Desiccation facilitates wave-induced mortality of the intertidal algaFucus gardneri

Physical factors such as desiccation and water motion can influence the abundance and performance of intertidal seaweeds. We addressed the hypothesis that the intertidal seaweedFucus gardnerican be dislodged or damaged by relatively small waves if weakened by the effects of desiccation. Stress-strain curves generated for desiccated and hydrated thalli revealed that desiccated stipes were more brittle than hydrated stipes and required less total work (energy) to break. Results from a dynamic loading experiment suggest that the oscillating motion of the waves may be an important factor in breaking brittle stipes. Therefore, seemingly benign water motion may provide sufficient force to damage or dislodge desiccated thalli. In the late spring and summer in this area, waves are generally small but desiccating conditions are common as low tides fall during midday. Mortality of field populations is high during these seasons; thalli experimentally protected from desiccation suffered many fewer losses. However, desiccatedF. gardnerican rehydrate rapidly (less than 1 min), minimizing the window of time that wave forces can break stipes. This study provides an example of how 2 sub-lethal physical factors, desiccation and benign waves, can interact to cause mortality in an upper intertidal seaweed.