Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs |
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Authors: | Joaquim-Justo, Celia Pirlot, Samuel Viroux, Laurent Servais, Pierre Thome, Jean-Pierre Descy, Jean-Pierre |
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Affiliation: | 1 Université de LiègeLaboratory of Animal Ecology and Ecotoxicology, Institut de Chimie, BÂT.B6C, allée du 6 Août 15, Sart-Tilman, B-4000 Liège, Belgium, 2 Facultés Universitaires Notre-Dame de la Paix, URBOLaboratory of Freshwater Ecology (LFE), rue de Bruxelles 61, B-5000 Namur, Belgium and 3 Université Libre de BruxellesEcology of Aquatic Systems, Campus de la Plaine, CP 221, B-1050 Brussels, Belgium |
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Abstract: | Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof 3H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m3 day1 (i.e. from 0.12 to 0.86 g C1bacteria g C1 protozoa day1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m3 day1, i.e. 0.084 g C1 flagellatesg C1 rotifers day1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m3 day1,i.e. 0.014 g C1 ciliates g C1 rotifers day1)in contrast to measurements performed in July (8.72 mg C m3day1, i.e. 0.242 g C1 ciliates g C1 rotifersday1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce. |
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