SR TRANSFER-FACTORS BETWEEN DIFFERENT LEVELS IN THE TROPHIC CHAIN IN 2 DAMS OF DOURO RIVER (PORTUGAL)

The study of Sr transfer in the trophic chain, was undertaken in the dams of Miranda do Douro and Regua belonging respectively to the upper and middle streams of Douro river. This study was carried out during 17 months, between January 1985 and May 1986, and Sr was measured in water, phytoplankton (seston) zooplankton, specimens belonging to two benthic families (Chironomidae and Tubificidae) and in representatives of four species of fish of Cyprinidae family: Chondrostoma polylepis-boce; Cyprinus carpio-carp; Carassius auratus-goldfish; Barbus bocagei-barbel. The results obtained showed the existence of strong seasonal variation in the levels of Sr in water and in the different levels of the food chain. The minimum values in water correspond to the rainy seasons, when the flow is higher, identifying a clear dilution effect. In fish, where these variation are slighter, the amount of Sr is proportional to the size of the body (constant concentration), possibly because the greatest part of this element is found on skeleton pieces (bones and scales). All the studied organisms present high Sr concentration factors from the water. However, taking into account annual averages, a great accumulation of Sr in the food chain, from the plankton to the fish itself, is not found, meaning that no important phenomena of biomagnification to Sr concentrations between prey and predator exist. The higher transfer factors (TFs) from benthic animals to fish are only due to the absence of skeleton in those animals. An estimation of TFs from benthic animals to the soft edible parts of fish show that no biomagnification exists. The determination of TFs between food-chain levels during the different seasons, together with other physico-chemical, biological and hydrological data presented, is essential to the development of protective measures against radioactive threats.