Habitat preferences and population dynamics of Ostracoda in a helocrene spring system

Habitat features and ostracod populations were monitored monthly between De cember 1995 and September 1996 in three contiguous pools of a Mediterranean helocrene spring. Principal-components analysis (PCA) of the physical and chemical properties of the water showed that seasonal variations were minor in the site closest to the source (pool 1). Conversely, the site the most distant from the source (pool 3) had the greatest variation in temperature and oxygen and calcium contents. Differences in limnological factors, espec ially the equilibrium system of carbonates, among sites were also clear fro m the results obtained with PCA. The cosmopolitan ostracod Cypridopsis vidu a predominated in pool 1, where variations in oxygen content and temperatur e were minimal, and showed a univoltine reproductive pattern. The crenophil ous species Potamocypris villosa and Herpetocypris intermedia were the most abundant ostracods at the other two sites, where the development of filame ntous algae and Characeae produced a high oxygen concentration and an incre ase in calcium carbonate precipitation. The latter two species both produce d two generations per year. The small size of springs and their position be tween interstitial and surface waters make them excellent systems for the s tudy of ostracod ecology and the influences of environmental factors upon t heir life-history traits.