Winters not too cold, summers not too warm: long-term effects of climate change on the dynamics of a dominant species in the Wadden Sea: the cockle Cerastoderma edule L.

The Wadden Sea experiences a warming climate with water temperatures having increased by about 2 °C during the last 50 years. We studied possible effects of this temperature rise on the dynamics of a cockle (Cerastoderma edule) population in the westernmost part of the Wadden Sea by monitoring the numbers of aged cockles since the early 1970s. We found increased rates of mortality at extreme climatic conditions. In particular, survival declined to close-to-0 in all age classes in severe winters. In the hottest summers, survival was somewhat reduced in older (> 1 year old) cockles. On the other hand, recruitment was enhanced after cold winters, but this was not a direct effect of low temperatures as it was mainly caused by a reduction of the abundance of predators on small spat. The observed decline of the long-term trend in recruitment was compensated by an increasing trend of winter survival, resulting in an absence of any long-term trend in adult numbers.