Resupply of mesopelagic dissolved iron controlled by particulate iron composition

The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

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An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Biogeosciences Discussions ◽

10.5194/bgd-6-6803-2009 ◽

2009 ◽

Vol 6 (4) ◽

pp. 6803-6837 ◽

Cited By ~ 1

Author(s):

F. Chever ◽

G. Sarthou ◽

E. Bucciarelli ◽

S. Blain ◽

A. R. Bowie

Keyword(s):

Southern Ocean ◽

Dissolved Iron ◽

Heard Island ◽

Natural Iron ◽

Particulate Iron ◽

The Difference ◽

The One ◽

Iron Pool ◽

Lateral Advection ◽

Very High

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L−1 above the plateau and from 0.34 to 2.23 nmol L−1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L−1). Apparent particulate iron (Feapp), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Feapp above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Feapp and DFe above the plateau, with a supply of 9.7±2.3×106 and 8.3±6.7×103 mol d−1, respectively. The residence times of 1.7 and 48 days estimated for Feapp and DFe, respectively, indicate a rapid turnover in the surface water. A comparison between Feapp and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)−1, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.

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Seasonal dispersal of fjord meltwaters as an important source of iron to coastal Antarctic phytoplankton

10.5194/bg-2021-79 ◽

2021 ◽

Author(s):

Kiefer Forsch ◽

Lisa Hahn-Woernle ◽

Robert Sherrell ◽

Joe Roccanova ◽

Kaixan Bu ◽

...

Keyword(s):

Phytoplankton Bloom ◽

Vertical Mixing ◽

Katabatic Wind ◽

Glacial Retreat ◽

Dissolved Iron ◽

Chemically Modified ◽

Ocean Coupling ◽

Particulate Iron ◽

Hydrologic System ◽

Wind Events

Abstract. Glacial meltwater from the western Antarctic Ice Sheet is hypothesized to be an important source of cryospheric iron, fertilizing the Southern Ocean, yet its trace metal composition and factors which control its dispersal remain poorly constrained. Here we characterize meltwater iron sources in a heavily glaciated western Antarctic Peninsula (WAP) fjord. Using dissolved and particulate ratios of manganese-to-iron in meltwaters, porewaters, and seawater, we show that glacial melt and subglacial plumes contribute to the seasonal cycle of bioavailable iron within a fjord still relatively unaffected by climate change-induced glacial retreat. Organic ligands derived from the phytoplankton bloom and the glaciers bind dissolved iron and facilitate the solubilization of particulate iron downstream. Using a numerical model, we show that plumes generated by outflow from the subglacial hydrologic system, enriched in labile particulate trace metals derived from a chemically-modified crustal source, can supply the surface through vertical mixing, and that prolonged katabatic wind events enhance export of meltwater out of the fjord. Thus, we identify an important atmosphere-ice-ocean coupling intimately tied to coastal iron biogeochemistry and primary productivity along the WAP.

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An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Biogeosciences ◽

10.5194/bg-7-455-2010 ◽

2010 ◽

Vol 7 (2) ◽

pp. 455-468 ◽

Cited By ~ 57

Author(s):

F. Chever ◽

G. Sarthou ◽

E. Bucciarelli ◽

S. Blain ◽

A. R. Bowie

Keyword(s):

Southern Ocean ◽

Dissolved Iron ◽

Heard Island ◽

Natural Iron ◽

Particulate Iron ◽

The Difference ◽

The One ◽

Iron Pool ◽

Lateral Advection ◽

Very High

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L−1 above the plateau and from 0.34 to 2.23 nmol L−1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L−1). Apparent particulate iron (Feapp), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Feapp above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Feapp and DFe above the plateau, with a supply of 9.7±3.6×106 and 8.3±11.6×103 mol d−1, respectively. The residence times of 1.7 and 48 days estimated for Feapp and DFe respectively, indicate a rapid turnover in the surface water. A comparison between Feapp and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)−1, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.

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Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago (Southern Ocean) during the natural fertilisation study, KEOPS-2

Biogeosciences ◽

10.5194/bg-12-4421-2015 ◽

2015 ◽

Vol 12 (14) ◽

pp. 4421-4445 ◽

Cited By ~ 39

Author(s):

A. R. Bowie ◽

P. van der Merwe ◽

F. Quéroué ◽

T. Trull ◽

M. Fourquez ◽

...

