Publication Date:
2018-04-13
Description:
eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML)
waters were characterized by high nitrate (~20 μM), low dissolved iron (DFe ~0.2 nM) and low silicate concentrations
(below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the
first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing
pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large
copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material
comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no
significant deeper export.
showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron
concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that
indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution
of copepod grazing to iron cycling we used two different approaches: first, we measured for the first
time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content
per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate
the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of
isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet
was ~5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed
leachable iron (pH 2.0) in 48 h (LFe48h) that, for the conditions during LOHAFEX, was shown to be an excellent
proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second
fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in
the upper 80 m.
We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy
of iron colloids freshly formed after the second fertilization by the predominant flagellate community
resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in
fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper
100–150 m, the accumulation of LFe48h in the upper 80 m after the second fertilization and provided the iron
required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative
evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate
depleted areas of the Southern Ocean.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf
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