Importantly, Fe regeneration was much lower during the early compared to late
bloom stage and was dominated by bacterial regeneration in spring
(60–90 % of total Fe regeneration). Strzepek et al. (2005) estimated Fe regeneration rates (during
FeCycle-II) for herbivores (16.5–18.4 pmol L
The mixed-layer phytoplankton intracellular Fe
Suspended mixed layer Fe
Vertical profiles of Fe
In addition to the ratio of “total” particulate (biogenic
At station R-2, the Fe
Interestingly, although Fe
The Fe
Since POC export fluxes during spring (KEOPS-2) were similar to late summer
(KEOPS-1), but pFe export fluxes were higher in spring compared to summer
(Table 2), this resulted in a generally higher carbon sequestration
efficiency (lower Fe
Morris and Charette (2013) presented a detailed synthesis of
A comparison of export fluxes of pFe versus POC in sinking particles
for natural iron fertilisation studies in the Southern Ocean. For details of
the sampling methods, refer to Table 2 and the original articles. The lines
indicate Fe
Biogeochemical iron budgets for the reference (R-2,
Using calculated flux estimates, a comparison of Fe supply and demand at the
three sites around the Kerguelen Archipelago in spring was possible (Fig. 7).
In our short-term iron biogeochemical budgets, the total dFe supply from
new sources (calculated as the sum of diffusion, upwelling, vertical and
lateral advection, and atmospheric dust) to surface waters of the plume was
more than twice that above the plateau and
Since Fe supply from new sources was greater than the Fe demand (uptake minus remineralisation as a recycled Fe source) at all stations (R-2, A3-2 and E-5), this resulted in a positive value for row k in Table 1 (i.e. there was no additional Fe required to balance the dissolved budget). This finding is consistent with other observations at both the plateau and plume sites which were Fe replete in early spring but somewhat surprising for the HNLC reference site R-2. This may partly be a result of an overestimate of the atmospheric supply used in calculations presented here from literature data. Another explanation is that the parameters used in our “short-term” iron budget calculations are decoupled in time (e.g. there will be an offset between the mechanisms for organism acquisition of Fe and the processes resulting in Fe-laden particles leaving the upper ocean), and the short-term Fe budget is based on an “instantaneous picture” of different fluxes that were not in steady state.
Interestingly, at station A3-2, the sink processes (Fe export and uptake) are
so large and the regenerated Fe flux so small that the total (dissolved
Applying a solubility of 2.5 % used for KEOPS-1 at A3 to enable Fe
supply to meet demand (Blain et al., 2007) would provide an extra
10–34
The release of Fe to biota via the conversion of lithogenic to biogenic Fe has been previously suggested (Lam et al., 2006; Frew et al., 2006; Borer et al., 2009; Planquette et al., 2011) and the present work strongly supports this hypothesis, with our data (Fig. 5) indicating that biogenic Fe has a longer residence time in the upper ocean than lithogenic Fe which is not accessed by biota. The role of pFe in supplying bioavailable Fe is also supported by the similarity of the pFe and dFe profile shapes in Fig. 3, which infer that pFe may be contributing to the control of dFe, either by supplying it or because biogenic particles are controlling both.
Finally, our estimation of Fe supply and regeneration allowed us to estimate
an
The complex regional circulation, multiple iron sources, and transport pathways above and downstream of the naturally fertilised Kerguelen Plateau region results in a mosaic of phytoplankton blooms. The budgets presented here result from direct measurements of the Fe inventories and fluxes between different pools. The system was not in steady state during the period of the KEOPS-2 observations, and the exchange of Fe between the dissolved, biogenic and lithogenic pools was highly dynamic in time and space. Our analysis highlights the important role of pFe, the inherent heterogeneity and biogeochemical differences associated with particulates within and exported below the mixed layer and the lithogenic to biogenic conversion pathways.
This study also highlights the significance not only of the mode of Fe
fertilisation on the plateau (predominantly vertical) versus the plume
(predominantly lateral) but also of the relative magnitude. Importantly,
since the Fe supply from new sources to the plume was more than double that
above the plateau, this implies that the waters that supply the plume are not the
same as those at station A3 on the southern plateau, and the plume must be
supplied with water from the northern part of the plateau or Kerguelen
coastal waters, which are richer in dFe (Quéroué et al., 2015; Trull
et al., 2015). This source of Fe, which will contain a large fraction of
particulate material (van der Merwe et al., 2015) that is transported off the
Kerguelen Plateau, is therefore an important but previously unquantified
contribution to the downward flux of Fe exiting the upper ocean in the plume.
Moreover, the KEOPS-2 results are tightly linked to the mode of Fe supply
that is different from dust deposition or purposeful additions and to the
concomitant supply of major nutrients, and this has consequences for the
carbon sequestration efficiency of the system. When Fe supply is
predominantly vertical (as it is at station A3), then the C sequestration
efficiency is lower (i.e. higher Fe
Future efforts should focus on the quantification of the full seasonal cycle of Fe delivery, which will be fundamental to closing the iron budget around the Kerguelen Archipelago on annual timescales. This will allow the assessment of the important longer-term climatic and ecosystem implications with changes in the nature and strength of Fe supply with physical (weakening overturning circulation, warming, increased stratification) and chemical (ocean acidification, deoxygenation) environmental forcings, together with increases in glacial melt, rainfall and dust deposition on a warming planet.
A. R. Bowie designed the iron budgets, performed the calculations and prepared the manuscript with contributions from all co-authors. P. van der Merwe, F. Quéroué, G. Sarthou, F. Chever and A. T. Townsend collaborated on trace metal sampling, analyses and interpretation; M. Fourquez and I. Obernosterer were responsible for biological cycling, T. Trull for carbon dynamics and the P-trap deployments, F. Planchon for Th-based export and J.-B. Sallée for vertical flux estimates. S. Blain designed the overall KEOPS-2 project and helped with budget calculations.
We thank the captain B. Lassiette, officers and crew of RV
This KEOPS-2 project was supported by the French Research program of INSU-CNRS LEFE–CYBER (“Les enveloppes fluides et l'environnement” – “Cycles biogéochimiques, environnement et ressources”), the French ANR (“Agence Nationale de la Recherche”, SIMI-6 program, ANR-2010-BLAN-614 KEOPS2 and, ANR-10-JCJC-606 ICOP), the French CNES program (“Centre National d'Etudes Spatiales”) and the French Polar Institute IPEV (Institut Polaire Paul-Émile Victor). The Australian participation in the project was supported by the Antarctic Climate and Ecosystems Cooperative Research Centre and a University of Tasmania “Rising Stars” award to the lead author.
We thank two anonymous reviewers for their very constructive comments, which improved our manuscript. Edited by: B. Quéguiner