Relationship between bacteria, phytoplankton and heterotrophic nanoflagellates along the trophic gradient

Abstract

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well as bacterial production and chlorophylla 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 grouped into trophic categories according to concentrations of chlorophylla. All the biotic-para-meters increased along the trophic gradient, leading to eutrophy, but they did not increase at the same rate. The bacterial biomass: phytoplankton biomass (BB: chla) 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) increased from 1000 bacteria per 1 flagellate in the oligotrophic system to 1700 bacteria flagellate⁴ in the eutrophic area. Decreasing BB: chla and increasing B: HNF ratios along the trophic gradient might reflect the different structures of the microbial food web. Relationships between bacterial abundance and production, and chla and HNF showed 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 chla than in the eutrophic system, suggesting 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, showed that the importance of bacteriovory increased in richer systems.