The role of environmental variability in determining physiological responses in ecologically important bivalve species

Abstract

Climate change is causing alterations to marine ecosystems with ocean acidification (OA) and warming widely considered to be some of the most pervasive anthropogenic threats to global marine biodiversity. To date the majority of climate change studies using intertidal organisms neglect the inherent natural variability experienced within coastal ecosystems and the substantial periods of emersion encountered over daily tidal cycles, thus limiting our ability to extrapolate responses of coastal organisms to near-future conditions. This project investigates how a variable intertidal environment can influence acid-base responses to future climatic conditions in the ecologically and economically important bivalve species Mytilus edulis and Perumytilus purpuratus. Fieldwork is used to provide novel and much needed data on the natural environmental conditions experienced by two populations of M. edulis and one population of P. purpuratus along with the acid-base response over 12-hour tidal cycles. This data then helps to parameterise experimental work investigating the influence of size (37 – 50 mm and 60 – 79 mm shell length), temperature (7 °C, 13 °C, 20 °C, 28 °C) and low pH exposure (pH 7.7) as drivers of acid-base change during environmentally realistic (6 hour) immersion and emersion periods. I then investigate the potential for populations inhabiting upwelling regions to have an altered acid-base response to short-term exposures to elevated pCO2 levels. Finally, I investigate if a variable pH/ pCO2 regime will incur greater physiological costs than a stable pH/ pCO2 regime over 14 days. My results add to further evidence suggesting mussels have a limited ability to regulate acid-base disturbances during both short- and medium- term exposures to elevated seawater pCO2. In addition, populations inhabiting upwelling regions showed no significant difference in acid-base response suggesting a low adaptation potential in acid-base regulatory abilities. Furthermore, my results clearly demonstrate for the first time that size (shell length) and temperature are the predominant drivers of acid-base change during emersion and pH/ pCO2 during immersion. Elevations in temperature had an increasing influence on acid-base balance, with size playing a crucial role in moderating these disturbances at increasingly higher temperatures. Perhaps most interesting, this project found a clear energetic cost of pH/ pCO2 variability over 14 days, suggesting the use of stable pH values currently used in OA studies may not reliably inform predictions of future organism responses. With seawater pH/ pCO2 variability expected to increase and intensify in addition to warming, sea level rise and other climatic changes, this work highlights the need to use environmentally realistic scenarios that reflect natural variability in order to predict future outcomes for marine biota.