Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia) |
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Authors: | A Sakka Hlaili B Grami Hassine Hadj Mabrouk M Gosselin D Hamel |
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Institution: | (1) Laboratoire de Cytologie végétale et Phytoplanctonologie, Département des Sciences de la Vie, Faculté des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia;(2) Institut des sciences de la mer de rimouski (ISMER), Université du Québec à Rimouski, 310 allée des Ursulines, QC, Rimouski, G5L 3A1, Canada |
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Abstract: | Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The
2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics.
High growth rates (0.9–1.04 day−1), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l−1) and carbon biomass (392–398 μg C l−1) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon
biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several
diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day−1) and biomass of algae (2.9–5.4 μg Chl a l−1 and 209–260 μg C l−1) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly
composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004
246–361 μg C l−1), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%)
in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton
grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during
the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling
of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to
the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom
dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day−1) and a higher carbon demand (165–470 μg C l−1 day−1) than dinoflagellates (0.1–0.5 day−1, 33–290 μg C l−1 day−1). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested
by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These
results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were
more vigorous grazers of the large algae (diatoms) than were dinoflagellates. |
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