Effects of water depth on survival, condition and stable isotope values of three invasive dreissenid species in a deep freshwater lake

To examine the effects of depth on the performance of the freshwater dreissenid species Dreissena polymorpha and D. rostriformis bugensis, and the brackish water dreissenid, Mytilopsis leucophaeata, a controlled experiment lasting four months was conducted in a manmade freshwater lake in the Netherlands. The three species used in the experiment were collected from other water bodies in the Netherlands. Cages containing each species in separate compartments were placed at water depths of 2, 5, 10 and 17 m from November 2009 until February 2010. Samples of individuals were retrieved at the start and at approximately four-week incubation intervals, for determination of overall survival, condition and soft tissue carbon and nitrogen stable isotope ratios. In contrast to the Dreissena species, survival of M. leucophaeata decreased strongly over time at each depth. Mortality increased with increasing depth and was greatest at 17 m. Survival of D. polymorpha at 17 m was much greater than that of D. rostriformis bugensis. The condition of all species after 3.5 weeks did not differ significantly from that at the start. Surviving M. leucophaeata showed a significantly reduced condition at all depths after 16 weeks. Both Dreissena species showed an improved condition at depths of 2, 5 and 10 m after 16 weeks, the greatest improvement being found at 2 m. After 16 weeks at 17 m, both Dreissena species had the lowest condition index. Stable isotope ratios of soft tissue for both Dreissena species showed a shift in δN within 3.5 weeks. The δN values of mussels held at 2-10 m fell. Those held at 17 m had values similar to that initially recorded, possibly indicative of very limited food intake at that depth or a diet similar to that at their location of origin. After 16 weeks, δC values became more negative than those at the start, in particular at 2 m. The stable isotope ratios of soft tissue of M. leucophaeata did not change at any depth throughout the experiment, possibly indicating lack of feeding in the freshwater lake. Changes in stable isotope values for both Dreissena species were reduced at 17 m. Mortality of D. polymorpha was lower at 17 m compared to the other two dreissenid species which indicated greater tolerance of habitat conditions a this deeper depth.