Seasonal and inter-annual variations of PCB and DDT contents in the oyster Crassostrea angulata from the Sado Estuary (Portugal)

Concentrations of 16 PCB congeners, pp'-DDT and its metabolites (DDE and DDD) and lipid content were determined in Crassostrea angulata. The oysters were collected monthly from a natural ground of the upper Sado Estuary, from November 1987 to November 1988, and between January and December 1990. The same compounds were also analysed in suspended particulate matter samples collected at the same site with the same periodicity. In 1987/88, the suspended particulate matter contained uniformly low levels of pp'-DDT and its metabolites (1–5 ng g–1), except in the winter runoff period when DDT compounds reached 48 ng g–1. The river flows remained low throughout 1990 and no seasonal variation of DDT contents was found in the suspended particulate matter. Concentrations of PCBs as the sum of congeners remained within the narrow interval of 5–12 ng g–1 in the two years. The seasonal profiles of DDT and PCB compounds in oysters were characterized by the presence of a spring peak associated with the sexual cycle, and another maximum in winter. The winter peak in 1987/88 results from the input of particles enriched in DDT and the downward displacement of the maximum turbidity zone, whereas the 1990 peak was related to nutritional alterations. These results show that the accumulation pattern of organochlorines in oysters varied seasonally and inter-annually according to the timing of environmental and biological factors.