25 research outputs found
The predation impact of juvenile herring Clupea harengus and sprat Sprattus sprattus on estuarine zooplankton
The consumption of estuarine copepods by juvenile herring and sprat during estuarine residency was estimated using fish biomass data and daily rations calculated from two models of feeding in fish: a bioenergetic model and a gastric evacuation model. The bioenergetic model predicted daily rations that were, on average, three times higher than those estimated by a model based on field records of stomach contents. The biomass of herring and sprat in the estuary was negatively correlated with the daily ration suggesting that the clupeid fish populations were resource-limited. Copepod production decreased towards the winter and peaked in spring and summer. The relative importance of predation changed seasonally in function of the migration pattern of herring and sprat. In the spring and the summer, in situ production ofâŁcopepod biomass was higher than the in situ consumption by fish. During the fall and the winter, consumption exceeded production. This suggests that topâdown control exerted by marine pelagic fish may be an important force structuring estuarine copepod populations
New occurrence of Lecane decipiens (Murray, 1913) and some other alien rotifers in the Schelde estuary (Belgium)
Three alien rotifers were found in the Schelde estuary in spring and summer 2002-2003. Lecane decipiens and Brachionus variabilis, with a maximum of respectively 6 and 4 ind L-1, were rare and quite localized in the freshwater tidal reach, corresponding to the stage III (non invasive species) in Colautti and Mac Isaacâs invasion terminology. This record of L. decipiens is the first confirmed one in Europe. Keratella tropica was generally present in the freshwater reach but, more tolerant to salinity, it was also found in brackish water. With its maximum abundance of 18 ind L-1, K. tropica appeared to be invasive, with a stage IVa (widespread but rare) at the scale of the Schelde estuary. A description with illustrations of these introduced species is given with some information about their ecology
Phytoplankton community structure and environment in the Kenyan waters of Lake Victoria
1. Phytoplankton species composition, numerical abundance, spatial distribution and total biomass measured as chlorophyll a concentration were studied in relation to environmental factors in September 1994 (dry season) and March 1995 (rainy season), respectively, in the Kenyan waters of Lake Victoria; 103 species were recorded. 2. Blue-green algae (Cyanophyceae) were most diverse, followed by diatoms (Bacillariophyceae), green algae (Chlorophyceae) and dinoflagellates (Dinophyceae). 3. Twinspan separated the phytoplankton communities in the Nyanza Gulf and those in the open lake during both seasons. During the dry season, the Nyanza Gulf was strongly dominated by blue-greens, while diatoms dominated in the open lake. During the rainy season, blue-greens remained dominant in the Nyanza Gulf although the number of species found was lower than during the dry season; in the open lake, blue-greens replaced diatoms as the dominant group and there were more species than in the dry season. 4. Canonical correspondence analysis indicated that the phytoplankton species distribution was significantly correlated with turbidity during the dry season and with SiO2 during the rainy season. Chlorophyll a concentrations ranging from 2.0 to 71.5 mg m-3 in the dry season and 2.0-17.2 mg m-3 in the rainy season confirm earlier reports of increasing phytoplankton biomass in Lake Victoria since the 1960s
Grazing by meso- and microzooplankton on phytoplankton in the upper reaches of the Schelde estuary (Belgium/The Netherlands)
In contrast with the marine reaches of estuaries, few studies have dealt with zooplankton grazing on phytoplankton in the upper estuarine reaches, where freshwater zooplankton species tend to dominate the zooplankton community. In spring and early summer 2003, grazing by micro- and mesozooplankton on phytoplankton was investigated at three sites in the upper Schelde estuary. Grazing by mesozooplankton was evaluated by monitoring growth of phytoplankton in 200 ÎŒm filtered water in the presence or absence of mesozooplankton. In different experiments, the grazing impact was tested of the calanoĂŻd copepod Eurytemora affinis, the cyclopoid copepods Acanthocyclops robustus and Cyclops vicinus and the cladocera Chydorus sphaericus, Moina affinis and Daphnia magna/pulex. No significant grazing impact of mesozooplankton in any experiment was found despite the fact that mesozooplankton densities used in the experiments (20 or 40 ind. lâ1) were higher than densities in the field (0.1â6.9 ind. lâ1). Grazing by microzooplankton was evaluated by comparing growth of phytoplankton in 30 and 200 ÎŒm filtered water. Microzooplankton in the 30â200 ÎŒm size range included mainly rotifers of the genera Brachionus, Trichocerca and Synchaeta, which were present from 191 to 1777 ind. lâ1. Microzooplankton had a significant grazing impact in five out of six experiments. They had a community grazing rate of 0.41â1.83 dayâ1 and grazed up to 84% of initial phytoplankton standing stock per day. Rotifer clearance rates estimated from microzooplankton community grazing rates and rotifer abundances varied from 8.3 to 41.7 ÎŒl ind.â1 hâ1. CHEMTAX analysis of accessory pigment data revealed a similar phytoplankton community composition after incubation with and without microzooplankton, indicating non-selective feeding by rotifers on phytoplankton