Keyword(s):

Southern Ocean ◽

Reference Site ◽

Standing Stock ◽

Minor Component ◽

Carbon Export ◽

Transport Pathways ◽

Dissolved Iron ◽

Iron Supply ◽

Particulate Iron ◽

Lateral Advection

Abstract. Iron availability in the Southern Ocean controls phytoplankton growth, community composition and the uptake of atmospheric CO2 by the biological pump. The KEOPS-2 (KErguelen Ocean and Plateau compared Study 2) "process study", took place around the Kerguelen Plateau in the Indian sector of the Southern Ocean. This is a region naturally fertilised with iron on the scale of hundreds to thousands of square kilometres, producing a mosaic of spring blooms which show distinct biological and biogeochemical responses to fertilisation. This paper presents biogeochemical iron budgets (incorporating vertical and lateral supply, internal cycling, and sinks) for three contrasting sites: an upstream high-nutrient low-chlorophyll reference, over the plateau and in the offshore plume east of the Kerguelen Islands. These budgets show that distinct regional environments driven by complex circulation and transport pathways are responsible for differences in the mode and strength of iron supply, with vertical supply dominant on the plateau and lateral supply dominant in the plume. Iron supply from "new" sources (diffusion, upwelling, entrainment, lateral advection, atmospheric dust) to the surface waters of the plume was double that above the plateau and 20 times greater than at the reference site, whilst iron demand (measured by cellular uptake) in the plume was similar to that above the plateau but 40 times greater than at the reference site. "Recycled" iron supply by bacterial regeneration and zooplankton grazing was a relatively minor component at all sites (< 8 % of new supply), in contrast to earlier findings from other biogeochemical iron budgets in the Southern Ocean. Over the plateau, a particulate iron dissolution term of 2.5 % was invoked to balance the budget; this approximately doubled the standing stock of dissolved iron in the mixed layer. The exchange of iron between dissolved, biogenic particulate and lithogenic particulate pools was highly dynamic in time and space, resulting in a decoupling of the iron supply and carbon export and, importantly, controlling the efficiency of fertilisation.

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The role of iron species on the competition of two coastal diatoms, <i>Skeletonema costatum</i> and <i>Thalassosira weissflogii</i>

Biogeosciences Discussions ◽

10.5194/bgd-10-19603-2013 ◽

2013 ◽

Vol 10 (12) ◽

pp. 19603-19631 ◽

Cited By ~ 2

Author(s):

S.-X. Li ◽

F.-J. Liu ◽

F.-Y. Zheng ◽

Y.-G. Zuo ◽

X.-G. Huang

Keyword(s):

Interspecific Competition ◽

Cell Density ◽

Skeletonema Costatum ◽

Density Ratio ◽

Iron Species ◽

Colloidal Iron ◽

Dissolved Iron ◽

Fe Species ◽

Particulate Iron ◽

Algal Exudates

Abstract. Coastal diatoms are often exposed to macronutrient (N and P) and Fe enrichment. However, how these exposures influence on Fe biogeochemical cycle and then on diatom interspecific competition is unknown. In this study, two non-toxic coastal diatoms, Skeletonema costatum and Thalassosira weissflogii were exposed to N, P, and Fe enrichment for four-day. The growth of algae was co-controlled by macronutrient and Fe species (Fe (III)-EDTA, Fe(OH)3, dissolved, colloidal, and particulate Fe from culture medium). The influence of Fe species on algal cell density was more significant than macronutrient. When S. costatum coexisted with T. weissflogii, their cell density ratios were ranged between 5.57–7.03 times, indicating that S. costatum was more competitive than T. weissflogii. There were not significant correlation between cell density ratio and iron requirement, including iron adsorption and absorption per cell, iron adsorption and absorption by all algal cells. As Fe complexing ligands, algal exudates can promote diatom growth itself and such promotion on S. costatum was more obvious than that on T. weissflogii. Iron species was a key determinant on interspecific competition of coastal diatom, and the degree of bioavailability was described as follows: dissolved iron from own exudates > colloidal iron from own exudates > particulate iron from own exudates > particulate iron from another algal exudates > colloidal iron from another algal exudates > dissolved iron from another algal exudates > Fe (III)-EDTA> Fe (OH)3.

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Effect of Drawdown on Ammonium and Iron Concentrations in a Coastal Mountain Reservoir

Water Quality Research Journal ◽

10.2166/wqrj.2000.015 ◽

2000 ◽

Vol 35 (2) ◽

pp. 231-244 ◽

Cited By ~ 4

Author(s):

C.J. Perrin ◽

K.I. Ashley ◽

G.A. Larkin

Keyword(s):

Water Quality ◽

Distribution System ◽

Management Strategies ◽

Dissolved Iron ◽

Particulate Iron ◽

High Concentrations ◽

Water District ◽

Coastal Mountain ◽

Iron Concentrations

Abstract Part way through a year-long water quality field survey, the Capilano Reservoir, located in the Greater Vancouver Water District, was drawn down to accommodate earthquake reinforcement work on the Cleveland Dam. This paper reports observations of the effect of drawdown on water quality in the reservoir, in the context of the original study. The drawdown exposed a large zone of deltaic material through which the Capilano River eroded a path up to 2.5 m deep. The deltaic material contained substantial amounts of nitrogen and iron (TKN, 4,470 mg kg-1; total iron, 21,800 mg kg-1). During drawdown, turbidity in the reservoir increased up to 25 times as compared to prior conditions. Ammonium (NH4+-N) and dissolved and particulate iron concentrations in the drawdown zone were up to two orders of magnitude higher than concentrations upstream. Both NH4+-N and dissolved iron were observed to travel the length of the reservoir. Persistence of NH4+-N in the highly oxygenated conditions may be explained by the presence of factors that inhibit nitrification. Persistence of NH4+-N and particularly dissolved iron may be due to adsorption and com-plexation reactions. Seasonally high concentrations of iron and ammonium at the water intake corresponded to the timing of complaints of taste, odour, turbid water, and staining in the distribution system. Observations made during this extreme drawdown suggest that transport of N and Fe from exposed deltaic sediments may play a role in the periodic increases in ammonium and iron observed in the Capilano Reservoir. Further understanding of the impacts associated with changing reservoir elevation should lead to the development of appropriate management strategies to maintain the high quality of GVWD water.

