Abstract

Relatively little is known about the causes of species distribution within macroalgal blooms occurring on or over soft substrata. We examine the roles of aerial exposure and desiccation in determining elevational patterns of two dominant bloom-forming ulvoid algae, Ulva lactuca and Ulvaria obscura, on the northeastern Pacific coast. These species were stressed with constant desiccation time, desiccation to a constant water potential, and desiccation to a fixed water loss. As a measure of health, we examined net photosynthesis by oxygen evolution or photosynthetic yield via pulse amplitude modulated fluorometry. By all measures, Ulva was physiologically superior in the intertidal zone. Under constant exposure, it desiccated more slowly than Ulvaria. When desiccated to a constant dryness, Ulva better maintained photosynthetic parameters. These observations are consistent with Ulva's usual dominance over Ulvaria in intertidal macroalgal blooms. Given these and other observed differences between the two species, natural resource managers should not regard the two as ecologically equivalent, even though they have similar functional forms.