The general distribution of the brown algaMacrocystis integrifoliaBory in British Columbia is related to seasonal seawater temperatures and salinities. This species is restricted to regions having little seasonal variation of seawater temperature and salinity or regions where the lower salinities occur during the winter. Locally,Macrocystisdistribution is restricted to moderately wave-exposed areas or areas subjected to appreciable tidal currents, provided solid substrate is available. The vertical distribution is usually determined by substrate availability and sea urchins.Macrocystisblade morphology varies regularly with wave exposure. However, transplant studies indicate that this may, in part, be genetically determined and thus may not be a plastic phenotypic response.