Assimilation efficlencies (AEs) of trace elements (Ag, Cd, CO, Se and Zn) in a marine deposit-feeding polychaete, Nerels succinea, from ingested sedinlents were measured using a pulsechase radiotracer feeding technique. Radiolabeled sediments were encapsulated and fed to the worms for 1 h, after which the worms were allowed to depurate their ingested materials for 3 d. The ranges of AEs were 1 2 to 36% for Ag, 5 to 44 % for Cd, 35 to 96% for Co. 29 to 60% for Se and 21 to 59% for Zn. Trace metal assimilation was little affected by sediment source and sediment grain size. Metals (Ag, Cd, CO and Zn) associated with anoxic sedirnents were assimilated with a significantly lower efficiency than metals from oxic sediments. The AE of Cd decreased with the duration of sediment radiolabeling; AEs of Ag. CO, Se and Zn were weakly affected by sediment aging. Metal uptake in worms from the dissolved phase was proportional to metal concentration in the dissolved phase, although there was some evidence of Cd and Zn regulation in response to an increase in ambient concentrations. Uptake rate constants were highest for Ag, followed by Zn > CO > Cd > Se. By incorporating metal influx from both the dissolved and particulate (sediment) phases, a bioenergetic-based kinetic model indicates that most (>98%) of the Cd. CO, Se and Zn in polychaetes arises from sediment ingestion due to the high ingestion rates of these animals and the low uptake rate of metals from the dissolved phase (porewater or overlying water). For Ag, approximately 5 to 35% is due to uptake from the dissolved phase. Our study suggests that the establishment of sediment quality critena must consider sediment as a potentially important source for metal uptake in benthic invertebrates.
We measured the assimilation efficiencies (AEs) from
various types of sediments and the uptake rate constants
from the dissolved phase of inorganic mercury (Hg(II))
and methylmercury (CH3Hg(II)) in the marine deposit-feeding
polychaete Nereis
succinea. AEs of Hg(II) ranged
between 7 and 30% and were unaffected by sediment
composition, whereas AEs of CH3Hg(II) ranged between
43 and 83% and were strongly affected by sediment
composition. Sediment grain size had no apparent effect
on Hg(II) and CH3Hg(II) assimilation. AEs for Hg(II)
associated with anoxic sediment were slightly lower than
with oxic sediment, whereas CH3Hg(II) displayed comparable
AEs for both oxic and anoxic sediment. Dissolved uptake
rate constants of CH3Hg(II) were 2.2 times those of
Hg(II). A bioenergetic-based kinetic model was used to
separate the pathways (solute vs sediment) and sources
[Hg(II) vs CH3Hg(II)] of Hg accumulation in N. succinea. The
model predicted that, under conditions typical of coastal
sediment environments, CH3Hg(II) accumulation contributes
about 5−17% of total Hg accumulation in polychaetes.
Most of the Hg(II) (>70%) accumulation is predicted to
derive from sediment ingestion, whereas for CH3Hg(II) the
relative importance of dissolved vs sediment ingestion
depends greatly on its partition coefficient for sediments.
Uptake from the dissolved phase and sediment ingestion
can be equally important for CH3Hg(II) accumulation in N.
succinea.
A f~rst order bioenergetic-based kinetic model has been developed to describe metal accumulation in marine copepods. To field-test predictions of the kinetic model regarding metal concentrations in marine zooplankton, we analyzed Cd, Zn, Ag. CO, and Se in water, phytoplankton, and zooplankton samples collected off the coast of Monaco. Mean concentrations in copepods were 1.3 nmol g.' for Ag, 22.2 nmol g-' for Cd, 9.5 nmol g-' for CO, 16.0 nmol g-' for Se, and 2570 nmol g.' for Zn; mean dissolved metal concentrations were 10.5 pM Ag, 72.3 pM Cd, 260 pM CO, and 4270 pM Zn. For each metal, model-predicted concentrations in zooplankton using kinetic parameters for influx rates from the dissolved phase, assimilation from ingested phytoplankton, and efflux rates measured in laboratory experiments generally matched field measurements, although discrepancies were noted, especially for Cd where a 4-fold difference between predicted and measured concentrations was observed. Overall, it appears that it is possible to account for the major processes governing metal accumulation from food and water in marine copepod populations.
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