Aim We tested whether the contraction of the equatorward boundary of an intertidal organism, the blue mussel, Mytilus edulis, was due to high summer temperatures limiting mortality.
Location The Atlantic coast of the United States.
Methods Field transplant experiments were conducted at three locations along the US Atlantic coast. Survival and heat shock protein 70 expression were determined at biweekly intervals. Air and water temperature profiles were used to model current and historical patterns of mortality, and to determine rates of temperature change.
Results High levels of mortality and expression of the inducible heat shock protein 70 were observed after multiple consecutive aerial exposures of 32 °C or greater. Since 1960, seasonal air and water temperatures have increased along the eastern US seaboard, and south of Lewes, DE (38.8° N) summer sea surface temperature increases have exceeded the upper lethal limits of this organism.
Main conclusions Along the southern portion of its range, intertidal populations of M. edulis have experienced catastrophic mortality directly associated with summer high temperatures. Over the past 50 years, a geographic contraction of the southern, equatorward range edge of M. edulis has occurred, shifting the range edge approximately 350 km north of the previous limit at Cape Hatteras, NC (35.2° N).