Looking for general trends in trophic interactions among estuarine micro- and mesozooplankton
Trophic interactions among micro- and mesozooplankton were investigated in the Schelde estuary. Field populations of plankton were separated by selective filtrations (30 and 200 mm). Predation was measured by comparing ciliate and rotifer abundance in treatments with and without potential predators (cladocerans, cyclopids, the calanoid Eurytemora affinis and rotifers). To deal with variability in the data, a cross-calculation method using all replicates separately is proposed. In order to look for general trends in predation behaviour, the predation rates were ranked and analysed in relation to ranked characteristics of the potential prey: numerical abundance, growth rate, individual and population biomass. Cyclopids fed selectively on the ciliates having the highest population biomass and growth rates among the ciliate population. Cyclopids selected the least abundant rotifers. No trends were detected for cladocerans and E. affinis. As predators, rotifers selected the least abundant ciliates in spring. Although no general selectivity patterns for the predators studied can be drawn yet, the potential of the method employed is illustrated and discussed. Its main advantage is the possibility to extend the obtained data set with data from new experiments as well as with extant data on selectivity of the predators
Response of zooplankton to improving water quality in the Scheldt estuary (Belgium)
Data obtained from 14 years of monthly samplings (1996e2009) were used to investigate the response of the crustacean zooplankton community to improving water quality in the Scheldt estuary. A strong reduction of poor water quality indicators, such as NH4ĂŸ and BOD5, as well as an increase in oxygen and in chlorophyll a concentrations were observed during the study period. During the study period, important changes were observed in the zooplankton community composition and spatial distribution. From 2007 onwards, most of the calanoid population, previously mainly found in the brackish water reach of the estuary, moved to the freshwater, where they reached higher abundances than previously observed. Simultaneously, cyclopoids populations strongly decreased in freshwater while cladocerans did not change their abundance, except during years with high chlorophyll a concentrations. Redundancy analyses (RDA) showed that the variability within the calanoid population can be explained by the improvement in water quality. Variability within the cyclopoids and cladoceran community is mainly explained by chlorinity and chlorophyll a concentrations. Their presence in the most polluted upstream area until 2007 suggests they are more tolerant to poor water quality than calanoids. Several hypotheses to explain the disappearance of cyclopoids after the move of calanoids to the freshwater are presented and discussed
Spatial spring distribution of the copepod Eurytemora affinis (Copepoda,Calanoida) in a restoring estuary, the Scheldt (Belgium)
The spatial spring distribution of Eurytemora affinis (adults and C5) in the Scheldt estuary (Belgium) brackish and freshwater reacheswas studied in between 1996 and 2007. The bulk of the E. affinis population being generally situated in the brackish water reach (salinity > 0.5); we studied which environmental factors are responsible for its recent sporadic occurrence in the freshwater estuarine reach. Using PLS analysis, it is shown that its presence upstream is limited by a sufficient oxygen concentration (>4mg l-1) that is associated with temperature. Not only are the environmental conditions in the upstream zone important, but also the frequent presence of an O2 minimum zone in the mid-estuary (O2 min < 1.3mg l-1) seems to block the movement of the downstream E. affinis population in an upstream direction. Occasionally, the bulk of the population is however situated upstream. During these periods, high E. affinis abundancewas also observed in the Durme tributary. Our findings suggest the possibility to use E. affinis as an âindicatorâ species ofwaterquality, but also lead us to stress the necessity to consider conditions over the entire estuary when studying restoration effects, not exclusively in the zone of interest
Retention of nutrients, suspended particulate matter and phytoplankton in a pondage associated with a run-of-the-river type hydroelectric power plant
