Atmospheric forcing on the annual iron cycle in the western Mediterranean Sea: A 1-year survey

A 1-year survey of simultaneous measurements of total atmospheric deposition and dissolved iron concentrations in surface waters (0-40 m) was performed in the northwestern Mediterranean Sea, an area with a marked seasonal hydrological regime. The total atmospheric iron flux was 1118 mg m(-2) (i.e., 20.4 mmol m(-2)). By using aluminium as a crustal marker the deposition was mainly attributed to Saharan dust deposition. Dissolved iron flux was estimated to be 42 mmol m(-2) yr(-1), of which 44% was anthropogenic in origin and 56% was of Saharan origin. Dissolved iron profiles revealed four typical situations throughout the year: (1) a winter situation with homogenous dissolved iron concentrations ranging from 0.8 to 0.9 nmol L-1, (2) a spring situation with uniformly low concentrations ranging from 0.2 to 0.5 nmol L-1, (3) a summer situation with enriched surface waters up to 1.2 nmol L-1, and (4) an autumnal situation with homogenous concentrations ranging from 0.9 to 1.1 nmol L-1. The results demonstrate that the iron enrichment in the mixed layer observed during the stratified period was of the same order of magnitude as the cumulative atmospheric inputs for the same period. The seasonal variability of dissolved iron (DFe) concentrations in surface waters was driven by a combination of factors, including aeolian Fe deposition, nature of aerosols, vertical mixing, phytoplankton uptake, and particle scavenging. Iron distribution can have a clear biogeochemical effect on the autotrophic communities: The low Fe: P ratio observed during the bloom indicates a possible iron limitation for phytoplankton, and the dissolved iron enrichment during summer is certainly at the origin of the development of diazotrophs populations in the system.