Seasonal variations of iron concentrations in the Ligurian Sea and iron budget in the Western Mediterranean Sea

Seasonal variations of three iron fractions (dissolved, total dissolvable and particulate) were studied during two cruises (EIMETO II and III, F-JGOFS program, beginning of October 1994 and beginning of May 1995) in the North-Western Mediterranean Sea (DYFAMED site). Concentrations of particulate iron varied between 0.8 and 14.5 nmol/kg, dissolved iron ranged from below detection limit (less than or equal to 0.13 nmol/kg) to 4.8 nmol/kg, and total dissolvable iron concentrations varied between 0.5 and 7.0 nmol/kg. These concentrations are coherent with previously reported ones in the Mediterranean Sea, although this is the first time that (1) such low values of particulate and dissolved iron are observed in the surface waters, and (2) a nutrient-like profile of dissolved iron is established. Seasonal variations are observed in the surface water for the three fractions. In the deep waters, concentrations are similar at both seasons. A simple calculation taking into account only the seasonal variations of hydrology and the strong atmospheric inputs gives a reasonable estimation of the accumulation of dissolved iron in the summer mixed layer. A two-box budget of dissolved and particulate iron allows us to identify the processes controlling the input and removal of this element to and from the water column. The net fluxes exchanged between dissolved and particulate iron are positive, suggesting a dominant transport of iron from the dissolved to the particulate fraction. In the surface waters, scavenging is found to be the main process of the dissolved/particulate exchange. In the deep waters, the net exchange flux is much lower (6% of the surface one), suggesting lower fluxes between dissolved and particulate fractions or/and increase of iron transport from the particulate to the dissolved fraction, which would yield a decrease of the net exchange flux. At the bottom of the surface and the deep boxes, calculated particulate iron fluxes are both equal to 3.1 +/- 0.4 mg/m(2)/day. The fluxes are coherent with the averaged measured fluxes at 200 and 1000 m, suggesting that the iron cycle is balanced in the Western Mediterranean Sea. (C) 2001 Elsevier Science B.V. All rights reserved.