Mass mortality of the invasive bivalve Corbicula fluminea induced by a severe low-water event and associated low water temperatures

The Asian clam Corbicula fluminea, originating from Southeast Asia, was first recorded in Lake Constance in 2003 and developed local mass occurrences afterwards. Effects of harsh winter conditions in 2005/2006 associated with a strongly decreasing water level were studied at three different depths at and below the mean low water level (MLL, MLL -1 m and MLL -3 m). Low winter temperatures produced a massive die-off of the C. fluminea population. The mortality of the clams was size class and depth dependent. At the mean low water level (MLL), all clams died because of lying dry. However, at MLL -1 m and at MLL -3 m, mortality was a consequence of water temperatures around 2 degrees C for nearly 3 months. At MLL -3 m, clams > 5 mm died later than young clams < 5 mm and later than clams of all sizes at MLL -1 m. But in late spring even the clams > 5 mm at MLL -3 m were dead and only about 1% of the overall population of C. fluminea survived the winter conditions until spring 2006. Lethal effects of low water temperatures on C. fluminea, which may become effective only after a time lag, were corroborated in an outdoor mesocosm experiment with constant water level and without predation.