| Summary: | Mussels of the genus Mytilus (M. edulis, M. trossulus, M. californianus) have
been proposed by numerous monitoring programs as indicators of pollution. It has been
assumed that these species exhibit similar responses to environmental changes in metals.
M. edulls, being the most abundant species, has been more thoroughly studied and often
the results for this species are generalized to the genus. M. californianus is found on the
Pacific coast in exposed rocky areas which are periodically exposed to natural inputs of
metals from upwelling.
The aim of this work was to study the natural variability of Cd and Pb due to
upwelling in M. californianus in the field and to investigate the capacity of this and a
similar species M. trossulus (this species was believed until recently to be M. edulis) to
release Cd during short periods of time in the laboratory.
Results from the field study showed high variability in the Cd and Pb
concentrations in the mussels, with concentrations varying on time scales of just a few
days. Most of the variability observed was explained by the variation in the condition
index of the mussel. After normalizing to remove the effect of the condition index, some
variability still persisted. The residual variability in the case of Cd, was explained by the
variations in the dissolved metal with upwelling. In the case of Pb, however, no
relationship with the variations in the dissolved metal in the environment was found. The
capacity of Cd release in M. californianus was striking since numerous laboratory studies
which showed that M. edulis is unable to release Cd over short periods of time.
To understand the reason for this discrepancy, several laboratory experiments
were designed to test the effect of exposure time on the mussel Cd release capacity, the
effect of 2 levels of dissolved Cd (1 µg L⁻¹ and 10 µgL⁻¹) on the release capacity (after 1
day of exposure) of the two species M. cal(fornianus and M. trossulus and the effect of 3
levels of Cd on the filtration rate of M. ca4fornianus. Results from these experiments
showed that M. trossulus can take up Cd (at 10 µgL⁻¹ exposure), at all the exposure
times tested (1, 2, 7 and 14 days), increasing significantly its concentrations, even at the
shortest period tested (1 day). No release was noticed after 7 days in seawater at
background levels after any of the exposure times tested. The level of exposure had a
different effect on M. trossulus than on M. cal(fornianus. M. trossulus increased its
concentrations at 10 µgL⁻¹ Cd exposure and did not decrease them after 5 days in
seawater at background levels. At the low level of exposure (1 µgL⁻¹Cd), however, M.
trossulus did not show any significant uptake. M. californianus, on the other hand,
increased its concentrations significantly at both levels of exposure. After 1 day in clean
seawater, the mussel decreased its concentrations to the control level. At the higher level
of exposure, Cd concentrations then increased again in the subsequent days and did not
return to the control levels even after 7 days in seawater at background levels. The
lowest Cd exposure experiment reproduced the behavior observed in the field in M.
californianus during upwelllng. The lowest level of Cd tested is only 10 times the
maximum Cd concentrations that can be expected in upwelling. These experiments
showed the importance of the laboratory design (exposure levels similar to natural) to
reproduce the behavior of the mussels in the field. The filtration rate of M. californianus
was not affected by the different Cd concentrations tested (0.5, 2 and 10 µgL⁻¹).
The results obtained are critical for monitoring studies and for a good assessment
of the data obtained from these species since the two species exhibited different
capacities to take up and release Cd. The high capacity of M. californianus to release Cd
casts serious doubts about its time-integrating ability and usefulness as a long term metal
bioindicator.
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