Publication Date:
2019-08-19
Description:
The genesis of phytoplankton blooms and the fate of their biomass in iron-limited,
high-nutrient−low-chlorophyll regions can be studied under natural conditions with ocean iron
fertilization (OIF) experiments. The Indo-German OIF experiment LOHAFEX was carried out
over 40 d in late summer 2009 within the cold core of a mesoscale eddy in the productive southwest
Atlantic sector of the Southern Ocean. Silicate concentrations were very low, and phytoplankton
biomass was dominated by autotrophic nanoflagellates (ANF) in the size range 3−10 μm.
As in all previous OIF experiments, the phytoplankton responded to iron fertilization by increasing
the maximum quantum yield (Fv/Fm) and cellular chlorophyll levels. Within 3 wk, chlorophyll
levels tripled and ANF biomass doubled. With the exception of some diatoms and dinoflagellates,
the biomass levels of all other groups of the phyto- and protozooplankton (heterotrophic nanoflagellates,
dinoflagellates and ciliates) remained remarkably stable throughout the experiment
both inside and outside the fertilized patch. We attribute the unusually high biomass attained and
maintained by ANF to the absence of their grazers, the salps, and to constraints on protozooplankton
grazers by heavy predation exerted by the large copepod stock. The resistance to change of
the ecosystem structure over 38 d after fertilization, indicated by homogeneity at regional and
temporal scales, suggests that it was locked into a stable, mature state that had evolved in the
course of the seasonal cycle. The LOHAFEX bloom provides a case study of a resistant/robust
dynamic equilibrium between auto- and heterotrophic ecosystem components resulting in low
vertical flux both inside and outside the patch despite high biomass levels.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf
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