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See detailPhytoplankton of the River Loire, France: a biodiversity and modelling study
Descy, Jean-Pierre; Leitao, Maria; Everbecq, Etienne ULg et al

in Journal of Plankton Research (2011)

Most studies of phytoplankton dynamics in lowland rivers have been conducted in regulated rivers. This study deals the phytoplankton of the River Loire, a relatively unregulated river in Western Europe ... [more ▼]

Most studies of phytoplankton dynamics in lowland rivers have been conducted in regulated rivers. This study deals the phytoplankton of the River Loire, a relatively unregulated river in Western Europe, and aims at investigating the factors controlling phytoplankton composition and dynamics in this river. Phytoplankton biodiversity was assessed by pooling data from monthly sampling for 8 years at different sites. A correspondence analysis showed temporal and longitudinal gradients, with a dominance of small centric diatoms and green algae for most of the year, in the middle and lower river sectors. Phytoplankton dynamics were further explored using the POTAMON simulation model, run for the year 2005. The simulations, in agreement with the observations, confirmed the virtual absence of a Stephanodiscus spring peak, and reproduced well the development of small centric diatoms and green algae. Production and loss rates calculated by the model helped us to explain the dynamics of the three main phytoplankton categories, which attained high net production rates due to the low river depth (∼1 m at low discharge), but were subject to high sedimentation losses. Model calculations also showed that P limitation was likely, particularly for green algae, with a reduction of growth rate of up to 35%. In addition, the simulations showed a significant impact of the invasive Asian clam, Corbicula spp., on phytoplankton biomass. This study shows that the factors determining phytoplankton diversity and dynamics in this unregulated river are basically the same as those identified in other lowland rivers, but that key factors are habitat diversity and variation of water level in the river channel. [less ▲]

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See detailThe utilization of polysaccharides by heterotrophic bacterioplankton in the Bay of Biscay (North Atlantic Ocean)
Piontek, Judith; Händel, Nicole; De Bodt, Caroline et al

in Journal of Plankton Research (2011), 33(11), 1719-1735

This study investigates the turnover of polysaccharides by heterotrophic bacterioplankton in the northern Bay of Biscay, a productive marine system on the shelf-break of the temperate Atlantic Ocean ... [more ▼]

This study investigates the turnover of polysaccharides by heterotrophic bacterioplankton in the northern Bay of Biscay, a productive marine system on the shelf-break of the temperate Atlantic Ocean. Field studies were conducted along the shelf-break south of Ireland and west of France (47° 07' 83'' N, 6° 92' 01'' E and 51° 34' 42'' N, 10° 49' 95'' E) during the development of phytoplankton blooms in late spring. Bacterial biomass production (BBP) near the surface ranged from 0.5 to 27.4 nmol C L-1 h-1 in both years. A direct relationship between BBP and the concentration of total polysaccharides strongly suggests the dependence of bacterial growth on the availability of semi-labile organic matter. Concentrations of combined glucose as well as rate constants of extracellular glucosidase activity and glucose uptake were determined to estimate the actual carbon fluxes from bacterial polysaccharide turnover. Results reveal that the degradation of polysaccharides n the upper 100 m of the water column sustained on average a glucose flux of 2.6 mmol C m-2 d-1 i that was available for bacterial uptake. The mean turnover time for polysaccharides was 170 and 165 days for alpha- and beta-glycosidic linked polymers, respectively. Free glucose, the labile hydrolysate of polysaccharides, had a mean turnover time of 4.5 days and supported 2.2 to 18.4% of BBP. The incorporation of glucose into bacterial biomass was stimulated by the availability of inorganic nitrogen. Overall, our results demonstrate that the bacterial recycling of polysaccharides in the Bay of Biscay generates a relevant flux of organic carbon in microbial food-webs and biogeochemical processes. [less ▲]

