Some like it cold: populations of the tellinid bivalve Macoma balthica (L.) suffer in various ways from a warming climate

Because of its relatively low tolerance to elevated temperatures, Macoma balthica (L.) may be one of the first important marine species in temperate coastal areas to suffer from a warming climate. For the last few decades, the abundance of the species in the Dutch Wadden Sea has been seriously declining. At lower latitudes, the southern edge of its range recently shifted several 100s of km to the north. To understand changes in abundance related to high temperatures, we studied short-term population responses to warmer than average seasons using data from a long-term (>35 yr) sampling program in the western Wadden Sea. The observed relationships included reduction of reproductive output and recruitment in years starting with mild winter-spring periods, reduction of adult survival in years with warm summers, reduction of annual migration in similar to 8 mo olds to more favourable areas in mild winters, reduction of growth rates in warmer than average growing seasons, and enhancement of seasonal weight loss in milder than average winters. We conclude that elevated temperatures negatively affect population dynamics of M. balthica in a number of ways, i.e. via recruitment or mortality as well as growth. These negative influences were observed already far below the upper lethal temperature of the species and in populations living similar to 1000 km poleward of the warm edge of the species' range. The outcome of M balthica's energy balance appears to be decisive for their continued existence by effects not only on individual weights but also on mortality as well as reproduction.