Sourcing the iron in the naturally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics

Abstract

Abstract. The KEOPS2 project aims to elucidate the role of natural Fe fertilisation on biogeochemical cycles and ecosystem functioning, including quantifying the sources and processes by which iron is delivered in the vicinity of the Kerguelen Archipelago, Southern Ocean. The KEOPS2 process study used an upstream high-nutrient, low-chlorophyll (HNLC), deep water (2500 m), reference station to compare with a shallow (500 m), strongly fertilised plateau station and continued the observations to a downstream, bathymetrically trapped recirculation of the Polar Front where eddies commonly form and persist for hundreds of kilometres into the Southern Ocean. Over the Kerguelen Plateau, mean particulate (1-53 µm) Fe and Al concentrations (pFe = 13.4 nM, pAl = 25.2 nM) were more than 20-fold higher than at an offshore (lower-productivity) reference station (pFe = 0.53 nM, pAl = 0.83 nM). In comparison, over the plateau dissolved Fe levels were only elevated by a factor of ∼ 2. Over the Kerguelen Plateau, ratios of pMn / pAl and pFe / pAl resemble basalt, likely originating from glacial/fluvial inputs into shallow coastal waters. In downstream, offshore deep-waters, higher pFe / pAl, and pMn / pAl ratios were observed, suggesting loss of lithogenic material accompanied by retention of pFe and pMn. Biological uptake of dissolved Fe and Mn and conversion into the biogenic particulate fraction or aggregation of particulate metals onto bioaggregates also increased these ratios further in surface waters as the bloom developed within the recirculation structure. While resuspension of shelf sediments is likely to be one of the important mechanisms of Fe fertilisation over the plateau, fluvial and glacial sources appear to be important to areas downstream of the island. Vertical profiles within an offshore recirculation feature associated with the Polar Front show pFe Published by Copernicus Publications on behalf of the European Geosciences Union. P. van der Merwe et al.: Kerguelen Plateau: particulate trace metal sourcesand pMn levels that were 6-fold and 3.5-fold lower, respectively, than over the plateau in surface waters, though still 3.6-fold and 1.7-fold higher respectively than the reference station. Within the recirculation feature, strong depletions of pFe and pMn were observed in the remnant winter water (temperature-minimum) layer near 175 m, with higher values above and below this depth. The correspondence between the pFe minima and the winter water temperature minima implies a seasonal cycle is involved in the supply of pFe into the fertilised region. This observed association is indicative of reduced supply in winter, which is counterintuitive if sediment resuspension and entrainment within the mixed layer is the primary fertilising mechanism to the downstream recirculation structure. Therefore, we hypothesise that lateral transport of pFe from shallow coastal waters is strong in spring, associated with snow melt and increased runoff due to rainfall, drawdown through summer and reduced supply in winter when snowfal...

Inferring source regions and supply mechanisms of iron in the Southern Ocean from satellite chlorophyll data

“…It is also widely recognised that many supply mechanisms of iron are highly episodic (Boyd et al, 2004). Consequently, in-situ measurements of dissolved iron from surface waters may not be indicative of the mean state (Croot et al, 2004;Bergquist and Boyle, 2006;Boyd and Ellwood, 2010;Chever et al, 2010;Ellwood et al, 2014;Van der Merwe et al, 2015). However, iron concentrations in deeper waters (200-6000 m) are thought to be more stable.…”

“…However, the photochemical reduction of ligands is thought to make organically complexed iron more bioavailable (Gledhill and Buck, 2012). There is also increasing evidence that particulate iron can be accessible to some phytoplankton under certain conditions (Nodwell and Price, 2001;Frew et al, 2006;Van der Merwe et al, 2015). The bioavailability of different iron species should therefore be treated as a spectrum, rather than available/unavailable (Shaked and Lis, 2012).…”

“…The challenge of understanding the largescale sources and supply mechanisms of iron is further complicated by the vast size of the Southern Ocean and eddying circulation. The residence time of iron in surface waters is thought to be relatively short, on the order of only weeks to months and newly available iron in surface waters may be quickly utilised by biota, or scavenged from the water column (Gerringa et al, 2012;Van der Merwe et al, 2015). It is also widely recognised that many supply mechanisms of iron are highly episodic (Boyd et al, 2004).…”

Early spring mesopelagic carbon remineralization and transfer efficiency in the naturally iron-fertilized Kerguelen area

“…particles from around 5 L of Milli-Q water, dissolved in a final volume of 13 mL as for the samples. Biogenic barium (hereafter called excess Ba or Ba xs ) was calculated as the difference between total particulate Ba and lithogenic Ba using Al as the lithogenic reference element (Dymond et al, 1992;Taylor and McLennan, 1985). At most sites and depths, the biogenic Ba xs represented > 95 % of total particulate Ba.…”

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

“…Downstream of island plateaus, such as those at Kerguelen (Blain et al, 2001(Blain et al, , 2007, Crozet ice melt (van der Merwe et al, 2011), glacial runoff (Hawkings et al, 2014;van der Merwe et al, 2015) and melting icebergs (Duprat et al, 2016). Primary production downstream of subantarctic island systems is typically dominated by diatoms (Smetacek et al, 2004;Quéguiner, 2013), a microphytoplankton group generally considered to promote carbon export out of the mixed layer directly by sinking (Smetacek, 1985;Newton, 1995, 1999) or indirectly by sustaining faecal pellet-productive food webs (Smetacek et al, 2004;Manno et al, 2014).…”