MICROPHYTOBENTHOS PRODUCTION IN THE GULF-OF-FOS, FRENCH MEDITERRANEANCOAST

Microphytobenthic oxygen production was studied in the Gulf of Fos (Fr ench Mediterranean coast) during 1991/1992 using transparent and dark benthic chambers. Nine stations were chosen in depths ranging from 0.5 to 13 m, which represents more than 60% of bottoms in the Gulf. Posit ive net microphytobenthic oxygen production was seasonally detected do wn to 13 m; the maximum value attained was 60 mg O-2 m(-2) h(-1) (0.7- 0.8 g O-2 m(-2) d(-1)) in sediments at 0.5 m depth during spring and w inter. Respiration rates were maximum in the sediments located at the mussel farm (5 m), in the center of the Gulf, with 135 mg O-2 m(-2) h( -1) in spring (3.2 g O-2 m(-2) d(-1)); in the other locations, it rang ed from 3.3 to 58.2 mg O-2 m(-2) h(-1) (0.08-1.4 g O-2 m(-2) d(-1)). C ompared to phytoplankton, microphytobenthos production was higher only in the bottoms < I m depth. In deeper bottom waters, phytoplankton pr oduction could be absent due to light limitation, while microphytobent hos was still productive. Phytoplankton production m(-2) was generally higher than microphytobenthic production. Microphytobenthic biomass, higher than phytoplanktonic, varied from 27 to 379 mg Ch1 a m(-2), the maximum in the mussel farm sediments, with the minimum in sandy shall ow bottoms. Pigment analysis showed that microphytobenthos consisted m ainly of diatoms (Chi c and fucoxanthin) but other algal groups contai ning ChI b could become seasonally important. A Principal Component An alysis suggested that the main statistical factors explaining the dist ribution of our observations may be interpreted in terms of enrichment in phaeopigments and light; the role of Chi a appearing paradoxically as secondary in benthic production rates. Phaeopigments are mainly co nstituted by phaeophorbides, which indicate grazing processes. The inf luence of the mussel farm on the oxygen balance is noticeable in the w hole Gulf.