Environmental Context. Trace metals play an essential role in the growth and function of aquatic organisms; however, enhanced metal concentrations can have detrimental effects on ecosystems. The distribution of metals between filterable and filter-retained phases is key to their bioavailability and biogeochemical cycling. Studies on rivers have shown that separating dissolved metals from particle-associated metals may introduce significant artefacts in the resulting filtrate concentrations. Comparable studies on turbid estuarine waters are necessary to improve the design of water sampling programmes.

Abstract. Total filterable and colloidal metals were measured in the Tamar Estuary. The colloidal fraction (M_c) was highly variable for each metal, both spatially and temporally. In April 2001, Al_c and Co_c were higher in the river than the estuary, Cd_c, Cr_c and Pb_c were lower, and Mn_c and U_c were similar. In September 2001, Cd_c was higher in the river than the estuary. Colloidal Al and Mn were positively correlated with concentrations of suspended particles, whereas the colloid contents of the other metals were primarily determined by additional factors. The effect of the colloidal metal content on resultant metal filtrate concentrations, as a function of sample volume filtered, was examined. The amount of Al, Cd, Cu, Pb, and U in the filtrate decreased significantly in at least one case, whereas Mn initially increased in some cases. There was no change for Co, Cr, and Zn. The implications of these results for the study of trace metals in moderately turbid estuaries are discussed.