Water is essential for life on Earth, but many organisms are subject to water loss under certain environmental conditions and this can cause biological stress. However, some cyanobacteria and algae are capable of coping with periodic exposure to potentially desiccating conditions. Thus, phototrophs in biological soil crusts can survive in desert environments, even when the only source of water is dew. Other aquatic plants and algae can be exposed to emersion following seasonal changes in water level in rivers or lakes and, importantly, during the daily emersion of intertidal species. Seaweeds living in the intertidal are poikilohydric, and each time they are emersed, they risk water loss. Dehydration can lead to inhibition of photosynthesis and respiration as well as disruption to nutrient availability and assimilation. However, intertidal seaweeds have evolved a range of adaptations/acclimations that allow them to cope with exposure to air. These include morphologies that minimise surface area:volume ratio and biochemical changes that involve, for example, enhanced capacity for detoxification of reactive oxygen species. The extent to which seaweeds can recover function following re-immersion and differences in their capacity for nutrient uptake during restricted periods of immersion appear to be correlated with the zonation of species in the intertidal.