Marine benthic communities function within the constraints of
changing environmental conditions. To better understand the
dynamics of this relationship, increased knowledge of
physiological mechanisms that link environmental conditions to
ecological responses is imperative. This thesis considers the
effects of increased sedimentation, hypoxia, food availability,
and habitat characteristics on bivalve physiology using two
temperate species, Austrovenus stutchburyi and Paphies
australis, and two Antarctic species, Adamussium colbecki and
Laternula elliptica. Methods for quantification of nucleic acid
ratios (RNA:DNA ratio, RNA:protein ratio, or RNA concentration
as a measure of protein synthesis capacity) were developed and
compared with other indicators of bivalve fitness, such as
conditions indices and scope for growth. Bivalve nucleic acid
ratios were analysed over several temporal and spatial scales in
a combination of laboratory experiments, field experiments,
transplants and surveys. The magnitude of response in RNA
concentration (RNA) varied seasonally, along environmental
gradients, and with bivalve species and size class. The major
factor affecting RNA was enhanced food availability, which
appeared to have the potential to mask any negative effects of
stressors. Over the short-term (10-14 d), elevated suspended
sediment concentrations or single deposition of terrigenous clay
did not affect RNA, but over the longer term (months),
transplantation of bivalves to turbid sites and repeated
deposition of clay had a slight negative effect on RNA. In the
laboratory, hypoxia did not affect the fast increase in RNA in
response to added food or the slow decrease in RNA in response
to starvation. In addition, a species-specific baseline RNA
concentration was indicated, possibly representing the basal
metabolism of a species. Thus, RNA was more sensitive to
factors facilitating growth than to factors inhibiting growth,
indicating that RNA could be a good indicator of factors related
to energy acquisition, while not being as sensitive to stressors.
By assessing the variability in response across time and space
scales, and considering the context and time scale of both the
stressor and the response, organism-level measures may be
used as part of an integrated approach in impact assessment
and ecological experiments.