DYNAMICS OF THE DOWNWARD FLUX OF PARTICLES AND CARBON IN THE OPEN NORTHWESTERN MEDITERRANEAN-SEA

Downward fluxes and the elemental composition of settling particles at 80, 200 and 1000 m depth measured during an 18-month sediment trap ex periment are presented, together with associated hydrographic and biol ogical data, in order to assess the relative importance of these facto rs in controlling particle flux. Mass, carbon and nitrogen fluxes were generally higher during the first half of the year. Mass flux maxima occurred at different times of the year depending on trap depth, where as the peak carbon flux was measured in late June. Most of the total c arbon flux was composed of organic carbon. On an annual basis the tota l particle flux was 32.9, 40.6 and 8.1 g m-2 year-1 at 80, 200 and 100 0 m depth, respectively. The organic carbon flux decreased from 4.8 an d 5.0 g m-2 year-1 at 80 and 200 m, respectively to 1.2 g m-2 year-1 a t 1000 m, and the corresponding nitrogen flux from 0.7 and 0.8 to 0.2 g m-2 year-1. Organic carbon and nitrogen content of sinking particles decreased with depth, whereas the carbonate carbon content remained c onstant. The contribution of zooplankton feces to the carbon flux was highly variable throughout the year, ranging from 1.6 to 62%. Swimmers , including fish, that entered the traps in the upper 200 m were abund ant and, on average, were twice the weight of the sinking particle flu x. Pelagic production was the main source of particles during the late spring and mid-summer sedimentation pulses; however physical forcing, particularly the winter mixing of the water column, was partially res ponsible for the high sedimentation during the earlier part of the yea r. Wind gusts and concomitant vertical mixing appeared responsible for the carbon rich particle flux in summer. Separation of data for the m ixed and stratified water column periods clearly indicated that most o f the downward flux occurred in mixed waters whereas the organic carbo n content was higher in particles sedimenting through stratified water s. The export of organic carbon from the euphotic zone (upper 200 m) r epresented only 5-7% of the estimated phytoplankton production in this region. Furthermore time-series measurements of POC flux averaged on an annual basis were only 14-42% of indirect estimates of carbon expor t or ''new production'' for this region. This discrepancy suggests tha t a good portion of the total downward carbon flux occurs through adve ction and/or diffusion of DOC.