Les variations annuelles du seston sont davantage influencée par les remontées des particules du dépôt sédimentaire du fond que par les poussées phytoplanctoniques. Il existe des corrélations plus fortes entre les cinq classes de particules qu'entre chacune de ces classes et chacun des groupes phytoplanctoniques.
Particles were counted, either by an optical microscope method or by using a “Coulter counter”, in water withdrawn daily from the Bay of Villefranche-sur-Mer. These estimations enabled the seasonal cycles of the seston to be described and the principal groups of nano- and micro-phytoplankton to be identified.
Nanoplankton, which is essentially characterized by naked flagellates, constitutes the main fraction of phytoplankton. The total number of phytoplankton cells is ≈109 cells/m3 for nanoplankton and 1.5 × 107 cells/m3 for the microplankton. The seston between 2.8 and 12 μm contains 20 % of nanoflagellates.
Comparing the numbers obtained by microscopic counting and the automatic Coulter counting it may be concluded that the use of the latter method gives the same results as the former as long as strong gusts of wind do not cause mixing.
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