Heatwaves in the intertidal: how microhabitat, thermal unpredictability, and food availability shape physiological performance in mussels

Abstract

A central prediction of recent climate change models is that there will be an increase in thermal unpredictability and heatwave events. Sessile intertidal organisms experience a high degree of thermal unpredictability due to combined effects of solar radiation and tidal movement. How thermal unpredictability operates in microhabitats with different media, such as tidepool (aquatic) or tidally exposed (aerial) environments is poorly understood, especially when coupled with other important determinants of performance, such as food availability. Understanding how these factors interact to influence performance is essential towards understanding the effects of climate change on intertidal communities. We examined how acclimation to different levels of thermal predictability (no, predictable, unpredictable) with high and low food availability shape performance during a three-day heatwave in Mytilus californianus in two microhabitat treatments: tidepool and tidally exposed. During the heatwave, mussels were warmed to 35C each daytime low tide in their respective medium (water or air). Tissues samples were taken for biochemical analyses the day before, each day during, and the day after the heatwave. We found that microhabitat was the strongest driving factor for shaping physiological performance in Mytilus californianus. Food availability played a larger role in shaping performance in tidally exposed mussels, and the influence of thermal predictability depended on heatwave day. Our results suggest that the predicted increases in heatwave events and thermal unpredictability from climate change will not have a uniform effect in different microhabitats, which could have severe consequences for intertidal community and composition.