Abstract
Climate change is an ever-growing global threat that affects various ecosystems
worldwide. One of the most vulnerable systems to this threat is the rocky intertidal
system. Rocky shores are highly dynamic and biodiverse environments, providing
habitats for numerous species that play critical roles in marine ecosystems. The
ongoing increase in greenhouse gas emissions combined with other human-induced
activities has resulted in numerous environmental stressors that pose significant
challenges to the survival of these species. The aim of the study was to determine the
effects of multiple stressors such as increased chemical and thermal stressors on an
indigenous pulmonate limpet, Siphonaria capensis. Cadmium (Cd) was selected as
an appropriate chemical stressor because it occurs naturally in low concentrations in
the oceans (ions in solution) but is also introduced through anthropogenic activities
such as mining and industrial discharge, such as that experienced in South Africa. Cd
can accumulate in various tissues of these organisms, when exposed to Cd ions in
solution, and can induce oxidative stress, damage DNA, and interfere with ion
regulation, ultimately affecting their growth, reproduction, and survival. The study
comprises integrated chapters that have different outcomes, the first of which was to
determine sublethal Arrhenius Breakpoint Temperatures (ABT) and lethal Flatline
Temperatures (FLT) thermal limits in S. capensis from three marine protected areas
(MPAs) along the southern coast at the Garden Route National Park (GRNP) and west
coast at Namaqua National Park (NNP) and the West Coast National Park (WCNP) of
South Africa, as a means of testing the effect of environmental conditions on the
pulmonate limpet with minimal anthropogenic influence. Limited variation in sublethal
(GRNP – 41.89ºC, NNP – 39.09ºC, and WCNP – 40.33ºC) and lethal thermal (GRNP
– 47.45ºC, NNP – 45.45ºC, and WCNP – 45.27ºC) limits were found between
populations of S. capensis along the warmer southern and cooler west coasts,
probably as a result of the wide mixing and lack of genetic variations in populations
due to their planktonic dispersal stage. Environmentally relevant concentrations
(ERCs) of cadmium were determined from two populations of field-fresh S. capensis
and water samples from the southern (0.240 μg ℓ-1) and west coasts (0.139 μg ℓ-1) prior
to running laboratory-controlled bioaccumulation studies on populations collected
along the southern coast at the GRNP and the west coast at NNP. Two, 96-hr,
sublethal exposures to cadmium chloride (CdCl2) were performed using two exposure
groups: ERC +10% and ERC +20%, both determined from collected field-fresh
samples. Following directly on from the acute sublethal exposure to CdCl2, limpets
from both coastal populations were exposed to one of five laboratory-controlled, acute
(2.5-hr) thermal treatments (24, 29, 34, 39, and 44ºC). Throughout the acute thermal
exposures, heart rates of individuals (n=10 per concentration) were measured to
investigate the physiological response to the concomitant exposure of varying
concentrations of CdCl2 and different thermal treatments. The bioaccumulation of Cd
in S. capensis varied between populations, with limpets from the southern coast
exhibiting greater bioaccumulation of Cd (mean ± SD, 6.68 ± 0.249 μg g-1) in
comparison to those from the west coast (mean ± SD, 5.28 ± 0.263 μg g-1). The results
from the bioaccumulation chapter show how local exposure and acclimation affect the
populations and may play an important role in their ability or lack thereof to mitigate
Cd uptake during acute chemical and thermal exposures. Limpets from the cooler west
coast (NNP) were more susceptible to Cd uptake with increased thermal stress,
measured by cardiac performance (mean heart rates). Limpets from the warmer
southern coast (GRNP) were however more tolerant to increases in thermal exposure
and therefore have a greater capacity of reducing and preventing Cd uptake through
reduced metabolic processes. Limpets from the warmer southern coast (GRNP)
exhibited greater cardiac performance with increased thermal treatments compared to
the cooler west coast (NNP) population. Local acclimation and phenotypic plasticity in
these populations have therefore led to varying cardiac responses to acute chemical
and thermal treatments. Relative expression of heat shock protein 70 (Hsp70) was
used to determine discrete alterations to homeostatic levels in S. capensis following
acute exposure to ERCs of Cd and varying thermal treatments. Individuals from the
warmer southern coast (GRNP) expressed overall lower Hsp70 levels when compared
to those from the cooler west coast (NNP) after acute CdCl2 and thermal exposures.
Given their biogeographic range differences and local acclimations, these two
populations are likely to show marked differences in their responses to predicted
climate change scenarios. Lastly, biomarker responses were used to determine
biochemical alterations between populations of S. capensis. Using univariate and
multivariate statistical analysis, discrete alterations were observed between
populations in response to Cd and thermal exposures. Combined exposure to ERCs
of Cd and varying thermal treatments resulted in oxidative stress (GSH and MDA),
metabolic (CS), and energetic (CEA and Ea) biomarkers all being strong drivers in the
southern coast population. Energetic (CEA and Ec) and oxidative stress (SOD and
CAT) biomarkers were all strong drivers in the biochemical response of organisms
from the west coast population when using the integrated biomarker response index
(IBRv2). Understanding the impacts of cadmium exposure and climate change on
intertidal organisms can provide valuable insights for predicting and preparing for
future climate change scenarios.