Environmental and morphological influences on thallus temperature and desiccation of the intertidal alga Mastocarpus papillatus Kützing

The upper limits of vertical zonation for intertidal macroalgae are largely determined by the algae's ability to withstand temperature and desiccation stress when exposed to air at low tide. However, very little is known about the processes that determine thallus temperature and desiccation rates in the field. This study provides a quantitative analysis of the effects of the physical environment (i.e. sun, wind, air vapor density) on the temperature and rate of desiccation of the intertidal macroalga Mastocarpus papillatus Kützing. Wind tunnel experiments measure directly the influence of single physical factors on thallus temperature and desiccation, and demonstrate how thallus temperature and thallus hydration are coupled. Under most physical conditions, fully hydrated thalli readily evaporate water and are consequently below air temperature. Desiccated thalli, however, have no water for evaporative cooling and can greatly exceed air temperature, particularly when wind velocity is <0.4 m · s−1. In addition, the effects of thallus morphology on thallus temperature and desiccation are examined. Increasing thallus thickness decreases the rate of desiccation, while increasing the degree of thallus dissection (i.e. branching) increases both the rate of desiccation and the rate of heat dissipation. Thus, while thallus temperature and desiccation are ultimately controlled by the physical environment, they can be influenced by thallus morphology.