BIOGEOCHEMICAL CYCLES OF NATURAL RADIONUCLIDES IN THE NW MEDITERRANEAN-SEA

Th-234, with its period of 24.1 days, is an ideal particle-reactive tr acer for studying the scavenging of trace elements in surface waters. Dissolved and particulate Th-234 activities, as well as phyto- and zoo plankton biomasses were periodically measured 8 miles off the coast of Nice (Mediterranean Sea) during spring 1987. The results show a stron g correlation between phytoplanktonic activity and particulate Th-234 content. However the export of Th-234 out of the surface layer seems t o be controlled by zooplankton grazing. Moreover, our results indicate the importance of salps, in particular as efficient removers of small suspended particles in surface waters.