Dissolved iron in the Cretan Sea (eastern Mediterranean)

Abstract

Dissolved iron (Fe) distributions in the water column of the Cretan Sea show consistent patterns of elevated surface mixed layer concentrations (mean values of 1.44 and 1.95 nmol L 21 in March and September, respectively) that rapidly decrease with depth to low and relatively uniform concentrations in deep waters (circa 0.5 nmol L 21 ). These features are consistent with inputs of dissolved Fe to surface waters and with rapid scavenging to low values in deep waters. The main source of surface dissolved Fe is probably Saharan atmospheric dust, the supply of which is variable depending on sporadic dust storm events and transport to the Cretan Sea. The short residence times estimated for dissolved Fe in the mixed layer (0.7‐2 yr) emphasize the rapid removal of this element from surface waters and are consistent with recent mass balance calculations for Fe in the western Mediterranean Sea, in which rapid removal of Fe to sediments is needed to provide a balance in the budget. The deep-water concentrations are strikingly constant and similar to or slightly lower than those reported for the open ocean. The high surface concentrations we observed would be ample to support substantial primary production, and it is likely that the macronutrients, and phosphorus in particular, limit production in this region. Iron (Fe) is an essential nutrient for phytoplankton, bacteria, and other marine microorganisms, and the role of this metal as a potentially limiting factor for phytoplankton has come under increasing scrutiny since the initial ‘‘iron hypothesis’’ was put forward by Martin et al. (1988) to explain limited productivity in high-nutrient low-chlorophyll (HNLC) oceanic systems. Experiments such as IRONEX and SOIREE have now clearly demonstrated that Fe additions can stimulate phytoplankton growth in HNLC zones (Watson 2001), although many questions still remain about the forms of Fe present, uptake mechanisms, and the impact of Fe on export carbon production. The distributions, sources, and sinks of Fe in seawater have recently been reviewed (De Baar and De Jong 2001). There is a coherent trend in reported data for all regions, with increasing concentrations approaching the continental margins (up to circa 100 nmol L 21 ), while in the open ocean, dissolved Fe has typically low concentrations in surface waters (0.02‐1 nmol L 21 ) and higher (0.4‐2.0 nmol L 21 ) concentrations in deep waters. The rapid removal of dissolved Fe through association with biotic and abiotic particles and its loss from the water column results in Fe having a short residence time (De Baar and De Jong 2001), and upper-ocean concentrations should largely reflect local inputs (e.g., aeolian and shallow benthic sourc1