Bleaching of an intertidal coralline alga: untangling the effects of light, temperature, and desiccation

Intertidal macroalgae must tolerate temperature, light, and desiccation stresses when the tide recedes, and differences in physiological tolerance to these environmental stresses contribute directly to zonation patterns and community structure along the shore. When low tides occur on particularly hot sunny days, seaweeds may sustain physiological damage, lose pigment, and ‘bleach.’ Because bleaching events often occur when temperature, light, and desiccation stresses coincide, their precise cause is not understood. We conducted fully factorial laboratory manipulations to explore the individual and interactive effects of temperature, light, and desiccation on acute pigment loss in the intertidal coralline Calliarthron tuberculosum (Postels & Ruprecht) E. Y. Dawson. Findings suggest that desiccation is the most significant contributor to bleaching; desiccating fronds even bleached in the dark at 15°C. Susceptibility to desiccation may explain why mid-intertidal C. tuberculosum fronds are rarely found outside tidepools. Light and temperature had only marginal effects on pigment loss, although stresses interacted with increasing significance through time. When combined, temperature, light, and desiccation stresses were capable of inducing, on average, 50% pigment loss in C. tuberculosum within 24 min of emersion. These physiological data could be used in conjunction with environmental datasets to generate ‘ecomechanical’ models to predict future bleaching events and their ecological consequences under hypothetical climate change scenarios.