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Iron budgets for three distinct biogeochemical sites around the Kerguelen archipelago (Southern Ocean) during the natural fertilisation experiment KEOPS-2

Biogeosciences Discussions ◽

10.5194/bgd-11-17861-2014 ◽

2014 ◽

Vol 11 (12) ◽

pp. 17861-17923 ◽

Cited By ~ 28

Author(s):

A. R. Bowie ◽

P. van der Merwe ◽

F. Quéroué ◽

T. Trull ◽

M. Fourquez ◽

...

Keyword(s):

Southern Ocean ◽

Standing Stock ◽

Minor Component ◽

Carbon Export ◽

Iron Dissolution ◽

Transport Pathways ◽

Dissolved Iron ◽

Iron Supply ◽

Particulate Iron ◽

Internal Cycling

Abstract. Iron availability in the Southern Ocean controls phytoplankton growth, community composition and the uptake of atmospheric CO2 by the biological pump. The KEOPS-2 experiment took place around the Kerguelen plateau in the Indian sector of the Southern Ocean, a region naturally fertilised with iron at the scale of hundreds to thousands of square kilometres, producing a mosaic of spring blooms which showed distinct biological and biogeochemical responses to fertilisation. This paper presents biogeochemical iron budgets (incorporating vertical and lateral supply, internal cycling, and sinks) for three contrasting sites: an upstream high-nutrient low-chlorophyll reference, over the plateau, and in the offshore plume east of Kerguelen Island. These budgets show that distinct regional environments driven by complex circulation and transport pathways are responsible for differences in the mode and strength of iron supply, with vertical supply dominant on the plateau and lateral supply dominant in the plume. Iron supply from "new" sources to surface waters of the plume was double that above the plateau and 20 times greater than at the reference site, whilst iron demand (measured by cellular uptake) in the plume was similar to the plateau but 40 times greater than the reference. "Recycled" iron supply by bacterial regeneration and zooplankton grazing was a relative minor component at all sites (<8% of "new" supply), in contrast to earlier findings from other biogeochemical iron budgets in the Southern Ocean. Over the plateau, a particulate iron dissolution term of 2.5% was invoked to balance the budget; this approximately doubled the standing stock of dissolved iron in the mixed layer. The exchange of iron between dissolved, biogenic and lithogenic particulate pools was highly dynamic in time and space, resulting in a decoupling of iron supply and carbon export and, importantly, controlling the efficiency of fertilisation.

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Effects of dust deposition on iron cycle in the surface Mediterranean Sea: results from a mesocosm seeding experiment

Biogeosciences ◽

10.5194/bg-7-3769-2010 ◽

2010 ◽

Vol 7 (11) ◽

pp. 3769-3781 ◽

Cited By ~ 54

Author(s):

T. Wagener ◽

C. Guieu ◽

N. Leblond

Keyword(s):

Water Column ◽

Sediment Traps ◽

Dust Particles ◽

Dust Deposition ◽

Dissolved Iron ◽

Surface Ocean ◽

Batch Dissolution ◽

Particulate Iron ◽

Dust Events ◽

Scavenging Ratio

Abstract. Soil dust deposition is recognized as a major source of iron to the open ocean at global and regional scales. However, the processes that control the speciation and cycle of iron in the surface ocean after dust deposition are poorly documented mainly due to the logistical difficulties to investigate in-situ, natural dust events. The development of clean mesocosms in the frame of the DUNE project (a DUst experiment in a low Nutrient low chlorophyll Ecosystem) was a unique opportunity to investigate these processes at the unexplored scale of one dust deposition event. During the DUNE-1-P mesocosm seeding experiment, iron stocks (dissolved and particulate concentrations in the water column) and fluxes (export of particulate iron in sediment traps) were followed during 8 days after an artificial dust seeding mimicking a wet deposition of 10 g m−2. The addition of dust at the surface of the mesocosms was immediately followed by a decrease of dissolved iron [dFe] concentration in the 0–10 m water column. This decrease was likely due to dFe scavenging on settling dust particles and mineral organic aggregates. The scavenging ratio of dissolved iron on dust particles averaged 0.37 ± 0.12 nmol mg−1. Batch dissolution experiments conducted in parallel to the mesocosm experiment showed a increase (up to 600%) in dust iron dissolution capacity in dust-fertilized waters compared to control conditions. This study gives evidences of complex and unexpected effects of dust deposition on surface ocean biogeochemistry: (1) large dust deposition events may be a sink for surface ocean dissolved iron and (2) successive dust deposition events may induce different biogeochemical responses in the surface ocean.

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