MEPS 192:181-193 (2000)  -  doi:10.3354/meps192181

ABSTRACT: The objective of this study was to determine the influence of a selective feeding strategy on the assimilation efficiency of ¹⁰⁹Cd (¹⁰⁹Cd-AE) by the blue mussel Mytilus trossulus. Two complementary experiments which used 5 seston matrices of different seston quality (SQ) were implemented: (1) algae labeled with ¹⁰⁹Cd was mixed with unlabeled silt, and (2) labeled silt was mixed with unlabeled algae. ¹⁰⁹Cd-AE was determined by a dual-tracer ratio (¹⁰⁹Cd/²⁴¹Am) method (DTR) and based on the ingestion rate of ¹⁰⁹Cd by the mussel (IRM) (total amount of ¹⁰⁹Cd ingested over the 4 h feeding period). As a result of the non-conservative behavior of ²⁴¹Am, the DTR underestimated mussel ¹⁰⁹Cd-AEs as compared to the IRM. Therefore only IRM-determined ¹⁰⁹Cd-AE was considered further. When only algae was spiked, ¹⁰⁹Cd-AEs were proportional to diet quality (DQ), (r = 0.98; p <\ 0.05) with maximum ¹⁰⁹Cd-AE occurring at the mussel's filter-feeding 'optimum' and where maximum carbon assimilation rates have been observed. However, for the spiked-silt exposures, ¹⁰⁹Cd-AE was independent of DQ, with maximum values of ~85% occurring in all diets except for silt alone. ¹⁰⁹Cd-AE for the silt-only exposure was 36%, suggesting that digestive processes which occur in diets of both algae and silt were not operating as effectively in the silt-only exposures.¹⁰⁹Cd-AE correlated with ¹⁰⁹Cd in mussel tissue (r = 0.63; p <\ 0.05), with the radiotracer assimilated from the silt-labeled matrices corresponding to the greatest amounts of ¹⁰⁹Cd activity within the mussel. These results suggest an active and passive assimilation of ¹⁰⁹Cd from the algae and silt components of seston respectively. Active ¹⁰⁹Cd-AE will be proportional to DQ with maximum assimilation possibly occurring at the mussel's filter-feeding optimum. Passive ¹⁰⁹Cd-AE will be dependent on amounts of metal associated with the inorganic component of seston, with digestive processes that are activated in the presence of algae concurrently desorbing inorganic cadmium. Although both components of the diet will be important for determining amounts of Cd that can be potentially assimilated from seston by filter-feeding organisms, the contribution from the inorganic component of seston will likely overwhelm that from the organic fraction. Therefore, predictive models of metal accumulation by seston-ingesting organisms need to consider the role of both seston components in contributing to amounts of metal ultimately assimilated by the organism.

KEY WORDS: Mytilus trossulus · ¹⁰⁹Cd-AE · Inorganic seston · Organic seston