Sediment Geochronologies for Fish Farm Contaminants in Lime Kiln Bay, Bay of Fundy

Abstract

Sedimentation rates were measured on gravity cores collected near finfish cages in Lime Kiln Bay, N.B. in the Western Bay of Fundy using the radionuclide tracers ²¹⁰Pb and ¹³⁷Cs. Sediment cores collected close to aquaculture sites that have undergone extensive salmon fish farm activity over the past 20 years exhibit elevated levels of Zn and Cu in the upper 50 cm of the cores. Sediment geochronologies indicate that the threshold horizons for elevated Zn and Cu levels conform to the initial introduction of fish farms into Lime Kiln Bay in 1981. The source of the Zn is the fish feed while Cu is associated with chemical agents used to reduce fouling of the cages by marine biota. The highest contaminant levels for Zn (>250 μg g⁻¹) and Cu (>70 μg g⁻¹) were measured in sediment cores collected within the “footprint” of previously abandoned sites. The contaminant signals decrease with increasing distance away from the cages to values approaching background levels at distances greater than 200 m from the original cage locations. Zn and Cu concentrations have remained elevated in sediments for the five-year period between the removal of the cages and the date of core collection, suggesting that remobilization of these metals from sediments following the termination of aquaculture operations may be minimal. P is present at elevated levels in sediments close to aquaculture sites because it is an important constituent of marine organic material associated with fish feed pellets and fish farm wastes. Elevated Cd, Mo and U levels were also observed in sediments deposited under salmon cages during past periods of aquaculture operations. These elements are soluble in seawater, but can be authigenically precipitated in sediments under reducing conditions. In the present study they have been used as indirect tracers of fish farm activities owing to their transfer from seawater to sediments under the anoxic sediment conditions generated by the high sediment flux of reactive organic material from aquaculture operations.