Dynamics of the downward flux of particles and carbon in the open northwestern Mediterranean Sea

Abstract

Downward fluxes and the elemental composition of settling particles at 80, 200 and 1000m depth measured during an 18-month sediment trap experiment are presented, together with associated hydrographic and biological data, in order to assess the relative importance of these factors 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, whereas the peak carbon flux was measured in late June. Most of the total carbon flux was composed of organic carbon. On an annual basis the total particle flux was 32.9, 40.6 and 8.1 g m −2 year −1 at 80, 200 and 1000 m depth, respectively. The organic carbon flux decreased from 4.8 and 5.0 g m −2 year −1 at 80 and 200 m, respectively to 1.2 g m −2 year −1 at 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 constant. 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 abundant and, on average, were twice the weight of the sinking particle flux. 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 responsible for the high sedimentation during the earlier part of the year. Wind gusts and concomitant vertical mixing appeared responsible for the carbon rich particle flux in summer. Separation of data for the mixed and stratified water column periods clearly indicated that most of the downward flux occured in mixed waterswhereas the organic carbon content was higher in particles sedimenting through stratified waters. The export of organic carbon from the euphotic zone (upper 200 m) represented 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 export or “new production” for this region. This discrepancy suggests that a good portion of the total downward carbon flux occurs through advection and/or diffusion of DOC.