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

We examined phytoplankton community re-
sponses to natural iron fertilisation at 32 sites over and down-
stream 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;
13
C-POC; particulate nitrogen – PN;
15
N-PN; and biogenic
silica – BSi) on size-fractionated samples from surface wa-
ters (300, 210, 50, 20, 5, and 1 µm fractions). Higher values
of
13
C-POC (vs. co-located
13
C values for dissolved inor-
ganic carbon – DIC) were taken as indicative of faster growth
rates and higher values of
15
N-PN (vs. co-located
15
N-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 wa-
ters over the plateau developed dominance by very large di-
atoms (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 am-
monium 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 com-
munities 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 re-
sponses differed between regions with persistent low levels
vs. intermittent high levels of iron fertilisation.

Publication year:2015