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

Abstract

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

Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the KEOPS2 survey

“…The most westerly stations were located over the inner (TEW1) and outer (TEW2) parts of the Kerguelen shelf. The most easterly stations (TEW7 and TEW8) were situated in Subantarctic Mode Water whereas the central section (TEW4 to TEW6) within the stationary meander was covered by mixed Antarctic surface water (Farías et al, 2015;Trull et al, 2015).…”

“…The resulting δ 13 C normalized provides an estimate of δ 13 C corrected for the effects of Post et al (2007). As δ 13 C values provided by Trull et al (2015) were not lipid-normalized, acidified δ 13 C values for all zooplankton size fractions are indicated in Table 2, along with lipid-normalized δ 13 C values, to allow comparisons between the two data sets.…”

“…To consider the relationships between zooplankton and phytoplankton, we used the groups of stations (T-groups) defined by Trull et al (2015) based on chemometric measurements of phytoplankton. The HNLC reference station R2 belonged to T-group 1, along with station TEW4.…”

“…Satellitebased chlorophyll images of this region highlight the intensive seasonal Kerguelen bloom and its south-east extension off the archipelago (Schlitzer, 2002;Thomalla et al, 2011;Blain et al, 2013;Trull et al, 2015). During the KEOPS1 survey (KErguelen Ocean and Plateau compared Study), the origin and fate of the elevated phytoplankton biomass over the Kerguelen Plateau were addressed (Blain et al, 2008), with a focus on the mechanisms supplying the surface waters with iron.…”

Carbon, oxygen and biological productivity in the Southern Ocean in and out the Kerguelen plume: CARIOCA drifter results

“…1). The average MLD at these stations, calculated with the criteria that depth where the potential density equals potential density at 10 m + 0.02 kg m −3 , was equal to 20 m (Park et al, 2014;Trull et al, 2015). We elect to use this depth as our MLD definition, as physical (T , S) characteristics at these stations are very similar to CARIOCA measurements (Fig.…”