end of the vernal bloom respectively, whereas just after an important
phytoplankton outburst in 1995, micro- and nanozooplankters (2200 |um) where responsible for the loss of almost 100% of primary
productivities (tab. 2; Sautour et al., 2000).
On the other hand, bacterial daily losses due to bactetivorism processes
were extremely variable from one dav to another (0-100% of bacterial
production). However, a significant correlation was found between bacterial growth rates with and whithout grazers, in all the situations considered:
a mean grazing pressure of 35% and 45% of both daily and hourly bacterial
productivities (respectively) was then deduced (Artigas et al., 1999).
In spite of some structural and dynamical changes in the microbial
communitv more and larger phytoplanktonic cells in late spring, enhanced bacterial productivity and microzooplanktonic grazing during
the spring bloom period (Sautour et al, 2000), great variability of both
microbial autotrophic and heterotrophic growth rates (Sautour et al,
1996a; Artigas et al., 2000), general increase of the organic pool with the
advanced season very low sedimentation rates were measured at the base
of the euphotic zone in late autumn (tab. 2; Laborde et al, 1996).
Moreover, primary production appeared to be limited by the phosphate concentration (< 0.05 pM; Herbland et ai, 1998) in spring,
pointing out the importance of recycling processes within the
euphotic layer.
Conclusion
The Gironde dilution plume system was characterized, in spring, as a
“maintenance” system (as defined by Riegman et al., 1993). In this svstem, in spite of the relatively low bacterial grazing losses, the entire microbial food web (including a more or less important viral lysis, not
measured), would represent the main trophic pathway responsible for
the fate ot microplanktonic biomasses (fig. 5). As micro-heterotrophs
can theoretically sustain from 40 to 70% of the nutritional requiremcnts of mesozooplankton in temperate seas (Koshikawa et al., 1996),
zooplankton would be potentially supported bv both heterotrophic
bacteria and autotrophs in this system.
The “microbial loop” and microbial food web processes in addition to an
important role in sustaining the “regenerated” primary production, represented a “microbial barrier” to the organic matter sedimentation (high
exoproteolytic activities measured on aggregates, Delmas et at, 2000;
83% of the fecal pellets cgested in the euphotic zone would not have
reached the deep layers, Sautour et at, 1996b). Thus, bacteria would
have indirectly contributed to the degradation and dissolution of organic particles, completing the phvsical and chemical actions (Lampitt
et al., 1990).
132
phytoplankton outburst in 1995, micro- and nanozooplankters (2200 |um) where responsible for the loss of almost 100% of primary
productivities (tab. 2; Sautour et al., 2000).
On the other hand, bacterial daily losses due to bactetivorism processes
were extremely variable from one dav to another (0-100% of bacterial
production). However, a significant correlation was found between bacterial growth rates with and whithout grazers, in all the situations considered:
a mean grazing pressure of 35% and 45% of both daily and hourly bacterial
productivities (respectively) was then deduced (Artigas et al., 1999).
In spite of some structural and dynamical changes in the microbial
communitv more and larger phytoplanktonic cells in late spring, enhanced bacterial productivity and microzooplanktonic grazing during
the spring bloom period (Sautour et al, 2000), great variability of both
microbial autotrophic and heterotrophic growth rates (Sautour et al,
1996a; Artigas et al., 2000), general increase of the organic pool with the
advanced season very low sedimentation rates were measured at the base
of the euphotic zone in late autumn (tab. 2; Laborde et al, 1996).
Moreover, primary production appeared to be limited by the phosphate concentration (< 0.05 pM; Herbland et ai, 1998) in spring,
pointing out the importance of recycling processes within the
euphotic layer.
Conclusion
The Gironde dilution plume system was characterized, in spring, as a
“maintenance” system (as defined by Riegman et al., 1993). In this svstem, in spite of the relatively low bacterial grazing losses, the entire microbial food web (including a more or less important viral lysis, not
measured), would represent the main trophic pathway responsible for
the fate ot microplanktonic biomasses (fig. 5). As micro-heterotrophs
can theoretically sustain from 40 to 70% of the nutritional requiremcnts of mesozooplankton in temperate seas (Koshikawa et al., 1996),
zooplankton would be potentially supported bv both heterotrophic
bacteria and autotrophs in this system.
The “microbial loop” and microbial food web processes in addition to an
important role in sustaining the “regenerated” primary production, represented a “microbial barrier” to the organic matter sedimentation (high
exoproteolytic activities measured on aggregates, Delmas et at, 2000;
83% of the fecal pellets cgested in the euphotic zone would not have
reached the deep layers, Sautour et at, 1996b). Thus, bacteria would
have indirectly contributed to the degradation and dissolution of organic particles, completing the phvsical and chemical actions (Lampitt
et al., 1990).
132