CHAPTER 60
Microbial Loop Structure along Trophic Gradients in the Adriatic Sea
P. Del Negrol, G. Civitarese2, P. RamaniI, and S. Fonda UmanP
ABSTRACT
Seasonal observations (May 1995-February 1996) carried out along the whole Adriatic basin
have significantly contributed to a better understanding of the distribution of bacterial and heterotrophic nanoplankton communities in different trophic areas. The average density values
along the water column ranged from 1 x 10 5 to 1.6 x 10 6 cell mL- 1 and from 1.2 x 10 2 to 1.9 x 10 3
cell mL-l for bacteria and nanoheterotrophs respectively.
The relationships between bacterial and heterotrophic nanoplankton abundance changed
along the trophic gradient suggesting that bacteria should be regulated more strongly by predation in eutrophic systems.
Introduction
Bacteria are the less variable component of the
plankton in terms of both total density and biomass (Cole et a1. 1993). Bacterial growth and
abundance may be tightly regulated within a relatively narrow range by a combination of
resource limitation (bottom-up control) and
grazing (top-down control). There has been an
antagonism between the two theories as to which
is the principal regulating force. Bottom-up control of bacterial community is supported by the
results of Billen et al. (1990), Simon et a1. (1992),
Weinbauer and Hofle (1998) that analysed several marine and freshwater systems characterized
by very different trophic conditions using various approaches. Pace and Cole (1994) found no
evidence in experimental studies that protozoa
effectively regulate bacterial abundance. On the
contrary, Berninger et a1. (1991) and Sanders et
al. (1992) described a high relationship between
bacterial and heterotrophic nanoflagellate
(HNF) abundance, and suggested that significant
predatory control of bacteria occurs. However, it
has also been shown that bacteria and HNFs are
not strongly coupled across systems, and, consequently, HNFs do not always control bacterial
abundance (Ga801 and Vaque 1993) probably
because of predatory control exerted on HNFs by
larger zooplankton (Gasol 1994). Evidence has
shown the potentially important role for dadocera, bacterivorous ciliates, mixotrophic algae in
removing bacteria from aquatic ecosystems
(Sanders et al.1992). By eliminating a fraction of
bacteria through cell lysis, also viruses are likely
to be significant agents in the control of bacteria
(Fuhrman 1999).
Much of the evidence used in support of bottom-up or top-down hypothesis in marine waters
has been obtained from estuarine or coastal systems and surface waters, and there are still few
data for the open sea, especially throughout the
water column (Dufour and Torreton 1996).
The aim of this study was to examine the
processes regulating bacterial abundance in the
whole water column along four transects of the
Adriatic Sea presenting different trophic status.
Study Area
The Adriatic Sea covers about six degrees of latitude, from 39.5 to 45.5°N, and about eight
. degrees of longitude. from 12 to 200E. Its NW -SE
oriented longitudinal axis is about 800 km long.
while its width varies from 100 to 200 kilometres.
Usually, the Adriatic Sea is divided into three
1 Laboratorio di Biologia Marina, Via A. Piccard 54, 340lO Trieste, Italy
1. Istituto Sperimentale Talassografico. CNR" Trieste, Italy
F.M. Faranda, L. Guglielmo, G. Spezie (OOs)
Mediterranean Ecosystems: Structures and Processes
e Springer-Verlag ltalia 2001
Microbial Loop Structure along Trophic Gradients in the Adriatic Sea
P. Del Negrol, G. Civitarese2, P. RamaniI, and S. Fonda UmanP
ABSTRACT
Seasonal observations (May 1995-February 1996) carried out along the whole Adriatic basin
have significantly contributed to a better understanding of the distribution of bacterial and heterotrophic nanoplankton communities in different trophic areas. The average density values
along the water column ranged from 1 x 10 5 to 1.6 x 10 6 cell mL- 1 and from 1.2 x 10 2 to 1.9 x 10 3
cell mL-l for bacteria and nanoheterotrophs respectively.
The relationships between bacterial and heterotrophic nanoplankton abundance changed
along the trophic gradient suggesting that bacteria should be regulated more strongly by predation in eutrophic systems.
Introduction
Bacteria are the less variable component of the
plankton in terms of both total density and biomass (Cole et a1. 1993). Bacterial growth and
abundance may be tightly regulated within a relatively narrow range by a combination of
resource limitation (bottom-up control) and
grazing (top-down control). There has been an
antagonism between the two theories as to which
is the principal regulating force. Bottom-up control of bacterial community is supported by the
results of Billen et al. (1990), Simon et a1. (1992),
Weinbauer and Hofle (1998) that analysed several marine and freshwater systems characterized
by very different trophic conditions using various approaches. Pace and Cole (1994) found no
evidence in experimental studies that protozoa
effectively regulate bacterial abundance. On the
contrary, Berninger et a1. (1991) and Sanders et
al. (1992) described a high relationship between
bacterial and heterotrophic nanoflagellate
(HNF) abundance, and suggested that significant
predatory control of bacteria occurs. However, it
has also been shown that bacteria and HNFs are
not strongly coupled across systems, and, consequently, HNFs do not always control bacterial
abundance (Ga801 and Vaque 1993) probably
because of predatory control exerted on HNFs by
larger zooplankton (Gasol 1994). Evidence has
shown the potentially important role for dadocera, bacterivorous ciliates, mixotrophic algae in
removing bacteria from aquatic ecosystems
(Sanders et al.1992). By eliminating a fraction of
bacteria through cell lysis, also viruses are likely
to be significant agents in the control of bacteria
(Fuhrman 1999).
Much of the evidence used in support of bottom-up or top-down hypothesis in marine waters
has been obtained from estuarine or coastal systems and surface waters, and there are still few
data for the open sea, especially throughout the
water column (Dufour and Torreton 1996).
The aim of this study was to examine the
processes regulating bacterial abundance in the
whole water column along four transects of the
Adriatic Sea presenting different trophic status.
Study Area
The Adriatic Sea covers about six degrees of latitude, from 39.5 to 45.5°N, and about eight
. degrees of longitude. from 12 to 200E. Its NW -SE
oriented longitudinal axis is about 800 km long.
while its width varies from 100 to 200 kilometres.
Usually, the Adriatic Sea is divided into three
1 Laboratorio di Biologia Marina, Via A. Piccard 54, 340lO Trieste, Italy
1. Istituto Sperimentale Talassografico. CNR" Trieste, Italy
F.M. Faranda, L. Guglielmo, G. Spezie (OOs)
Mediterranean Ecosystems: Structures and Processes
e Springer-Verlag ltalia 2001