ENVIRONMENTAL AND MORPHOLOGICAL INFLUENCES ON THALLUS TEMPERATURE ANDDESICCATION OF THE INTERTIDAL ALGA MASTOCARPUS-PAPILLATUS KUTZING

The upper limits of vertical zonation for intertidal macroalgae are la rgely determined by the algae's ability to withstand temperature and d esiccation stress when exposed to air at low tide. However, very littl e is known about the processes that determine thallus temperature and desiccation rates in the field. This study provides a quantitative ana lysis of the effects of the physical environment (i.e. sun, wind, air vapor density) on the temperature and rate of desiccation of the inter tidal macroalga Mastocarpus papillatus Kutzing. Wind tunnel experiment s 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, ful ly hydrated thalli readily evaporate water and are consequently below air temperature. Desiccated thalli, however, have no water for evapora tive 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. Increa sing thallus thickness decreases the rate of desiccation, while increa sing 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 p hysical environment, they can be influenced by thallus morphology.