Bioavailability of Metals in Sediments of the Dogger Bank (Central North Sea): A Mesocosm Study

Abstract

There are conflicting arguments surrounding the nature and origins of metal enrichment in sediments from the Dogger Bank (central North Sea) and much speculation as to its biological significance. To help resolve this controversy, a mesocosm approach was evaluated to test whether metal loadings in sediments from the Dogger Bank region display enhanced bioavailability, relative to reference sites off south-west England. This involved the combination of physicochemical characterization of sediments (including porewaters) with bioaccumulation studies, using sediment cores seeded with benthic organisms (bivalves Spisula solida and Venus striatula, the gastropod Turritella communis and the polychaete Melinna palmata).There was little evidence of As, Cu, Hg or Pb bioaccumulation from Dogger cores. In contrast, all species accumulated Cd; Ag concentrations rose by up to fourfold in most bioindicators; and Ni, Cr and Mn burdens also increased, occasionally by as much as 10-fold. Variable, but generally smaller increases in Fe and Zn were observed. Physiological variations in metal bioaccumulation processes, including the ability to regulate essential elements, were responsible for species differences in response—a feature which may contribute to uncertainty in the interpretation and comparison of biomonitoring data. Mesocosm results nevertheless complement earlier field reports of unexpectedly enriched levels of certain metals (notably Cd) in biota from this part of the central North Sea.Characterization of sediments provided some physicochemical explanations for enhanced metal uptake in biota and helped, partly, to define bioavailable and anthropogenic fractions. Thus, whilst total sediment-metal concentrations were not exceptional in Dogger samples, for some metals there was a significant proportion in non-refractory (readily extractable) form, together with relatively high concentrations in interstitial waters—both presumably available for assimilation. Normalization of sediment metals, with respect to grain size and Fe, indicated a homogeneous population of fines (<100μm) in this part of the central North Sea, in terms of anthropogenic influence. This was reflected in uniform bioaccumulation patterns, between sites, making it impossible to establish whether porewaters or sediment extracts were the better surrogates of bioavailable metal. However, both measures appear complementary, and invoke the greater lability of metals in Dogger sediments as the explanation for incidences of increased metal bioaccumulation. The combination of mesocosm exposure of biota and geochemical characterization of sediments, is not a substitute for large-scale survey but is suggested as a cost-effective means of investigating unusual episodes of metal uptake, particularly in areas where intensive field programmes are impractical. The approach may also be useful for screening the biological impact of sediments and wastes destined for disposal in the marine environment.