Bioavailability of metals in sediments of the Dogger Bank (central North Sea): A mesocosm study

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 controv ersy, a mesocosm approach was evaluated to test whether metal loadings in s ediments from the Dogger Bank region display enhanced bioavailability, rela tive to reference sites off south-west England. This involved the combinati on of physicochemical characterization of sediments (including porewaters) with bioaccumulation studies, using sediment cores seeded with benthic orga nisms (bivalves Spisula solida and Venus striatula, the gastropod Turritell a communis and the polychaete Melinna palmata). There was little evidence of As, Cu, Hg or Pb bioaccumulation from Dogger c ores. In contrast, all species accumulated Cd; Ag concentrations rose by up to fourfold in most bioindicators; and Ni, Cr and Mn burdens also increase d, occasionally by as much as 10-fold. Variable, but generally smaller incr eases in Fe and Zn were observed. Physiological variations in metal bioaccu mulation processes, including the ability to regulate essential elements, w ere responsible for species differences in response-a feature which may con tribute to uncertainty in the interpretation and comparison of biomonitorin g data. Mesocosm results nevertheless complement earlier field reports of u nexpectedly enriched levels of certain metals (notably Cd) in biota from th is part of the central North Sea. Characterization of sediments provided some physicochemical explanations fo r enhanced metal uptake in biota and helped, partly, to define bioavailable and anthropogenic fractions. Thus, whilst total sediment-metal concentrati ons were not exceptional in Dogger samples, for some metals there was a sig nificant 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 (<100 mu m) in this part of the central North Sea, in terms of anthropogenic influen ce. This was reflected in uniform bioaccumulation patterns, between sites, making it impossible to establish whether porewaters or sediment extracts w ere the better surrogates of bioavailable metal. However, both measures app ear complementary, and invoke the greater lability of metals in Dogger sedi ments as the explanation for incidences of increased metal bioaccumulation. The combination of mesocosm exposure of biota and geochemical characteriza tion of sediments, is not a substitute for large-scale survey but is sugges ted as a cost-effective means of investigating unusual episodes of metal up take, particularly in areas where intensive field programmes are impractica l. The approach may also be useful for screening the biological impact of s ediments and wastes destined for disposal in the marine environment. (C) 19 99 Academic Press.