Trace metals (Cd, Cu, Zp, Pb, Ni) were determined in different zooplankton taxa to assess their role in the biogeochemical cycle in the North Sea. Results of semi-static bioaccumulation experiments on board ship using collectives of Calanus finmarchicus/C. helgolrindicris were in agreement with a net accumulation strategy, thus meeting an inevitable precondition for their utilization as biomonitors. Cd, Cu and Zn concentrations in field samples of zooplankton displayed a pronounced variability between taxa, with Cd ranging from 0.13 mg kg-1 (d.w.) in fish larvae to 5 I mg kg-1 in hyperiid amphipods. Almost no information is available about metal requirements of copepods. However, most of our and worldwide reported Cu concentrations in calanoid copepods (6-22 mg kg-1) coincided with estimates of enzymatic requirements of decapod crustaceans. Cd levels in Calurzrrs collectives increased significantly from 0.68 mg kg-1 in the German Bight to 11 mg kg-1 at some stations around Scotland above 59" N, in line with literature data reported for benthic decapod crustaceans. within the PRISMA multidisciplinary research project. The station grid is shown in Figure 1 (A, B). More information on the cruises is available in MEINCKE and BECKER (1993). HAARlCH et a[. (1993a) and PRISMA ( I 994).Samples for the taxonomic determination and quantification of zooplankton were taken with a vertically towed WP-2 net (mesh size 200 pm, from 3 rn above bottom to the surface), whereas samples for the determination of metals in zooplankton were obtained from a ring trawl (Californian Copepod Fishing Net, CALCOFI net, mesh size 300 pm), trawled with 2.5 knots from about 30 m depth to the surface. The CALCOFI net was equipped with a collecting jar made of polypropylene, fitted to the cod end of the net. Procedures for the biological assessment of the samples were followed as reviewed in KRAUSE and RADACH (1989) and FRANSZ et al. (1 99 I).On board ship, the zooplankton catches was transferred to polyethylene buckets and kept alive in aerated seawater at 12 "C, at least for 1-3 h. We assume completion of defaecation within this period, since gut transit times in these animals are less than 0.5 h (at 18 "C, REINFELDER and FISHER, 1991) and re-ingestion of food particles is most probably prevented by the turbulence of the water. Subsequently, the samples, still alive, were sorted under a binocular microscope, shortly rinsed with demineralized water and then stored frozen at -18 "C in acid cleaned Eppendorf reaction tubes ( I .5 mi, polypropylene). Whenever possible, samples were sorted to the species level. However, when preparing zooplankton samples for metal analyses (requiring several milligrams of dry weight, respectively), complicated procedures for biological assessment are not applicable, for instance, inspection of morphological structures of each specimen taken under a microscope. In such cases, only higher taxa were noted. A total of about 440 sorted samples (referred to as zooplankton collectives) were obtained from all cruises for ...
