Parallel changes of Limecola (Macoma) balthica populations in the Dutch Wadden Sea

Long-term (>40 yr, started in the 1970s) data series of Limecola (Macoma) balthica dynamics in 4 distant (>100 km) areas of the Dutch Wadden Sea are described and compared, including annual recruitment, total abundance, age composition (recruits/adults) and adult survival. The aim was to determine to what extent long-term changes in L. balthica population attributes were synchronous in populations throughout the Dutch Wadden Sea and to identify possible underlying causes. At all 4 sites studied, changes in annual recruitment (n m(-2) of spat-sized animals in summer) was synchronous with long-term declining trends and peaks and lows occurring in the same years at all sites. Adult survival was high for the first 15 to 20 yr, but irregular after 1990 with low values for periods of varying length. In contrast to the strongly synchronized recruitment, the low-survival periods were not synchronous over the Wadden Sea. Adult density and total biomass varied in parallel over most sites and were low throughout the Dutch Wadden Sea (less than a quarter of the earlier long-term average) from 2005 onwards. The factors suggested to underlie these changes in abundance were (1) negative effects of climate warming on recruitment, which includes higher abundance of predators on young bottom stages, and (2) an unknown cause of enhanced adult mortality, possibly disease. Synchronization in recruitment, adult density and total biomass of L. balthica populations throughout the Dutch Wadden Sea indicates that they are part of a Wadden Sea-wide metapopulation.