FOOD-WEB STRUCTURE VARIABILITY IN THE SURFACE-LAYER, AT A FIXED STATION INFLUENCED BY THE NORTH-WESTERN MEDITERRANEAN CURRENT

The structure of the planktonic community and the influence of mesozoo plankton migration on the microbial food web were investigated during six diel studies from June 92 to June 93 in the surface waters of a st ation in the North-Western Mediterranean Sea. Each diel study consiste d of sampling at 5 and 40 m every 3 h over 24 h. Most of the times die l cycles did not show any convincing diel patterns in any of the varia bles studied. Clear zooplankton migration was evident in only two diel studies. The ratio of heterotrophic/autotrophic biomasses varied from 0.68 to 3.0, with a strong dominance of the heterotrophic biomass und er oligotrophic conditions. Differences in food web structure were pro bably related to the influences of coastal water and the North-Western Mediterranean Current. Thus we found that the planktonic food web var iability relatable to hydrodynamic variability, to be greater than die l variability. However, very large differences in food web structure a mong dates were evident. For example proportion of Chi a found in the <10 mu m fraction varied from 18 to 96%.