Long-term changes in annual growth of bivalves in the Wadden Sea: influences of temperature, food, and abundance

We report on results of a long-term field study on between-year variability in annual weight growth of 1-y old individuals of 4 dominant bivalve species on tidal flats in the western part of the Wadden Sea: 3 filter feeders (Cerastoderma edule, Mytilus edulis, and Mya arenaria) and 1 filter/deposit feeder (Macoma balthica).Relationships between individual weight gain during the growing season and 2 environmental factors (temperature, food supply) in the growing season (March -August) were studied.Weight gains varied strongly from year to year (by an order of magnitude in all species) and showed significant correlations with water temperatures (negative in M. balthica, but positive in the other 3 species).Chlorophyll concentrations showed a significant (and positive) relationship with growth only in M. balthica.In the other 3 species, year-to-year fluctuations in growth were found to be synchronized, showing a consistent pattern with elevated values in the years between 1991 and 2005.The abrupt change to faster growth in the early 1990s may have been caused by the nearly-complete disappearance in the western Wadden Sea of mussel beds for a period of several years, starting in 1990.The change to declining growth rates in the early 2000s took place simultaneously with a decline in chlorophyll concentrations and a rapid increase of the stocks of the invasive bivalve Ensis directus.In all 4 studied species, growth rates were seriously reduced in the 1 year (2012) with exceptionally high numbers of filter-feeding bivalves on the tidal flats.