LINKING THE SEDIMENT GEOCHEMISTRY OF AN INTERTIDAL REGION TO METAL BIOAVAILABILITY IN THE DEPOSIT FEEDER MACOMA-BALTHICA

Surficial sediment and biota samples were collected from 26 locations in the intertidal region of the Eraser River estuary (British Columbia , Canada). Sediment samples were collected in May and July of 1995. Be nthic samples of Macoma balthica, a deposit feeding bivalve, were coll ected from each site in July. Sediments were characterized by a simult aneous extraction procedure that characterized the sediment matrix int o oxides of manganese and iron (easily reducible Mn, ER Mn, and reduci ble Fe, RED Fe) and organic matter and separated the trace metals, cad mium, copper, lead, nickel and zinc among these 3 'biologically releva nt' sediment components, i.e. metals associated with ER Mn, RED Fe and organic matter. Total mercury was also determined in each sediment sa mple. Bivalve samples were separated into shell and tissue and analyse d for the same metals. An R-2 MAX procedure was applied to determine i f the concentration of trace metals in the shell and tissue of M. balt hica was related to sediment geochemistry using sediment geochemistry and bivalve metal concentrations as the independent variable and depen dent variable respectively. The partitioning of trace metals was depen dent on the location-specific geochemistry (i.e. amounts of ER Mn, RED Fe and organic matter recovered at each location). This in turn led t o differences in metal uptake by M. balthica that were related to sedi ment geochemistry. The relationships with tissues were highly signific ant (p less than or equal to 0.001), except for mercury (p less than o r equal to 0.05). Sediment geochemistry accounted for 31% of the varia bility for cadmium, 39% for zinc, 51% for copper and 54% for lead. Rel ationships were not as strong for shells; sediment geochemistry explai ned 12 % of the variation for mercury, 15 % for zinc, 21 % for nickel and 43 % for copper. Overall, metal levels in the tissue and shell of M. balthica were best related to the concentration of metal associated with the ER Mn component of sediment. The sediment geochemical matrix as defined by ER Mn, RED Fe and organic matter and the partitioning o f trace metals among the 3 fractions was highly site-specific. This in turn led to site-specific patterns in metal bioavailability to M, bal thica. The spatial heterogeneity that occurs within an estuary, on the scale of metres, must be considered when assessing the impact of meta ls on such regions.