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

Abstract

Abstract. We 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. 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-DIC source values) 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. 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 as biomass built up. In contrast, downstream Polar Frontal waters with 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 exported similar amounts of nitrogen to the high biomass blooms over the plateau and north of the Polar Front. This suggests that trophodynamic and export responses differed between regions with persistent low levels vs. punctual high levels of iron fertilisation.

The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau

“…1 and 2). For more information on the complex spatio-temporal evolution of the phytoplankton bloom over the full 2011-2012 annual cycle, we refer the reader to an animation of NASA MODIS Aqua chlorophyll images, provided as a supplementary material in Trull et al (2014). Combination of sediment trap collection with volume-to-carbon conversion factors allowed to determine preferential modes of carbon export (Ebersbach and Trull, 2008;Ebersbach et al, 2011).…”

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

“…As a result of the mixing, eddies commonly form in this region. Also within this mixing zone, surface filaments, identified by elevated Chlorophyll a, can be seen in SeaWiFS images diverging from the PF and entering the eastern boundary of the recirculation structure (see the Supplement in Trull et al, 2014). The Kerguelen Archipelago is isolated, being a relatively small and localised source of Fe fertilisation surrounded by the large and deep, HNLC, low Fe, Southern Ocean.…”

Mesoscale Variability of Conditions Favoring an Iron‐Induced Diatom Bloom Downstream of the Kerguelen Plateau

“…This area is characterized by an annual phytoplankton bloom that extends for more than 250,000 km 2 downstream, with chlorophyll concentrations up to 10 times that of surrounding waters. Previous studies have attributed the iron enrichment to sedimentary sources from the plateau (Blain et al, ; d'Ovidio et al, ; Quéroué et al, ; Trull et al, ; Van Der Merwe et al, ; Zhang et al, 2008). As in other cases of blooms developing in the wake of sub‐Antarctic islands, the shape and extension of the Kerguelen bloom is known to be strongly related to the circulation (Borrione et al, ; d'Ovidio et al, ; Graham et al, ; Grenier et al, ; Mongin et al, ; Robinson et al, ; Trull et al, ).…”

“…Satellite images of surface chlorophyll concentrations show that the Kerguelen plume is highly heterogeneous, displaying strong gradients even within the plume itself, in response to eddies and frontal structures (Figure c)). The community structure of the bloom's primary producers has been studied during ship‐based oceanographic campaigns, which have focused mainly on differentiating diatoms types and on pigment and size analyses (Armand et al, ; Lasbleiz et al, ; Mosseri et al, ; Quéguiner, 2013; Trull et al, ; Uitz et al, ). However, in‐situ observations are limited in spatial extent by the large size of the plume and the remoteness of the area.…”

“…This dynamical complexity compounds the problems of the vastness, remoteness and harsh weather conditions of most Southern Ocean regions, because it means that the sparse in‐situ biological (i.e., taxonomical observations, pigment sampling) and chemical (i.e., iron concentrations) observations cannot be readily extrapolated regionally or from one year to another. Thus, approaches that address this mesoscale complexity are important to gain insight into the mechanisms regulating Southern Ocean diatom abundances, distinguish regions characterized by different biogeochemical processes, and identify oceanic provinces that are likely to support large predators and fisheries (Allen et al, ; Barton et al, ; Dutkiewicz et al, ; Falkowski et al, ; Follows et al, ; Longhurst, ; Oliver & Irwin, ; Trull et al, ).…”