Ecological and biogeochemical response of Antarctic ecosystems to iron fertilization and implications on global carbon cycle

Abstract

The European Iron Fertilization Experiment EIFEX studied the growth and decline of a phytoplankton bloom stimulated by fertilising 10 km2 in the core of a mesoscale (80 × 120 km) cyclonic eddy south of the Antarctic Polar Front with about 2 times 7 tonnes of iron sulphate. The phytoplankton accumulation induced by iron fertilization did not exceed 3 μg chl a l-1 despite a draw down of 5 μM of nitrate that should have resulted in at least double to triple the amount of phytoplankton biomass assuming regular Redfield-ratios for draw down after phytoplankton growth in the Southern Ocean. During EIFEX the fertilized core of the mesoscale eddy evolved to a hotspot for a variety of small and medium sized mesozooplankton copepods. In contrast to copepods, the biomass of salps (Salpa thompsoni) that dominated zooplankton biomass before the onset of our experiment decreased to nearly extinction. Most of the species of the mesozooplankton community showed extremely high feeding rates compared to literature values from Southern Ocean summer communities. At the end of the experiment, massive phytoplankton sedimentation reached the sea floor at about 3800 m water depth.