Water samples were collected hourly at three depths from two twenty-five hour stations in the Fraser River estuary. Sampling stations were selected to represent both minimal saline intrusion and prominent salt wedge environments. A bioassay using the diatom Thalassiosira pseudonana was run to evaluate biological availability of the dissolved metals. Inter-element ratios indicate that changes in bulk composition of suspended particulates occur throughout the tidal cycle as a result of resuspension and mixing of particulates from different sources. However, Fe:Ti ratios and leachable Fe indicate loss of about 1 wt.% Fe from river-borne particulates mixing with saline waters. This loss from particulates is accompanied by a corresponding increase of “dissolved” Fe concentrations, from 10 ppb in both fresh waters and the salt water intrusion, to more than 80 ppb in mixed waters of intermediate salinity. Co, Cu, Mn and Pb show similar trends to Fe which are attributed to their association with “dissolved” Fe colloids. This is consistent with bioassay data which suggests that the biological availability of metals changes abruptly with the change in dissolved Fe concentrations.