The role of ciliates within the microbial food web in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

Natalia Bojanic,Stefanija Šestanovic,Živana Nincevic Gladan,Ivona Marasovic,Nada Krstulovic,Igor Brautovic,Mladen Šolic

doi:10.3989/scimar.2006.70n3431

Abstract

Interactions among phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNF), ciliated protozoa and copepod nauplii were studied in the eutrophicated part of Kas?tela Bay from May 1998 to November 1999. Special emphasis was placed on relationships between size categories of nonloricate ciliates (NLC) and other microbial food web components. Biomasses of phytoplankton and bacteria were primarily influenced by abiotic parameters. Temperature indirectly controlled variation in HNF biomass through the changes in biomass of bacteria and the smaller phytoplankton fraction. Besides HNF, bacterial biomass was affected by the NLC 3 (Cell Length 3 , CL 104 µm 3 (CL >40 µm) had a strong mutual correlation and they seemed to be controlling the microphytoplankton fraction. During the colder part of the year, HNF abundance was regulated by ciliate grazing. The high impact of ciliates in summer 1998 could have been influenced by the taxonomic composition of the phytoplankton community changing, as well as the higher eutrophication level in the study area. Predation by copepod nauplii on ciliates and carbon transfer to higher trophic levels appear to be of relative importance only in the period when they are most abundant. This paper outlines the dominant relationships within the microbial food web and suggests that a significant amount of bacterial production, phytoplankton and HNF biomass could be transferred to higher trophic levels through the microbial food web.

Highlights

IntroductionTwo regulation mechanisms of organism abundance and biomass are generally recognised in planktonic ecosystems: control by resources (bottom-up) and control by predators (top-down control)
Two regulation mechanisms of organism abundance and biomass are generally recognised in planktonic ecosystems: control by resources and control by predators
SUMMARY: Interactions among phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNF), ciliated protozoa and copepod nauplii were studied in the eutrophicated part of Kastela Bay from May 1998 to November 1999

Summary

Introduction

Two regulation mechanisms of organism abundance and biomass are generally recognised in planktonic ecosystems: control by resources (bottom-up) and control by predators (top-down control). The bottom-up control of ciliates is conditioned by the biomasses of phytoplankton, bacterioplankton and heterotrophic nanoflagellates (HNF) (Hagström et al, 1988; Rassoulzadegan et al, 1988; Solicand Krstulovic, 1995). Ciliates as the main HNF consumers decrease the HNF abundance and their grazing impact on bacteria (Conover, 1982; Solicand Krstulovic, 1994; Sherr and Sherr, 2002; Urrutxurtu et al, 2003), which in turn increases the bacterial biomass. Several studies have dealt with trophic relationships within the microbial food web (Sanders et al, 1989; Verity and Smetacek, 1996; Rodríguez et al, 2000), information about microbial interactions is still very limited

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