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.
Bibliographical noteFunding Information:
Acknowledgements. We are indebted to the captain and crew of RV ‘Polarstern’ and the scientific team of LOHAFEX. The Council of Scientific and Industrial Research (CSIR), India and the Helmholtz Foundation, Germany, equally shared the costs of the experiment. This work was funded through the DFG Research Center / Cluster of Excellence ‘The Ocean in the Earth System’. I.S. was supported by GLOMAR Bremen International Graduate School for Marine Sciences. We thank Diana Sarno and Adriana Zingone (Stazione Zoologica Anton Dohrn, Napoli, Italy) for SEM preparations and support in taxonomical identification.
© The authors 2018.
- Carbon:chlorophyll ratios
- Ecology–biogeochemistry relationship
- Ecosystem stability
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Aquatic Science