Expected Effects of Changing Seawater Temperatures on the Geographic Distribution of Seaweed Species

Abstract

Seaweeds are generally kept within their geographic boundaries by limiting effects of temperature. Northern boundaries are set by low lethal winter temperatures, or by summer temperatures too low for growth and/or reproduction. Southern boundaries are set by high lethal summer temperatures, or by winter temperatures too high for induction of a crucial step in the life cycle. Characteristic thermal response types, as identified in laboratory experiments, were found to be responsible for characteristic distribution patterns in the North Atlantic Ocean. Changes in seawater temperature have therefore easily predictable effects on the geographic distribution of seaweed species. Locally, species composition (and community structure) will be altered. Apart from latitudinal displacement and regional extinction, changing seawater temperatures may also cause a shift in selection pressure at a boundary, particularly when summer and winter temperatures change to a different extent. For instance, southern boundaries of several cold temperate brown algae in Europe, which are presently set by ‘winter reproduction’ limits will become ‘summer lethal’ limits following a rise in summer temperatures of only a few degrees. Reconstruction of thermal regimes during glacial and interglacial periods shows that such shifts in selection pressure have probably occurred more often on eastern than on western Atlantic coasts.