Phytodetritus at the sediment-water interface, NW Mediterranean Basin: spatial repartition, living cells signatures, meiofaunal relationships
Keywords:deep-sea phytodetritus, meiofauna, phytoplankton, NW Mediterranean Basin
The deep-sea phytodetritus at the sediment-water interface was investigated in the continental margin, canyons and interfluves from 260 to 2070 m depth in the northwestern Mediterranean in June 1995. Overlying phytoplankton biomass, hydrodynamics and relationships with meiofauna densities were also evaluated. The phytoplankton biomass was low (< 0.2 µg Chl a l-1), with the lowest standing stocks on the eastern part of the Balearic Basin. Two sedimentary entities were identified: 1) the west coast where pigment concentrations were higher, with an average of 0.38 µg Chl a g-1 dry weight and 4.60 Phaeo a g-1 dw on the first half cm of sediments; and 2) the eastern part of the North Balearic basin where pigment concentrations were significantly lower (0.15 µg Chl a g-1 dw). A north to south gradient was observed, with higher pigment amounts on the Rhodanian slope. Differences were found in the different parts of the studied canyons, with a slight increase in the Axis sites. The vertical pigment repartition was a gradual decrease in the Interfluve sites, whereas concentrations decreased sharply below the first cm in the Axis sites, attesting differences in sedimentation, grazing or bioturbation within the different canyon parts. The north to south gradient in the phytoplankton Chl a/Chl b ratio and sedimented phytodetritus attest geographical differences within phytoplankton assemblages (i.e. diatoms are absent in the south). The epifluorescence tests from the interface revealed the presence of living microphytes at the northern sites, associated with aggregates and faeces. No consistent fluff was evidenced anywhere. The revival tests also agree with a north to south trend. The Meiofaunal density was positively related to the quality of the sedimented layer. A PCA, within the major parameters and factors, illustrates these relationships. All results allow two distinct geographical sectors to be discriminated around the frontal zone: the NW and E sides of the NW Mediterranean Basin, following in particular their contrasted hydrodynamics. The results indicated the relationships within phytoplankton input, underlying phytodetritic layer and meiofauna. They confirm the well-documented statement that demonstrates the deep-sea floor chloropigment concentration to be a good proxy for the overlying primary productivity, but also that some pigment ratios representative of plankton assemblages are preserved on surficial deep sediments, even after sedimentation and grazing.
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