Iron-Stimulated Phytoplankton Blooms in the Southern Ocean: a Brief Review

Abstract

Nine iron fertilization experiments have been carried out in the Southern Ocean (SO) so far comprising of seven artificial fertilization and two naturally enriched events. The first artificial iron addition, SOIREE, was conducted south of the polar front in February 1999 followed by six more experiments (EisenEx, SOFeX-N, SOFeX-S, EIFEX, SOLAS-SAGE, and LOHAFEX) took place in different sectors of the SO involving multinational and trans-disciplinary efforts. Besides, two naturally fertilized studies (KEOPS and CROZEX) were carried out in the downstream of the SO islands. This article summarizes the significant findings of all the iron-enrichment experiments carried out in the SO and explains the phytoplankton bloom dynamics as observed by satellite data and recapitulates the possible sources of iron entrainment to the water column. Findings from the earlier artificial ocean fertilization experiments revealed strong influence of iron on phytoplankton biomass, community composition, and export production in the SO. Satellite-derived chlorophyll-a concentrations (2002–2016) are utilized to characterize the monthly evolution of phytoplankton blooms in the SO. Results suggest that the areal extent of the bloom varied from 1.1 to 18.1 million km2 during July (austral winter) and January (austral summer), respectively. The blooms are pronounced in a conducive environment with the optimal light condition, sedimentary source of iron from shallow bathymetric region (< 1 km), continental dust advection, and supply of iron from the marginal ice zone through sea-ice melting. In toto, the SO contributes up to 60% of global ocean phytoplankton blooms during December and January (austral summer), and the dominant region of bloom occurrence is located in the Atlantic sector of the SO, which could be ascribed to iron-rich dust input from Patagonia and regional physical processes.