Reservoirs associated with run-of-the-river type hydroelectrical power plants (i.e. pondages) have short water residence times. For this reason, pondages are thought to have a limited impact on the fluxes of particles and solutes transported by rivers. The Malause reservoir (South West France) is such a pondage. Fed by both the Garonne and Tarn rivers, it has a water residence time of only a few days. Incoming and outgoing fluxes of nutrients, suspended particulate matter (SPM) and phytoplankton were measured weekly over the course of 1 year. Mass balance calculations showed that Malause pondage retained 24% of soluble reactive phosphorus (SRP) supplied by both rivers on an annual basis. SRP retention occurred mostly in springâsummer,pointing to biological uptake. In addition, the pondage was a sink for SPM and phytoplankton, retaining 39% of SPM and 14% of chlorophyll a supplied by both rivers on an annual basis. The retention efficiency appeared to be constrained by water temperature and residence time. The pondage was a source of phytoplankton during summer, when temperature and water residence time was high. The pondage was a sink for SPM when water residence time was low (<1 day). Our observations highlight the need to reconsider the impact of minor hydrological discontinuities on the functioning of the river continuum. The shallow depth of the pondage and the presence of dense stands of submerged macrophytes have probably favoured the retention of nutrients and the sedimentation of particles within the pondage
Dynamique de transfert des matiĂšres organiques et inorganiques le long du continuum fluvial de la Garonne (impact de la retenue de Malause)
Rivers are mainly responsible for organic and inorganic matter transfers from continents to oceans. These matters can be separated by filtration (generally at 0,45 or 0,22 m) into particulate and dissolved phases. The behaviour of organic fractions depends on their origin. The labile fractions are often rapidly mineralized during their transfer, whereas the refractory fractions have a conservative behaviour. We studied the dynamic of all organic and inorganic fractions (dissolved and particulate) along the river continuum of the middle Garonne as soon as accross the Malause reservoir. This dam represents the last hydrological discontinuity along the river continuum. Our results show that, over all the hydrological periods, the river continuum of the middle Garonne is an important sink for suspended matters, POC and PON, while it is a source for phytoplankton. Chla and dissolved silica are stored during the low water periods and remobilized during the high flows. TDP only shows important remobilizing downstream from the urban agglomeration of Toulouse. Retentions of matters during the high water periods take place along the river continuum in meanders and in areas covered by dense riparian vegetation. During low water periods, retentions are due to biological uptakes (vegetation, phytoplankton, biofilm, bacteria ). Adsorption processes on periphyton-sediment complexes are also responsible for storage of some elements, particularly phosphorus. Exportations towards the macroporous riverine environment also contribute to matter storage (TSM, DOC). The phytoplanctonic production remains very low along the river continuum. It is supplied to the Garonne river by benthic algae drift which could be produced by biofilm. Nevertheless, the main phytoplankton production originates from the different tributaries. The isotopic signatures ( 13C) of organic matters (POC, DOC) carried by the Garonne river show very few differences between the different hydrological periods. On the opposite, nitrogen isotopic signatures ( 15N) exhibit a strong enrichment during the low water periods on the different tributaries. This 15N increase is due to a high microbial activity which is responsible for biodegradation of nitrogen from sewage effluents. Specific surface areas of Garonne river bottom sediments are very low compared to other world rivers (Amazon, Mississippi) because of low clay contents in the Garonne bottom sediments. These low clay contents did not allow us to put in evidence a relationship between POC content and specific surface areas. The Malause reservoir shows on a yearly basis retentions of TSM, POC, phytoplankton, Chla and phosphorus, contrary to the river continuum, and, to a lesser extend, of dissolved silica. We can observe a low remobilizing of DOC and TDN. In the reservoir, the phytoplankton is mainly supplied by the Tarn tributary. Even if we can observe a net Chla production within the reservoir during low water periods, we never found specific phytoplanctonic species in the reservoir. Those measured during low water periods, originate from spring and summer blooms in the Tarn tributary. The Garonne contribution to the Malause reservoir phytoplankton is low and originate mainly from benthic diatoms. Whereas in the Tarn tributary, the production is rather pelagic.POC contents in the reservoir bottom sediments show a clear increase. The Malause reservoir represents a discontinuity which is trapping the POC, contrary to other sediment deposition areas like estuaries and deltas in which POC is partly mineralized. Carbon and nitrogen isotopic signatures exhibit a sequence of sediment deposits (13C impoverished and 15N enriched, and vice versa), showing an overlapping of sedimentary materials from different origins (soil erosion, phytoplankton production). It corresponds also to a mixed influence of Garonne and Tarn river contributions to the sedimentary deposits of Malause reservoirLes fleuves sont Ă l'origine des transferts de matiĂšres