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. 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.
Highlights
Natural iron fertilisation from islands, shelves, and plateaus in the Southern Ocean produces local and downstream elevations of phytoplankton biomass that are ∼ 10-fold higher than in surrounding high-nutrient low-chlorophyll (HNLC) waters; see, e.g., de Baar et al (1995)
To determine nitrate depletion by the biological pump, we explored both temperature and salinity-based approaches to estimate initial winter surface water concentrations and evaluated the fraction of the observed depletion that still remained in the water column for potential future export using particulate nitrogen and biogenic silica stocks from CTD casts (Blain et al, 2014; Lasbleiz et al, 2014)
There were some differences in particulate organic carbon (POC) results across the multiple sample methodologies of the entire KEOPS2 program, e.g. from underway supply, Niskin bottles, and in situ pumps (Dehairs et al, 2014; Lasbleiz et al, 2014; Tremblay, 2014), these remain to be fully assessed, and here we focus on our own internally consistent results
Summary
Natural iron fertilisation from islands, shelves, and plateaus in the Southern Ocean produces local and downstream elevations of phytoplankton biomass that are ∼ 10-fold higher than in surrounding high-nutrient low-chlorophyll (HNLC) waters; see, e.g., de Baar et al (1995) In some of these systems, carbon export has been observed to be elevated ∼ 2–. 3 fold, e.g over the Kerguelen Plateau (Blain et al, 2008; Savoye et al, 2008) and to the north of Crozet Island (Pollard et al, 2007) These studies produced order-of-magnitude variations in estimates of the amount of carbon export per unit of iron supply, as have deliberate iron fertilisation studies (Boyd et al, 2007). These variations appear to reflect both observational limitations and system complexity, including the possibility of variations in initial communities prior to fertilisation (as a result of north–south oceanographic variations or the extent of connection to coastal habitats)
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call