Chemometric perspectives on plankton community responses to natural iron fertilisation over and downstream of the Kerguelen Plateau in the Southern Ocean

International audienceWe examined phytoplankton community responses to natural iron fertilisation at 32 sites over and downstream from the Kerguelen Plateau in the Southern Ocean during the austral spring bloom in October–November 2011. The community structure was estimated from chemical and isotopic measurements (particulate organic carbon – POC; 13C-POC; particulate nitrogen – PN; 15N-PN; and biogenic silica – BSi) on size-fractionated samples from surface waters (300, 210, 50, 20, 5, and 1 μm fractions). Higher values of 13C-POC (vs. co-located 13C values for dissolved inorganic carbon – DIC) were taken as indicative of faster growth rates and higher values of 15N-PN (vs. co-located 15N-NO3 source values) as indicative of greater nitrate use (rather than ammonium use, i.e. higher f ratios).Community responses varied in relation to both regional circulation and the advance of the bloom. Iron-fertilised waters over the plateau developed dominance by very large diatoms (50–210 μm) with high BSi / POC ratios, high growth rates, and significant ammonium recycling (lower f ratios) as biomass built up. In contrast, downstream polar frontal waters with a similar or higher iron supply were dominated by smaller diatoms (20–50 μm) and exhibited greater ammonium recycling. Stations in a deep-water bathymetrically trapped recirculation south of the polar front with lower iron levels showed the large-cell dominance observed on the plateau but much less biomass. Comparison of these communities to surface water nitrate (and silicate) depletions as a proxy for export shows that the low-biomass recirculation feature had exported similar amounts of nitrogen to the high-biomass blooms over the plateau and north of the polar front. This suggests that early spring trophodynamic and export responses differed between regions with persistent low levels vs. intermittent high levels of iron fertilisation

Resupply of mesopelagic dissolved iron controlled by particulate iron composition

The dissolved iron supply controls half of the oceans’ primary productivity. Resupply by the remineralization of sinking particles, and subsequent vertical mixing, largely sustains this productivity. However, our understanding of the drivers of dissolved iron resupply, and their influence on its vertical distribution across the oceans, is still limited due to sparse observations. There is a lack of empirical evidence as to what controls the subsurface iron remineralization due to difficulties in studying mesopelagic biogeochemistry. Here we present estimates of particulate transformations to dissolved iron, concurrent oxygen consumption and iron-binding ligand replenishment based on in situ mesopelagic experiments. Dissolved iron regeneration efficiencies (that is, replenishment over oxygen consumption) were 10- to 100-fold higher in low-dust subantarctic waters relative to higher-dust Mediterranean sites. Regeneration efficiencies are heavily influenced by particle composition. Their make-up dictates ligand release, controls scavenging, modulates ballasting and may lead to the differential remineralization of biogenic versus lithogenic iron. At high-dust sites, these processes together increase the iron remineralization length scale. Modelling reveals that in oceanic regions near deserts, enhanced lithogenic fluxes deepen the ferricline, which alter the vertical patterns of dissolved iron replenishment, and set its redistribution at the global scale. Such wide-ranging regeneration efficiencies drive different vertical patterns in dissolved iron replenishment across oceanic provinces

Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the Th-234 approach

This study examined upper-ocean particulate organic carbon (POC) export using the Th-234 approach as part of the second KErguelen Ocean and Plateau compared Study expedition (KEOPS2). Our aim was to characterize the spatial and the temporal variability of POC export during austral spring (October-November 2011) in the Fe-fertilized area of the Kerguelen Plateau region. POC export fluxes were estimated at high productivity sites over and downstream of the plateau and compared to a high-nutrient low-chlorophyll (HNLC) area upstream of the plateau in order to assess the impact of iron-induced productivity on the vertical export of carbon. Deficits in Th-234 activities were observed at all stations in surface waters, indicating early scavenging by particles in austral spring. Th-234 export was lowest at the reference station R-2 and highest in the recirculation region (E stations) where a pseudo-Lagrangian survey was conducted. In comparison Th-234 export over the central plateau and north of the polar front (PF) was relatively limited throughout the survey. However, the Th-234 results support that Fe fertilization increased particle export in all iron-fertilized waters. The impact was greatest in the recirculation feature (3-4 fold at 200 m depth, relative to the reference station), but more moderate over the central Kerguelen Plateau and in the northern plume of the Kerguelen bloom (similar to 2-fold at 200 m depth). The C : Th ratio of large (> 53 mu m) potentially sinking particles collected via sequential filtration using in situ pumping (ISP) systems was used to convert the Th-234 flux into a POC export flux. The C : Th ratios of sinking particles were highly variable (3.1 +/- 0.1 to 10.5 +/- 0.2 mu mol dpm(-1)) with no clear site-related trend, despite the variety of ecosystem responses in the fertilized regions. C : Th ratios showed a decreasing trend between 100 and 200 m depth suggesting preferential carbon loss relative to Th-234 possibly due to heterotrophic degradation and/or grazing activity. C : Th ratios of sinking particles sampled with drifting sediment traps in most cases showed very good agreement with ratios for particles collected via ISP deployments (> 53 mu m particles). Carbon export production varied between 3.5 +/- 0.9 and 11.8 +/- 1.3 mmol m(-2) d(-1) from the upper 100 m and between 1.8 +/- 0.9 and 8.2 +/- 0.9 mmol m(-2) d(-1) from the upper 200 m. The highest export production was found inside the PF meander with a range of 5.3 +/- 1.0 to 11.8 +/- 1.1 mmol m(-2) d(-1) over the 19-day survey period. The impact of Fe fertilization is highest inside the PF meander with 2.9-4.5-fold higher carbon flux at 200 m depth in comparison to the HNLC control station. The impact of Fe fertilization was significantly less over the central plateau (stations A3 and E-4W) and in the northern branch of the bloom (station F-L) with 1.6-2.0-fold higher carbon flux compared to the reference station R. Export efficiencies (ratio of export to primary production and ratio of export to new production) were particularly variable with relatively high values in the recirculation feature (6 to 27 %, respectively) and low values (1 to 5 %, respectively) over the central plateau (station A3) and north of the PF (station F-L), indicating spring biomass accumulation. Comparison with KEOPS1 results indicated that carbon export production is much lower during the onset of the bloom in austral spring than during the peak and declining phases in late summer

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

00000 ăWOS:000390514800002International audienceThe Kerguelen Plateau is characterized by a naturally Fe-fertilized phytoplanlcton bloom that extends more than 1000 km downstream in the Antarctic Circumpolar Current. During the KEOPS2 study, in austral spring, we measured particulate nitrogen (PN), biogenic silica (BSi) and particulate iron (PFe) export fluxes in order to investigate how the natural fertilization impacts the stoichiometry and the magnitude of export fluxes and therefore the efficiency of the biological carbon pump. At 9 stations, we estimated elemental export fluxes based on element concentration to Th-234 activity ratios for particulate material collected with in-situ pumps and Th-234 export fluxes (Planchon et al., 2015). This study revealed that the natural Fe-fertilization increased export fluxes but to variable degrees. Export fluxes for the bloom impacted area were compared with those of a high-nutrient, low-chlorophyll (HNLC), low-productive reference site located to the south-west of Kerguelen and which had the lowest BSi and PFe export fluxes (2.55 mmol BSi m(-2) d(-1) and 1.92 mu mol PFem(-2) d(-1)) and amongst the lowest PN export flux (0.73 mmol PN m(-2) d(-1)). The impact of the Fe fertilization was the greatest within a meander of the polar front (PF), to the east of Kerguelen, with fluxes reaching 1.26 mmol PN m(-2) d(-1); 20.4 mmol BSi m(-2) d(-1) and 22.4 mu mol PFe m(-2) d(-1). A highly productive site above the Kerguelen Plateau, on the contrary, was less impacted by the fertilization with export fluxes reaching 0.72 mmol PN m(-2) d(-1); 4.50 mmol BSi m(-2) d(-1) and 21.4 mu mol PFe d(-1). Our results suggest that ecosystem features (i.e. type of diatom community) could play an important role in setting the magnitude of export fluxes of these elements. Indeed, for the PF meander, the moderate productivity was sustained by the presence of large and strongly silicified diatom species while at the higher productivity sites, smaller and slightly silicified diatoms dominated. Interestingly, our results suggest that PFe export fluxes can be driven by the lithogenic pool of particles, especially over the Plateau where such inputs from the sediments are important. Finally, for the Plateau and the PF meander, the comparison between PFe export and the particulate PFe stock integrated over the mixed layer depth revealed an efficient PFe export out of the mixed layer at these sites. Export efficiencies (i.e. the ratio between export and uptake) exhibit a very efficient silica pump especially at the HNLC reference station where heavily silicified diatoms were present. On the contrary, the increase with depth of the C:N ratio and the low nitrogen export efficiencies support the idea of a strong remineralization and nitrification activity

Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Events: Learning from the Past to Predict the Future

Despite interest as early as in the 1880s, it was not until 1953 that Tokimi Tsujita (Seikai Fisheries Research Laboratory, Japan) was able to carefully collect and describe the matrix of microorganisms embedded in suspended organic matter (Tsujita, J Oceanogr Soc Jpn 8:1–14, 1953) that today we call marine snow. Subsequent studies reported that marine snow consisted of phytoplankton, small zooplankton, fecal material, and other particles (Nishizawa et al., Bull Fac Fish, Hokkaido Univ. 5:36–40, 1954). Across the ocean, Riley (Limnol Oceanogr 8:372–381, 1963) called this material “organic aggregates” which in addition to the organic material included nonliving material that was a “substrate for bacterial growth.” More than a decade later, Silver et al. (Science 201:371–373, 1978) quantified the abundance of marine snow, and its contribution to the total community in situ, and showed that marine snow particles were “metabolic hotspots,” with concentrations of microorganisms 3–4 orders of magnitude greater than those in the surrounding seawater. Alldredge and Cohen (Science 235:689–691, 1987) emphasized the importance of marine snow as unique chemical and physical microhabitats. The importance of transparent exopolymer particles (TEP), which form the matrix that embeds the individual component particles of marine snow, were described and quantified in the early 1990s (Alldredge et al., Deep-Sea Res I 40: 1131–1140, 1993; Passow and Alldredge, Mar Ecol Prog Ser 113:185–198, 1994; Passow et al., Deep-Sea Res Oceanogr Abstr 41:335–357, 1994). The long-held belief that marine snow was both a specialized habitat and potential food source for those living in the deep ocean was also demonstrated at that time (Silver and Gowing, Prog Oceanogr 26:75–113, 1991). More recently it was confirmed that marine snow does indeed contribute significantly to the metabolism of the deep sea and provides hotspots of microbial diversity and activity at depth (e.g., Burd et al., Deep-Sea Res II 57:1557–1571, 2010; Bochdansky et al., Sci Rep 6:22633, 2016). Moreover, marine snow is now considered a transport vehicle for its biota and associated particulate matter (Volk and Hoffert, The carbon cycle and atmospheric CO: natural variations archean to present. American Geophysical Union, Washington, D.C., pp. 99–110, 1985; Alldredge and Gotschalk, Limnol Oceanogr 33:339–351, 1988). Rapidly sinking marine snow is important in the marine carbon cycle as it is responsible for vertical (re)distribution and remineralization of carbon. The transport of carbon from the surface to the deep sea is known as the “biological carbon pump” (De La Rocha and Passow, Deep Sea Res II 54:639–658, 2007; De La Rocha and Passow, Treatise on Geochemistry. Vol. 8, Elsevier, Oxford, 2014). This pump, which leads to the uptake and sequestration of atmospheric CO2 (e.g., Volk and Hoffert, The carbon cycle and atmospheric CO: natural variations archean to present. American Geophysical Union, Washington, D.C., pp. 99–110, 1985; Finkel et al., J Plankton Res 32:119–137, 2010; Zetsche and Ploug, Mar Chem 175:1–4, 2015), also plays an important role in the biogeochemical cycling of elements (e.g., Quigg et al., Nature 425:291–294, 2003; Quigg et al., Proc R Soc: Biol Sci 278:526–534, 2011). How climate change will change these processes is the subject of intense interest but beyond the scope of this chapter