Feeding response and carbon assimilation by the blue mussel Mytilus trossulus exposed to environmentally relevant seston matrices

The feeding response of the blue mussel Mytilus trossulus exposed to environmentally relevant seston matrices were studied in the laboratory under flow-through conditions. Twelve seston matrices varying in quality and quantity were prepared by mixing 3 microalga Thalassiosira pseudonana (5, 20 and 150 x 10(6) cells l(-1)) and 4 silt concentrations (0, 5, 20 and 50 mg l(-1)) to represent an increasing seston organic quality of 10 to 71% and an increasing seston quantity of 1.4 to 56.6 mg l(-1). Clearance rates (CRs), pseudofaeces (PF) production, sorting efficiency (SEF), ingestion rates (IRs) of particulate organic matter (POM) and particulate inorganic matter (PIM) and apparent and true carbon assimilation efficiencies (C-AE%) were determined for the various exposure regimes. Under conditions of 0 and 5 mg l(-1) silt, CRs decreased by 3-fold [14.4 to 4.9 l h(-1) g(-1) dry wt (gdw(-1))] and 6-fold (18.0 to 3.3 l h(-1) gdw(-1)) respectively, with increasing seston quality. Under conditions of high silt loads (20 and 50 mg l(-1)) CRs were independent of increasing seston quality with maximum CRs (21.8 +/- 2.2 l h(-1) gdw(-1)) observed at the 20 x 10(6) cells l(-1) and 20 mg l(-1) silt exposures. PF production was dependent on seston quantity (r(2) = 0.63; p < 0.05) with mussels preferentially rejecting the inorganic versus the organic component of the seston. This SEF was optimized at a seston quality of 40 % organic matter. As a consequence of this selective feeding strategy, determined POM IRs under high algae/high silt (high quantity/quality) exposure regimes were comparable to those of the high quality Oust algae) exposures (POM IRs of 48.0 as compared to 38.1 and 91.3 mg h(-1) gdw(-1) for high quality and high quantity/quality respectively). In contrast, mussels exposed to low quality/quantity seston ingested both seston components (SEF <24%), i.e. the mussel was non-selective, possibly ingesting both seston components to meet nutrient requirements. Apparent C-AE% expressed as C-seston - C-faeces/C-seston correlated with seston quality (% POM) (r(2) = 0.64, p < 0.05). However, carbon assimilation expressed as a true C-AE%, C-diet - C-faeces/C-diet, was independent of seston quality as was carbon assimilation rate (the product of the amount of ingested organic matter and carbon assimilation efficiency). Hence, through a dynamic interplay between CRs and carbon assimilation efficiency, the blue mussel was able to maintain a constant rate of carbon assimilation, regardless of the quality and quantity of seston to which it was exposed. This compensatory feeding strategy displayed by the blue mussel has important implications for predictive models which relate contaminant uptake to a diet/energy pathway. Contaminant uptake may be either underestimated or overestimated if the ability of the blue mussel to optimize nutrient gain under a variety of seston compositions has not been taken into account.