On the role of colloids in trace metal solid-solution partitioning in continental shelf waters: a comparison of model results and field data

Model calculations and field data have been used to investigate the role of high molecular weight colloidal matter (10,000 NMW-0.2 μm) in the solid-solution partitioning of Cu, Pb, Cd, Zn, Ni, Hg, Fe and Mn in shelf waters. Trace metal data from the literature and estimated metal solid-solution partition coefficients are used to calculate metal partitioning between dissolved, colloidal and particulate size classes as a function of suspended particle concentration (Cp). The calculated fraction of metal associated with colloids ranges from approximately 1% to 20% of the total metal concentration forCp ∼0.01–100 mgl−1. These results are largely consistent with metal concentrations determined in the 10,000 NMW-0.2μm size range collected using crossflow filtration in shelf surface waters in the northwest Atlantic and with recent literature data. The fraction of metal associated with colloids is dependent on metal reactivity (Kd) and particle concentration, with highest colloidal values observed for more reactive metals (Pb, Fe, Hg) in particle-rich nearshore waters. The model and field results indicate that a significant fraction of “dissolved” (<0.2 μm) trace metals is associated with colloids in productive shelf waters.