THE LIFE CYCLE, POPULATION DYNAMICS, GROWTH AND SECONDARY PRODUCTIONOF BITHYNIA GRAECA (WESTERLUND, 1879) (GASTROPODA) IN LAKEKERKINI, NORTHERN GREECE

The life cycle, population dynamics, growth and secondary production of the freshwater prosobranch snail Bithynia graeca (Westerlund 1879) were studied in the artificial Lake Kerkini (Serres, Northern Greece) where water volume and depth fluctuate dramatically. B. graeca faces deterministic and stochastic aspects of this environment. Growth and reproduction were studied for two populations, one being covered by the water almost throughout the year and the other not. B. graeca is a semelparous annual freshwater snail whose life expectancy decreased with increasing age. Maturity was attained 4 months earlier for the first population. Reproduction took place in spring. The majority of adults died after egg-laying. Growth rate was rapid during spring, and activity ceased during winter. B. graeca is a prosobranch snail whose short turnover time (77.9 and 84.3 days) was similar to that seen in freshwater pulmonate snails. Productivity was assessed in two ways, by a size frequency method and by using cumulative biomass. Observed values, for the two methods respectively, were for mean biomass 18.53 and 22.8 g dry body weight/m2, for annual production (P) 86.81 and 88.38 g dry body weight/m 2 or 9.56 gC/m2·year respectively, and for annual turnover ratio (P/B̅) 4.68 and 3.87/year. Net reproductive rate Ro was 2.99 and per capita rate of increase (rc) was 0.091.