Fe(III) is the thermodynamically stable form in
2
oxygenated seawater, existing predominantly as insoluble
oxy-hydroxides or colloidal matter [7±9]. Fe(II)
is a transient species in surface oxic waters, existing
via chemical or photochemical Fe(III) reduction [10±
12], or via atmospheric deposition [13±15]. At seawater
pH, Fe(II) is oxidised rapidly by O
2
[16]. Recently, organic complexation has been thought
to occur to a signi®cant extent in marine systems [17±
19]. Laboratory studies have shown that phytoplankton
are only able to utilise dissolved Fe
a,*
and H
2
species, and that uptake of colloidal or particulate Fe is