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See detailTrophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs
Joaquim-Justo, Célia ULg; Pirlot, S.; Viroux, L. et al

in Journal of Plankton Research (2006), 28(9), 857-870

Trophic interactions within the plankton of the lowland River Meuse (Belgium) were measured in spring and summer 2001. Consumption of bacteria by protozoa was measured by monitoring the disappearance of H ... [more ▼]

Trophic interactions within the plankton of the lowland River Meuse (Belgium) were measured in spring and summer 2001. Consumption of bacteria by protozoa was measured by monitoring the disappearance of H-3-thymidine-labelled bacteria. Metazooplankton bacterivory was assessed using 0.5-mu m fluorescent microparticles (FMPs), and predation of metazooplankton on ciliates was measured using natural ciliate assemblages labelled with FMPs as tracer food. Grazing of metazooplanklon on flagellates was determined through in situ incubations with manipulated metazooplankton densities. Protozooplankton bacterivory varied between 6.08 and 53.90 mg C m(-3) day(-1) (i.e. from 0.12 to 0.86 g C-1 bacteria g C-1 protozoa day(-1)). Metazooplankton, essentially rotfiers, grazing on bacteria was negligible compared with grazing by protozoa (similar to 1000 times lower). Predation of rotfiers on heterotrophic flagellates (HFs) was generally low (on average 1.77 mg C m(-3) day(-1), i.e. 0.084 g C-1 flagellates g C-1 rotfiers day(-1)), the higher contribution of HF in the diet of rotfiers being observed when Keratella cochlearis was the dominant metazooplankter. Predation of rotfiers on ciliates was low in spring samples (0.56 mg C m(-3) day(-1), i.e. 0.014 g C-1 ciliates g C-1 rotfiers day(-1)) in contrast to measurements performed in July (8.72 mg C m(-3) day(-1), i.e. 0.242 g C-1 ciliates g C-1 rotfiers day(-1)). The proportion of protozoa in the diet of rotfiers was low compared with that of phytoplankton (< 30% of total carbon ingestion) except when phytoplankton biomass decreased below the incipient limiting level (ILL) of the main metazooplantonic species. In such conditions, protozoa (mainly ciliates) constituted similar to 50% of total rotfier diet These results give evidence that microbial organisms play a significant role within the planktonic food web of a eutrophic lowland river, ciliates providing an alternative food for metazooplankton when phytoplankton becomes scarce. [less ▲]

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See detailIn situ filtration responses of Daphnia galeata to changes in food quality
Darchambeau, François ULg; Thys, I.

in Journal of Plankton Research (2005), 27(3), 227-236

In the stoichiometric study of phyto-zooplankton interactions, a controversy exists about how Daphnia species regulate their feeding rate when submitted to low-quality food (i.e. high dietary carbon ... [more ▼]

In the stoichiometric study of phyto-zooplankton interactions, a controversy exists about how Daphnia species regulate their feeding rate when submitted to low-quality food (i.e. high dietary carbon:phosphorus [C:P] ratio). In this study, we gathered data over 3 years on in situ clearance rates of a Daphnia galeata population, by conducting grazing experiments from April 1998 to October 2000 in the Esch-sur-Sure reservoir (Grand-Duchy of Luxembourg). Observed clearance rates (2.5-13.5 mL individual(-1) day(-1), mean 7.0 mL individual(-1) day(-1)) were correlated with population and environmental variables. Mean body size of Daphnia individuals was the best predictor of clearance rate (r(2) = 0.639), followed by water temperature (r(2) = 0.262) and P concentration in the seston (r(2) <= 0.241). When combined with body size, seston C:P or nitrogen:P ratios provided the best predictive regression model of clearance rate (r(2) = 0.852-0.897). The clearance rate was always correlated negatively with P concentration in the food. Thus, Daphnia reacts to a decrease of food quality (i.e. higher C:P ratio) by an increase of its feeding rate. However, the filtration response to food quality was inhibited by high population density. We suggest that, when food quality is low and population density high, this inhibition may favour fitness of future conspecific generations. In conclusion, our results emphasize the effect of P availability on transfer rate of organic matter between lacustrian autotrophs and herbivores and pose questions about consequent effects on nutrient biogeochemical cycles. [less ▲]

