The distribution of colloidal and particulate bioactive metals in Narragansett Bay, RI

The bioactive metals Fe, Mn, Cu, Zn and Ni in Narragansett Bay, RI, were partitioned into soluble, colloidal and particulate size fractions using a combination of conventional and cross-flow filtrations. Particulate samples (0.2–8.0 μm; >8 μm) were chemically fractionated into acetic-acid reactive and non-reactive metals. Conventional “dissolved” samples (<0.2 μm) were separated into soluble (<1 kDa) as well as smaller (1–8 kDa) and larger (8 kDa–0.2μm) colloidal size classes. The particulate phase comprised 2%–98% of the total metal loads decreasing in the order Fe>Mn>Zn>Cu>Ni with concentrations in the 0.2–0.8 μm fraction being generally higher than in the >8.0 μm fraction. The acid leachable fraction of the particulate phase increased from 32%–80% in the order Fe8.0 μm) being generally less labile than small particulates (0.2–8.0 μm). The colloidal phase represented an average 4%–96% of the “dissolved” metals, ranging in importance from Fe (96%)>Cu (44%)>Ni (25%)>Zn (7%)>Mn (4%). Although generally small, the colloidal fraction of Zn and Mn was highest in a region of the bay where biomass typically is high. Changes in soluble and colloidal fractions along a transect through the bay indicate that a significant proportion of Fe, Cu and Ni were transferred from “dissolved” to particulate size fractions via colloid aggregation. Predicting colloidal metal concentrations from measurements of particulate mass (Cp) and literature values of colloid metal partition coefficients (Kc) underestimated the measured concentrations by 5–50×. Acetic acid leachable metal concentrations in the small particle (0.2–8 μm) phase correlated well with metal concentrations in the larger (8 kDa–0.2 μm) colloid fraction (r=0.91–0.99). In contrast, metals in the smaller colloid fraction (1–8 kDa) were for the most part independent of any measured parameters. Metals were not distributed equally between colloidal size classes; colloidal Zn was associated with larger colloids (>90%), Fe and Ni were associated primarily with larger colloids ( 70–85%) but also with the smaller colloid fraction, while 70% of colloidal Cu was associated with smaller colloids. The non-uniform distribution of metals within colloidal size classes indicates that metal:colloid associations are regulated by specific interactions. These findings suggest that it is inappropriate to employ single, non-specific sorbing metal tracers (e.g. Th) to delimit the pathways and kinetics of bioactive metal interactions with marine colloids.