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We conducted field investigations in a canal connected to Lake Kasumigaura (Japan) twice weekly during a summer algal bloom to understand the temporal variations in dissolved iron and its speciation, and the controlling factors. Dissolved iron (D-Fe), which is operationally defined as that passing through a 0.2-μm pore filter, acid-labile particulate iron (LP-Fe), and chemical speciation of dissolved iron were analyzed. LP-Fe was extracted from the suspended solids (SS) on 0.2-μm pore-size filters after a 2 h soak in 25% (4.5 M) acetic acid. The chemical speciation of dissolved iron was determined using cathodic stripping voltammetry. The concentration of D-Fe ranged from 72 to 1354 nM and that of filterable labile iron (FL-Fe) from 5 to 881 nM, and both increased markedly after two heavy rains. The increased D-Fe and FL-Fe appeared to be removed within a few days. The concentrations of LP-Fe ranged from 4.1 to 16.1 μM, were much higher than those of D-Fe, and did not increase after the rains. There was a significant correlation (|r| = 0.88, P < 0.01) between ln[D-Fe] and pH. This suggests that the dynamics of D-Fe in the canal depend upon the cycle of adsorptive removal onto the surface of SS at high pH and desorptive supply from the surface of SS at low pH. Moreover, D-Fe concentration would also be controlled by the formation of iron oxyhydroxides amorphous particles at high pH and dissolution at low pH. The percentage of organically complexed iron ranged from 35% to 99% and varied markedly at time scales of days to weeks. There was a significant correlation (|r| = 0.60, P < 0.01) between the percentage of organically complexed iron and the log of the cell density of Microcystis spp.. This suggests that the growth of specific algal species affects iron speciation in the canal.