Consequences of altered temperature and food conditions for individuals and populations: a Dynamic Energy Budget analysis for Corbicula fluminea in the Rhine

On the basis of simulations at two levels of organisation (individuals and populations), we analysed the impact of altered food and temperature conditions on the Asian clam Corbicula fluminea. In particular, we addressed the role of food and temperature in explaining occasional mass mortalities. A Dynamic Energy Budget Model (DEBM) was used for simulations at the individual level. We calibrated the DEBM successfully using experimental data on growth in length and mass of C.fluminea under four combinations of temperature and food. The calibrated DEBM was used to simulate the annual growth in length and mass and the reproductive success under different environmental scenarios. In general, an increase in temperature and food concentration resulted in larger and heavier clams and a higher per capita rate of reproduction. However, phytoplankton densities in rivers often fluctuate strongly and densities are high only briefly. Under such conditions, our simulations suggest that the temporal interactions of temperature and food concentration are particularly important and can decisively influence annual growth. With regard to occasional mass mortalities, it has been hypothesised that increased temperature might induce an unmet metabolic demand and hence starvation. To test this, we simulated the conditions observed in the Rhine in 2003, when there was a severe heat wave and a mass mortality. For this purpose, we used the calibrated DEBM and, in addition, integrated it into a Physiologically Structured Population Model (PSPM) to account for processes at the population level. Based on these simulations, it seems unlikely that starvation was the main cause of the mass mortality, although not all mechanisms affecting the energy budget are fully understood. However, a prolonged negative energy budget during heat waves can increase the vulnerability of clams to direct effects of high temperatures or additional stressors and might thus be regarded as an important factor indirectly increasing mortality rates.