Seasonal changes of ¹³⁷Cs in benthic plants from the southern Baltic Sea

¹³⁷Cs activity concentrations were determined in samples of macrophytes Polysiphonia fucoides (red algae) and Zostera marina (vascular plant) collected during the entire vegetation season in the Gulf of Gdańsk in the southern Baltic Sea. The measurements showed considerable seasonality of ¹³⁷Cs activity in both species; an increase of cesium concentrations was observed from spring to autumn with maximal levels 49.1 ± 1.4 Bq kg_d.w.^-1 (P. fucoides) and 14.5 ± 1.0 Bq kg_d.w.^-1 (Z. marina) in late autumn. ¹³⁷Cs concentrations observed in a given season are the result of a number of processes, the intensity of which can differ depending on external environmental conditions. The effects of these processes can differ and their directions can frequently be opposite to one another. The examined macrophytobenthic plant species could serve as bioindicators of radionuclide pollution for monitoring purposes on condition that the samples of plants are taken within a strictly defined period of the year to give comparable results and to supply realistic information about pollution levels.