Salinity effects on cadmium concentrations in blue mussels in the Baltic Sea

Abstract: This bachelor’s essay is a review written at the Swedish University of Agricultural Sciences on suggestion from the Swedish Museum of Natural History. The purpose of this study was to increase the understanding of how the salinity in the Baltic Sea affects the concentrations of cadmium (Cd) observed in blue mussels, Mytilus edulis, and to be able to predict more accurate future trends of the bioavailable Cd. Cadmium is a heavy metal that deposits to the Baltic Sea by runoff, point sources and atmospheric deposition. Cadmium is toxic to aquatic organisms and may bioaccumulate. Blue mussels feed by filtering the water. Through the filtration the mussel is contaminated by chemical pollutants, which means that chemical analysis of mussels can reflect how contaminated a habitat is. The mussels are suitable biomonitors since they are sedentary, easy to collect, abundant and large enough for tissue analysis. In the Baltic Sea there is a salinity gradient with decreasing salinity from west to east and from south to north. The surface water has lower salinity compared to the deep water. The mussel size is affected by the salinity with smaller mussels in less salty waters due to stress. The amount of soluble cadmium in water is increasing with salinity since the salt is competing with Cd for adsorption sites on particles and the formation of soluble chloride complexes are increasing. However, since the chloride complexes are not bioavailable, the bioavailability of cadmium is increasing with decreasing salinity. At the surface zooplanktons take up Cd from the water and in deeper layers it is released again by the decomposition by detritus, creating a vertical gradient. In deeper waters, below the detritus, the concentration is relatively constant. While using organisms to examine pollutants, biological variations must be considered. One way to eliminate influences from individual differences is to put the cadmium concentration in the soft tissue in relation to the mussel shells. Different body parts contain different concentrations of Cd. How the concentration is divided between the body parts is however indefinitely, since different experiments have shown different results.