Rotating disk electrodes to assess river biofilm thickness and elasticity

The present study examined the relevance of an electrochemical method based on a rotating disk electrode (RDE) to assess river biofilm thickness and elasticity. An in situ colonisation experiment in the River Garonne (France) in August 2009 sought to obtain natural river biofilms exhibiting differentiated architecture. A constricted pipe providing two contrasted flow conditions (about 0.1 and 0.45 m s−1 in inflow and constricted sections respectively) and containing 24 RDE was immersed in the river for 21 days. Biofilm thickness and elasticity were quantified using an electrochemical assay on 7 and 21 days old RDE-grown biofilms (t7 and t21, respectively). Biofilm thickness was affected by colonisation length and flow conditions and ranged from 36 ± 15 μm (mean ± standard deviation, n = 6) in the fast flow section at t7 to 340 ± 140 μm (n = 3) in the slow flow section at t21. Comparing the electrochemical signal to stereomicroscopic estimates of biofilms thickness indicated that the method consistently allowed (i) to detect early biofilm colonisation in the river and (ii) to measure biofilm thickness of up to a few hundred μm. Biofilm elasticity, i.e. biofilm squeeze by hydrodynamic constraint, was significantly higher in the slow (1300 ± 480 μm rpm1/2, n = 8) than in the fast flow sections (790 ± 350 μm rpm1/2, n = 11). Diatom and bacterial density, and biofilm-covered RDE surface analyses (i) confirmed that microbial accrual resulted in biofilm formation on the RDE surface, and (ii) indicated that thickness and elasticity represent useful integrative parameters of biofilm architecture that could be measured on natural river assemblages using the proposed electrochemical method

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 Isaac’s 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

When attempts at robbing prey turn fatal

Because group-hunting arboreal ants spreadeagle insect prey for a long time before retrieving them, these prey can be coveted by predatory flying insects. Yet, attempting to rob these prey is risky if the ant species is also an effective predator. Here, we show that trying to rob prey from Azteca andreae workers is a fatal error as 268 out of 276 potential cleptobionts (97.1 %) were captured in turn. The ant workers hunt in a group and use the “Velcro®” principle to cling firmly to the leaves of their host tree, permitting them to capture very large prey. Exceptions were one social wasp, plus some Trigona spp. workers and flies that landed directly on the prey and were able to take off immediately when attacked. We conclude that in this situation, previously captured prey attract potential cleptobionts that are captured in turn in most of the cases

Les genres Adlafia, Kobayasiella, Fallacia, Microstatus et Naviculadicta (Bacillariophycées) recensés dans quelques lacs des Pyrénées françaises

Les diatomées du périphyton de huit lacs des Pyrénées françaises ont été étudiées. Quelques espèces plutôt rares appartenant aux genres Kobayasiella, Fallacia, Microstatus, Naviculadicta ainsi qu’Adlafia bryophila (Petersen) Lange-Bertalot, plus connue, ont été recensées surtout dans les lacs d’Anglade et du Pé d’Estibère. La morphologie détaillée d’Adlafia bryophila, Kobayasiella jaagi (Meister) Lange-Bertalot, Kobayasiella micropunctata (Germain) Lange-Bertalot, Fallacia vitrea (Oestrup) D.G. Mann et Naviculadicta tridentula (Krasske) Lange-Bertalot a été examinée à l’aide du microscope photonique et du microscope électronique à balayage. Kobayasiella subtilissima (Cleve) Lange-Bertalot et Microstatus maceria (Schimanski) Lange-Bertalot ont été observées seulement au microscope photonique

Novel ligature methods for studying sublethal effects of sit-and-wait predators: test using Cordulegaster boltonii (Donovan, 1807) larvae (Anisoptera: Cordulegasteridae)

A novel method of labial palp ligature was tested as a substitute for palp ablation for studying sublethal effects of larvae of C. boltonii on prey populations and their consequences for ecosystem functioning. Two alternative types of ligature were designed to test for neutral or aggressive, but non-lethal, predator-prey interaction effects. Ligature efficiency in preventing prey capture was very high and the effects on larval survival and emergence success were negligible. Potential advantages and drawbacks, compared to other methods, are discussed. The results indicate that this fully reversible method should be applied whenever possible, especially for naturally rare or endangered odon. spp

Etude morphologique de quelques Cymbellaceae des Pyrénées françaises incluant la description d'une espèce nouvelle : Delicata couseranensis sp. nov

Les lacs et les rochers suintants des Pyrénées françaises abritent des communautés diatomiques diversifiées. Les taxons de la famille des Cymbellaceae sont particulièrement nombreux et l’étude morphologique, à l’aide du microscope photonique et du microscope électronique à balayage, de quelques espèces sélectionnées est présentée dans ce travail. L’une des espèces, appartenant au genre Delicata est décrite comme étant nouvelle pour la science. Delicata couseranensis sp. nov. est proche de Delicata judaica (Lange-Bertalot & Krammer) Krammer & Lange-Bertalot par la morphologie ; elle en diffère par les dimensions, le rapport longueur (L) / largeur (l) et le nombre de puncta en 10 µm. Par ailleurs, sept espèces récemment décrites sont examinées : Cymbella perparva Krammer, Cymbella maggiana Krammer, Cymbella simonsenii Krammer, Cymbella neoleptoceros Krammer var. tenuistriata Krammer, Cymbella subhelvetica Krammer, Cymbella lange-bertalotii Krammer et Cymbopleura laeviformis Krammer. Ces espèces sont comparées aux espèces morphologiquement proches et des données sur leur distribution géographique et leur écologie sont fournies

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 μm filtered water in the presence or absence of mesozooplankton. In different experiments, the grazing impact was tested of the calanoïd 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. l−1) were higher than densities in the field (0.1–6.9 ind. l−1). Grazing by microzooplankton was evaluated by comparing growth of phytoplankton in 30 and 200 μm filtered water. Microzooplankton in the 30–200 μm size range included mainly rotifers of the genera Brachionus, Trichocerca and Synchaeta, which were present from 191 to 1777 ind. l−1. Microzooplankton had a significant grazing impact in five out of six experiments. They had a community grazing rate of 0.41–1.83 day−1 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 μl ind.−1 h−1. 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

Cymbopleura pyrenaica sp. nov. (Bacillariophyceae) et d'autres espèces du même genre rarement recensées dans quelques lacs des Pyrénées françaises

Lors d’une étude des diatomées des lacs français des Pyrénées, une nouvelle espèce de Cymbopleura, C. pyrenaica sp. nov., a été mise en évidence. La morphologie détaillée de ce taxon a été examinée à l’aide du microscope photonique (MP) et du microscope électronique à balayage (MEB). La nouvelle espèce est très proche de Cymbella laevis Naegeli mais l’absence de champs apicaux de pores la place dans le genre Cymbopleura. Le caractère ultra-structural le plus frappant est la variabilité morphologique des aréoles. Cymbopleura pyrenaica sp. nov. est présente dans plusieurs lacs mais avec une très faible occurrence; elle tolère un large éventail de la conductivité. Une courte revue des autres espèces de Cymbopleura recensées dans les lacs prospectés est présentée

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 NH4þ 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