RELATIONSHIP BETWEEN BACTERIA, PHYTOPLANKTON HETEROTROPHIC NANOFLAGELLATES ALONG THE TROPHIC GRADIENT

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well a s bacterial production and chlorophyll a levels, were measured at five sites extending from the coastal zone toward the open Adriatic in the period from March to October 1995. The investigated areas were groupe d into trophic categories according to concentrations of chlorophyll a . All the biotic-parameters increased along the trophic gradient, lead ing to eutrophy, but they did not increase at the same rate. The bacte rial biomass : phytoplankton biomass (BB : chl a) ratio decreased from about 10 in the very oligotrophic area to 0.8 at the eutrophic site. In contrast, the bacterial abundance : HNF abundance ratio (B : HNF) i ncreased from 1000 bacteria per 1 flagellate in the oligotrophic syste m to 1700 bacteria flagellate(-1) in the eutrophic area. Decreasing BE : chl a and increasing B : HNF ratios along the trophic gradient migh t reflect the different structures of the microbial food web. Relation ships between bacterial abundance and production, and chl a and HNF sh owed that bacterial abundance along the trophic gradient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacterial production and chl a than in the eutrophic system, sugge sting stronger control of bacterial abundance by substrate supply. On the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial abundance and production in the eutrophic system, sh owed that the importance of bacteriovory increased in richer systems.