Particulate organic matter composition in a semienclosed Periantarctic system: the Straits of Magellan

The elemental and biochemical composition of particulate organic matter (PO M) was investigated in the Straits of Magellan during February-March 1991. Twenty-two stations were selected in order to identify different areas of t he Magellan ecosystem from a trophic point of view. The Strait of Magellan can be divided into three subsystems characterized by different hydrologica l and geomorphological conditions. Seston concentrations were mostly constr ained by physical events, particularly the influence of oceanic and land ru n-off water inputs and the strong vertical mixing and resuspension events. POM composition displayed quali-quantitative differences between the three areas. In the first subsystem, influenced by Pacific waters, the low seston and POM concentrations and the high POC/Chl-a ratio values indicated the g eneral predominance of the detrital and heterotrophic fractions. In the sec ond subsystem, characterized by superficial stratification, higher seston a nd organic matter concentrations and lower values of POC/Chl-a ratio were f ound, indicating that this subsystem was influenced by an active autotrophi c component. Shallow waters with intense tidal regime and strong vertical m ixing characterized the third subsystem, connected to the Atlantic Ocean, w hich displayed an increasing importance of the inorganic fraction (values o f the POC/TSM ratio lower than in the other systems). Moreover, the third s ubsystem showed higher values of the RNA/DNA ratio, possibly indicating tha t resuspension events may enhance the metabolic stare of the organic partic les mainly dominated by heterotrophic components.