Fluctuating seawater pCO₂/pH induces opposing interactions with copper toxicity for two intertidal invertebrates

Global ocean pCO₂ is increasing as a result of anthropogenic CO₂ emissions, driving a decline in seawater pH. However, coastal waters already undergo fluctuations in pCO₂/pH conditions over far shorter timescales, with values regularly exceeding those predicted for the open ocean by the year 2100. The speciation of copper, and therefore its potential toxicity, is affected by changing seawater pH, yet little is known concerning how present-day natural fluctuations in seawater pH affect copper toxicity to marine biota. Here, we test the hypothesis that a fluctuating seawater pCO₂/pH regime will alter the responses of the mussel Mytilus edulis and the ragworm Alitta virens to sub-lethal copper, compared to a static seawater pCO₂/pH scenario. Mussels and worms were exposed to 0.1 and 0.25 μM copper respectively, concentrations determined to produce comparable toxicity responses in these species, for two weeks under a fluctuating 12-hour pCO₂/pH cycle (pH 8.14-7.53, pCO₂ 445-1747 μatm) or a static pH 8.14 (pCO₂ 432 μatm) treatment. Mussels underwent a haemolymph acidosis of 0.1-0.2 pH units in the fluctuating treatments, alongside two-fold increases in the superoxide dismutase activity and DNA damage induced by copper, compared to those induced by copper under static pH conditions. Conversely, ragworms experienced an alkalosis of 0.3 pH units under fluctuating pH/pCO₂, driven by a two-fold increase in coelomic fluid bicarbonate. This mitigated the copper-induced oxidative stress to slightly reduce both antioxidant activity and DNA damage, relative to the static pH + copper treatment. These opposing responses suggest that differences in species acid-base physiology were more important in determining toxicity responses than the pH-induced speciation change. With variability in seawater chemistry predicted to increase as climate change progresses, understanding how fluctuating conditions interact with the toxicity of pH-sensitive contaminants will become more crucial in predicting their risk to coastal biota.