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See detailFeeding of planktonic rotifers on ciliates: a method using natural ciliate assemblages labelled with fluorescent microparticles
Joaquim-Justo, Célia ULg; Detry, Cédric ULg; Caufman, F. et al

in Journal of Plankton Research (2004), 26(11), 1289-1299

A method was developed to allow direct measurements of predation exerted by metazooplankton on ciliates. The method relied on the use of ciliates labelled with fluorescent microparticles (FMP). Optimal ... [more ▼]

A method was developed to allow direct measurements of predation exerted by metazooplankton on ciliates. The method relied on the use of ciliates labelled with fluorescent microparticles (FMP). Optimal labelling conditions were determined with ciliates from cultures (Tetrahymena pyriformis) and with natural ciliate assemblages sampled in a river. Labelled T. pyriformis were used as tracer food to determine gut passage time (GPT) and ingestion rates of the rotifer Brachionus calyciflorus in the laboratory. Predation of metazooplankton from the lowland river Meuse (Belgium) was determined by labelling natural assemblages of ciliates and using them as tracer food for metazooplankters sampled in the river. Optimal labels of ciliates, i.e. sharp distribution of FMP in cells, were obtained with short incubations (10 min) and low FMP concentrations (1 x 10(5) mL(-1)). GPT varied between 30 and 45 min for B. calyciflorus and from 25 up to >35 min for rotifers from the river. The ingestion rate of B. calyciflorus fed with T. pyriformis was 3.3 +/- 0.6 ciliate rot(-1) h(-1), i.e. 1.4 +/- 0.3 ngC rot(-1) h(-1). Metazooplankton species for which the ingestion of ciliates could be measured were the rotifers Keratella cochlearis, Euchlanis dilatata and Synchaeta spp. Ingestion rates measured ranged from 0.4 to 12.5 ngC rot(-1) h(-1). The method proposed proved to be useful in estimating the predation of microplankton on ciliates in semi- in situ conditions; in further developments, labelled natural assemblages of ciliates could be used for in situ incubations with the Haney chamber. [less ▲]

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See detailDynamics of copepod faecal pellets in relation to a Phaeocystis dominated phytoplankton bloom: characteristics, production and flux
Frangoulis, Costa; Belkhiria, Sami; Goffart, Anne ULg et al

in Journal of Plankton Research (2001), 23(1), 75-88

Copepod faecal pellet characteristics and production were measured in spring 1995, 1996 and 1997 in the North Sea Southern Bight in order to define changes due to the development of the phytoplankton ... [more ▼]

Copepod faecal pellet characteristics and production were measured in spring 1995, 1996 and 1997 in the North Sea Southern Bight in order to define changes due to the development of the phytoplankton bloom. Changes were related to the succession from diatoms to a Phaeocystis sp. bloom. Mean monthly pellet production decreased during the Phaeocystis bloom development to 0.27 pellets ind. –1 h –1, approximately 80% lower than before and after the bloom. Although phytoplanktonic pigments showed significant differences between inshore and offshore stations, there was no such significant difference for faecal pellet production. Faecal pellet sinking-rate decreased from 100 to 70 m day –1 during the transition from a diatom- to a Phaeocystis-dominated bloom. This was due to a decrease in pellet density and/or a decrease of the pellet volume. These results supported the idea of lower feeding of copepods under Phaeocystis bloom conditions. As mean seasonal density of copepod faecal pellets was higher (1.37 g cm –3) than in other seas, accidental ingestion of sedimentary material as the cause of this high density is discussed. [less ▲]

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See detailScanning electron microscope study and computer analysis of taxonomic distances of the marin Podonidae (Cladocera)
Meurice, Jean-Claude; Dauby, Patrick ULg

in Journal of Plankton Research (1983), 5

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