Sub-lethal and lethal effects of chronic and extreme multiple stressors on a critical New Zealand bivalve under hypoxia

Abstract

Warming of coastal regions due to climate change is exerting chronic thermal stress on intertidal organisms, many of which are already living at or near their limits for thermal tolerance. In addition, increasing frequency of marine heat waves and algal blooms leading to hypoxic events in estuaries exert acute stresses on aerobic organisms. Hypoxia and warming rarely occur in isolation, and when combined, likely have synergistic detrimental effects on bivalves. Further, midday low tides have been identified as important drivers of mass mortality events for bivalves as a consequence of the combined effects of thermal stress and desiccation. Accordingly, the current study focuses on the sublethal and lethal effects of these combined stressors on the critical estuarine bivalve Austrovenus stutchburyi. Experiments were used to test the effects of simulated midday low tides combined with chronic warming and oxygen depletion on activity, respiration and survival of A. stutchburyi. A lethal emersion thermal threshold was surpassed at 33°C for treatments exposed to both hypoxic and normoxic treatments, suggesting extreme thermal events such as heatwaves during midday low tides could have substantial detrimental effects on bivalve populations and their vital ecosystem functions. Further, the ability of bivalves to survive heat and desiccation of midday low tide events was much reduced when they were exposed to even minor heat and hypoxia stress when immersed at high tide. The findings of the current study highlight important biological consequences of coastal warming combined with extreme temperature and hypoxic events observed in estuaries.