Balance of organic fluxes in the Gironde
dilution plume: ecological importance
of “microbial loop” and microbial food web
processes in spring, in the photic zone
Luis Felipe Artigas
(1) , Pierre Laborde
(2) , Benoit Sautour
(2) ,
Daniel Delmas ”, Alain Herbland'"
(1) Unjversité du Littoral Côte d’Opale, maison de la recherche en environnement naturel,
UPRES-A 8013 Elico, 32 avenue Foch, 62930 Wimereux, France
(2) Université de Bordeaux 1, laboratoire d'océanographie biologique,
UMR-CNRS-EPOC 5805, 2 rue du Pr Jolyet, 33120 Arcachon, France
(3) Crema L’Houmeau, BP 5, 17137 L’Houmeau, France
artigas@loalit.univ-littoral.fr
Abstract
Characteristics of the pelagic ecosystem of the French bay of Biscay
submitted to the influence of the Gironde dilution plume were defined,
in spring, taking into account the structure and magnitude of both microplanktonic stocks and dynamics, the importance of trophic transfers,
and the magnitude of carbon downward fluxes. Notwhithstanding the
differences in both biomass and production rates measured during the
three different spring periods, nano- and picoplanktonic cells were predominant, and autotrophy appeared to be limited by the phosphate
concentration during the whole period. The bacterial heterotrophic
production represented, as a mean, from 10% to 33% of the autotrophic production according to the period considered. On the other
hand, the herbivorous grazing on microautotrophs was responsible for
the loss of 15% to 100% of the daily primarv production, whereas an
average of 45% of the net daily bacterial production was consumed by
bacterivores in spring. Finally, very low sedimentation rates were measured at the base of the euphotic zone in the three spring periods.
The inner-shelf euphotic zone constituted, in spring, a “maintenance”
system where “microbial loop” and microbial food web processes, in
addition to playing an important role in sustaining the “regenerated”
primary production, seemed to act as a potential “microbial barrier” to
the organic matter sedimentation (intense bacterial hydrolysis measured
in sinking aggregates).
Introduction
The turnover rate and ultimate fate of biogenic carbon in planktonic
marine systems is directly related to the modality of matter and energy
transfer from primary producers to the different groups of heterotrophs
(Calbet et al., 1996), and is strongly determined by the size structure of
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dilution plume: ecological importance
of “microbial loop” and microbial food web
processes in spring, in the photic zone
Luis Felipe Artigas
(1) , Pierre Laborde
(2) , Benoit Sautour
(2) ,
Daniel Delmas ”, Alain Herbland'"
(1) Unjversité du Littoral Côte d’Opale, maison de la recherche en environnement naturel,
UPRES-A 8013 Elico, 32 avenue Foch, 62930 Wimereux, France
(2) Université de Bordeaux 1, laboratoire d'océanographie biologique,
UMR-CNRS-EPOC 5805, 2 rue du Pr Jolyet, 33120 Arcachon, France
(3) Crema L’Houmeau, BP 5, 17137 L’Houmeau, France
artigas@loalit.univ-littoral.fr
Abstract
Characteristics of the pelagic ecosystem of the French bay of Biscay
submitted to the influence of the Gironde dilution plume were defined,
in spring, taking into account the structure and magnitude of both microplanktonic stocks and dynamics, the importance of trophic transfers,
and the magnitude of carbon downward fluxes. Notwhithstanding the
differences in both biomass and production rates measured during the
three different spring periods, nano- and picoplanktonic cells were predominant, and autotrophy appeared to be limited by the phosphate
concentration during the whole period. The bacterial heterotrophic
production represented, as a mean, from 10% to 33% of the autotrophic production according to the period considered. On the other
hand, the herbivorous grazing on microautotrophs was responsible for
the loss of 15% to 100% of the daily primarv production, whereas an
average of 45% of the net daily bacterial production was consumed by
bacterivores in spring. Finally, very low sedimentation rates were measured at the base of the euphotic zone in the three spring periods.
The inner-shelf euphotic zone constituted, in spring, a “maintenance”
system where “microbial loop” and microbial food web processes, in
addition to playing an important role in sustaining the “regenerated”
primary production, seemed to act as a potential “microbial barrier” to
the organic matter sedimentation (intense bacterial hydrolysis measured
in sinking aggregates).
Introduction
The turnover rate and ultimate fate of biogenic carbon in planktonic
marine systems is directly related to the modality of matter and energy
transfer from primary producers to the different groups of heterotrophs
(Calbet et al., 1996), and is strongly determined by the size structure of
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