organiques et inorganiques des continents vers les ocĂ©ans. Ces matiĂšres peuvent ĂȘtre sĂ©parĂ©es par filtration (gĂ©nĂ©ralement Ă 0,45 ou 0,22 m) entre une phase particulaire et une phase dissoute. Le devenir des formes organiques va dĂ©pendre de leurs origines. Les formes labiles sont souvent rapidement minĂ©ralisĂ©es lors de leur transfert tandis que les formes rĂ©fractaires ont un comportement conservatif. Nous avons Ă©tudiĂ© la dynamique de toutes les formes (dissoutes et particulaires) de matiĂšres organiques et inorganiques le long du continuum fluvial de la Garonne moyenne ainsi que dans la retenue de Malause. Cette retenue constitue la derniĂšre discontinuitĂ© hydrologique sur le continuum fluvial. Nos rĂ©sultats montrent que le continuum fluvial de la Garonne moyenne est un important puits de matiĂšres en suspension, COP, NOP et une source de phytoplancton, toutes pĂ©riodes hydrologiques confondues. Chla et silice dissoute sont stockĂ©es en pĂ©riodes de basses eaux et remobilisĂ©es en pĂ©riodes de hautes eaux. Seul le phosphore total dissous montre une forte remobilisation Ă l'aval de l'agglomĂ©ration toulousaine. Les rĂ©tentions de matiĂšres ont lieu en hautes eaux sur le continuum au niveau des mĂ©andres et dans les zones couvertes de vĂ©gĂ©tation riparienne dense. En Ă©tiage les rĂ©tentions sont dues Ă la consommation biologique (vĂ©gĂ©tation, phytoplancton, biofilm, bactĂ©ries, ). Les phĂ©nomĂšnes d'adsorption sur le complexe pĂ©riphyton-sĂ©diments sont Ă©galement responsables du stockage de certains Ă©lĂ©ments, notamment le phosphore. Des exportations vers le milieu macroporeux du fleuve contribuent Ă©galement aux stockages de matiĂšres (MES, COD). La production phytoplanctonique reste faible sur le continuum fluvial. Elle est apportĂ©e sur la Garonne par la dĂ©rive des algues benthiques qui pourraient provenir du biofilm. L'essentiel de la production du phytoplancton est cependant apportĂ©e par les affluents. Les signatures isotopiques ( 13C) des matiĂšres organiques (COD, COP) transportĂ©es par la Garonne montrent une trĂšs faible diffĂ©rence entre les pĂ©riodes hydrologiques. Les signatures isotopiques de l'azote ( 15N) montrent par contre un trĂšs fort enrichissement en Ă©tiage au niveau des affluents. Cet enrichissement traduit une forte activitĂ© microbienne qui est Ă l'origine d'une biodĂ©gradation de l'azote issu des rejets anthropiques. L'analyse des surfaces spĂ©cifiques des sĂ©diments de fond du continuum montre que la Garonne possĂšde des surfaces faibles par rapport Ă d'autres fleuves du monde (Amazone, Mississipi) du fait des teneurs faibles en argiles dans ses sĂ©diments de fond. Ces faibles teneurs en argiles n'ont pas permis de mettre en Ă©vidence de relation entre les teneurs en COP et les surfaces spĂ©cifiques. La retenue de Malause montre Ă l'Ă©chelle annuelle une rĂ©tention de MES, COP, phytoplancton, Chla et phosphore, contrairement au continuum fluvial, et dans une moindre mesure, de la silice dissoute. On observe une faible remobilisation de COD et NTD. Le phytoplancton de la retenue est apportĂ© principalement par le Tarn, bien que nous observions une nette production de Chla en Ă©tiage dans la retenue, nous n'avons pas trouvĂ© d'espĂšces spĂ©cifiques Ă la retenue. Celles qui se dĂ©veloppent en basses eaux sont issues des blooms printanier et estival du Tarn. La contribution de la Garonne au phytoplancton de Malause est faible et vient des diatomĂ©es essentiellement benthiques alors que le Tarn montre une production plutĂŽt pĂ©lagique. Les teneurs en COP des sĂ©diments de fond de la retenue montrent une nette augmentation. On est donc dans le cas d'une discontinuitĂ© qui piĂšge le COP contrairement Ă d'autres zones d'accumulation de sĂ©diments tels les estuaires ou les deltas oĂč le COP est en partie minĂ©ralisĂ©. L'Ă©tude des signatures isotopiques du carbone et de l'azote montre une succession de dĂ©pĂŽts sĂ©dimentaires (appauvris en 13C et enrichis en 15N puis l'inverse), mettant en Ă©vidence une superposition de matĂ©riaux d'origines diffĂ©rentes (Ă©rosion des sols, production phytoplanctonique). Cette succession traduit aussi la double influence de la Garonne et du Tarn sur les dĂ©pĂŽts sĂ©dimentaires de la retenue de Malause.TOULOUSE-ENSAT-Documentation (315552324) / SudocSudocFranceF
New occurrence of Lecane decipiens (Murray, 1913) and some other alien rotifers in the Schelde estuary (Belgium)
International audienceThree alien rotifers were found in the Schelde estuary in spring and summer 2002-2003. Lecane decipiens and Brachionus variabilis, with a maximum of respectively 6 and 4 ind L-1, were rare and quite localized in the freshwater tidal reach, corresponding to the stage III (non invasive species) in Colautti and Mac Isaacâs invasion terminology. This record of L. decipiens is the first confirmed one in Europe. Keratella tropica was generally present in the freshwater reach but, more tolerant to salinity, it was also found in brackish water. With its maximum abundance of 18 ind L-1, K. tropica appeared to be invasive, with a stage IVa (widespread but rare) at the scale of the Schelde estuary. A description with illustrations of these introduced species is given with some information about their ecology