Structure of planktonic food web in the Gulf of Gabès (Southeastern Mediterranean): potential importance of heterotrophic and mixotrophic microzooplankton

The planktonic food web structure was investigated in a productive Mediterranean gulf during spring and fall. Plankton communities, rates of primary production (PP), bacterial production (BP) and grazing by microzooplankton and mesozooplankton were examined in a station influenced by Saharan dust, St1; a station located in the Boughrara Lagoon, St2; and a polluted station, St3, close to a phosphate industrial site. The high nutrient (12–17 µM) and chlorophyll a (Chl a ) concentrations (3.7–16.9 mg m −3 ) as well as the high rates of PP (1123–2638 mg C m −2 d −1 ) and BP (128–1337 mg C m −2 d −1 ), recorded throughout the sampling period, indicated the eutrophic character of the study site. Microzooplankton and mesozooplankton mostly showed seasonal changes in their composition and grazing rates. During spring, PP was dominated by nano- and micro-sized fractions, but pico-, nano- and microphytoplankton contributed equally to Chl a . Heterotrophic and mixotrophic microzooplankton consumed significant proportions of the daily production for bacterioplankton (50–72%) and size-fractioned phytoplankton (25–86%), whereas herbivorous copepods grazed daily on 13–15% of PP. These trophic links suggested that the multivorous food web prevailed in spring. During fall, picophytoplankton, mainly Synechococcus , dominated the Chl a and PP in St1, where microbivorous microzooplankton (mainly mixotrophic dinoflagellates) grazed ≥ 50% of the production of bacterioplankton and picophytoplankton, while mesozooplankton, dominated by detritivorous copepods, removed only 5% of PP, suggesting a microbial food web. In St2 and St3, Chl a and PP were dominated by large phytoplankton, which was substantially grazed by heterotrophic and mixotrophic microzooplankton (42–62% grazed d −1 ) and copepods (12–25% grazed d −1 ), indicating a carbon channeling throughout the herbivorous food web. The seasonal and spatial change in the planktonic food web implies different efficiencies in the export of carbon. Even in productive waters, picophytoplankton along with microzooplankton, including mixotrophic and heterotrophic organisms, significantly contribute to the ecological functioning of these systems and play a central role in structuring the carbon transfer pathway.