Impact of different iron input regimes on phytoplankton dynamics and biogeochemistry in the Atlantic Sector of the Southern Ocean downstream of South Georgia

Primary production in the Southern Ocean is largely limited by iron (Fe) availability, resulting in
persistently high macronutrient concentrations, thus making it a high-nutrient, low-chlorophyll
(HNLC) region. Localized exceptions occur near islands, where natural Fe fertilization from
upwelled deep waters, shelf sediments, or glacial meltwater sustain intense phytoplankton
blooms (Island Mass Effect). South Georgia Island in the Atlantic sector of the Southern Ocean
is a prominent example of such a high productivity area and constitutes a major sink for
atmospheric CO₂. While bloom extent and intensity in this region are well documented by
satellite observations, in situ data on phytoplankton physiology is scarce and the bioavailability
of relevant natural Fe sources in the region remains unknown. Recent research has shown that,
in addition to Fe, manganese (Mn) can also limit phytoplankton growth in the Southern Ocean.
However, the role of Mn limitation in naturally Fe fertilized regions has not yet received
attention. This thesis addresses these knowledge gaps by investigating the Fe bioavailability of
different nearshore sources, spatial distribution patters of Fe concentrations in conjunction
with primary production and photophysiology, as well as Mn availability and its impact on
primary production and carbon export.
Publication I quantified the bioavailability of Fe from glacial meltwater, estuarine water, and
groundwater from South Georgia by measuring Fe uptake rates of a natural phytoplankton
community amended with each source. Glacial meltwater and estuarine water contained
bioavailable Fe and increases primary production in low-chlorophyll waters by 75–105%. On
the other hand, groundwater-derived Fe was not bioavailable, contrasting with earlier
geochemical assumptions. In high-chlorophyll waters, aggregates formed from dissolved
organic matter scavenged Fe added from natural sources, making it unavailable to
phytoplankton. These results demonstrate that Fe bioavailability depends on the chemistry of
both the source and the receiving seawater.
Publication II revealed distinct spatial distribution patterns of dissolved Fe (dFe) concentrations,
suggesting different Fe supply mechanisms around the Island. On the shelf and in fjords, high
dFe concentrations and productivity were sustained by the proximity to sedimentary and glacial
Fe sources. Downstream, shelf-influenced waters exhibited low dFe but high productivity,
indicating a continuous supply of dFe from the island while high Fe uptake rates by
phytoplankton prevented an accumulation of dFe in surface waters. In contrast, waters
upstream of the island, lacking shelf influence showed low dFe and low productivity, reflecting
typical HNLC conditions. These findings highlight the central role of the Island Mass Effect in
shaping regional phytoplankton dynamics.
Publication III demonstrated the importance of Mn availability around South Georgia as in 5 out
of 6 amendment experiments with natural phytoplankton communities, Mn-related growth
responses were found. This was especially evident in species-specific responses with different
Chaetoceros species being highly susceptible to Mn limitation. In subsequent roller tank
incubations, the aggregation capacity of the six phytoplankton communities was tested and
species composition exerted a stronger control on aggregate formation and carbon export than
total biomass.
Overall, this thesis demonstrates that primary production, phytoplankton community
composition, and carbon export around South Georgia Island are governed by distinct Fe supply
regimes and source-specific Fe bioavailability. Additionally, it highlights a contributing role of
Mn availability in regulating phytoplankton growth around the island.