MMHg production and export from intertidal sediments to the water column of a tidal lagoon (Arcachon Bay,France)

Hg cycling in biologically productive coastal areas is of special importance given the potential for bioaccumulation of monomethylmercury (MMHg) into aquatic organisms. Field experiments were performed during three different seasons in Arcachon Bay, a mesotidal lagoon (SW France), to assess the variability of the water column concentrations, sediment–water exchanges and potential formation and degradation of MMHg. The objectives were to evaluate the contribution of intertidal mudflats to MMHg production and the various pathways of Hg species export. Dissolved and bulk concentrations of Hg species in the water column downstream of tidal flats were measured throughout several tidal cycles. The Hg benthic fluxes at the sediment–water interface were determined by means of benthic chambers for three different stations. Hg methylation and demethylation potentials were determined in surficial sediments and the water column using isotopic tracers. The tidal surveys demonstrated that benthic remobilization of Hg occurs primarily in association with sediment erosion and advection during ebb tide. However, elevated dissolved Hg concentrations observed at low tide were found to be caused by a combination of pore-waters seeping, benthic fluxes and methylation in the water column. Benthic fluxes were more intense during late winter conditions (median MMHg and inorganic Hg (IHg) fluxes: 64 and 179 pmol m^?2 h^?1, respectively) and subsequently decreased in spring (median 0.7 and ?5 pmol m^?2 h^?1, respectively) and fall (median ?0.4 and ?1.3 pmol m^?2 h^?1, respectively). The trends in methylation and demethylation potentials were at the opposite of the fluxes, two times lower during winter than for spring or fall conditions. In this tidal environment, MMHg production in surface sediments and its subsequent release is estimated to be the major source of MMHg to the water column during winter and spring time. However, during the more productive summer period, the Hg methylation extent in the water column may be very significant and equivalent to the sediment contribution.     

Diel and seasonal vertical distribution of meiobenthic copepods in muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay)

The vertical distribution of meiobenthic copepods was investigated within muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay, France). The aim of the study was to determine if in muddy sediments, as previously established in sandy sediments, meiobenthic copepods migrate vertically according to the seasons or diel periods. Two experimental approaches were used, viz: a three-season comparison was made of the diel vertical distribution of the harpacticoid Canuella perplexa T. & A. Scott (1893) and secondly the depth distribution of a meiobenthic copepod assemblage was followed for a 24 h period, in shallow water subtidal locations. The harpacticoid C. perplexa vertically migrated through the top three centimeters of the sediment, showing diel and seasonal variations in depth distribution. The differential vertical distributions shown by the dominant meiobenthic populations suggest that emergence into the water column may mainly concern surface dwelling copepods. The physical and biological factors affecting seasonal and diel changes in the copepod assemblage of the fish ponds are discussed.     

In situ microbial ecology of hydrothermal vent sediments

Abstract Lipid analysis of a preliminary sampling of the Endeavor Ridge hydrothermal vent site was performed in order to estimate the extent and nature of the diversity of the microbial community. The vent microbial community was found to be highly variable in density and composition. Evidence was found for a dense microbial community of archaebacteria and possibly Thiobacilli in the interior of the flange of a black smoker, a red Beggiatoa -type colony in a sediment sample, and large amounts of polyenoic fatty acids of the type previously found in barophilic eubacteria. Lipid analysis provided a 'snapshot' of the in situ biomass, community structure, and metabolic status of the microbial community, a valuable addition to the techniques available to the microbial ecologist.     

Evaluation of a eutrophic coastal lagoon ecosystem from the study of bottom sediments

Morphological reconstruction and biogeochemical characterisation of the lagoon of Comacchio (Italy) were carried out in order to provide recommendations for the recovery, conservation and sustainable management of the Fattibello-Spavola coastal lagoon ecosystem. Samples were taken in two seasons: July 1997 and November 1998. The irregular morphology of the Fattibello lagoon affects depositional processes and seawater exchange (tidal currents). Several depressions retain part of the dense water of the saline wedge; these stagnant waters became sinks for fine terrigenous and organic matter. The basin is already extensively supplied with N and P compounds from land. The inflow is demonstrated by the large quantity of organic C, N and organic and inorganic P compounds in the superficial sediment. The ratios between the various forms of macronutrients indicate that the organic matter is primarily of autochthonous origin, with relatively low C/N ratios (8.4±0.6 and 8.1±0.6 in July and November, respectively). Shallow areas were almost always oxygenated by tidal currents and thus rich in organisms, with a predominance of molluscs and Ficopomatus enigmaticus. However, the trophic equilibrium of the ecosystem was affected by the accumulation of organic matter in the depressions, favoured by the increased hydrodynamics during the autumn. These accumulations generate high oxygen consumption and release considerable quantities of nutrients into the water column, with the risk of serious dystrophy throughout the basin during the summer. Local dredging and an improvement of the circulation have been suggested and carried out to contain these processes. Reclamation measures in the longer term were proposed.     

In situ sampling of interstitial water from lake sediments

A sampler with a relatively high resolution has been developed, which allows interstitial water to be obtained from lake sediments at well defined depths, without serious disturbance of sediment structure. Oxidation effects are excluded. Sampling time is in the order of a day. Installation requires little additional equipment. The instrument has been developed for use in shallow lakes.     

In situ micro- and macro-Raman investigation of the redox couple behavior in DSSCS

Solar cells based on nanocrystalline TiO₂ sensitized by dyes that contain poly-pyridyne rings have been examined by in situ micro- and macro-Raman spectroscopy under positive and negative bias. Four modes in the low frequency range appear for nearly short-circuit conditions and are attributed to the vibrations of triiodide, bound on the oxidized form of the dye. The intensity and frequency of these modes depend on the applied potential and their behavior is correlated with characteristic regions of the current-potential curves. Our results are supported by polarized Raman experiments and basic theoretical simulations.     

The kinetics of denitrification in permeable sediments

Permeable sediments comprise the majority of shelf sediments, yet the rates of denitrification remain highly uncertain in these environments. Computational models are increasingly being used to understand the dynamics of denitrification in permeable sediments, which are complex environments to study experimentally. The realistic implementation of such models requires reliable experimentally derived data on the kinetics of denitrification. Here we undertook measurements of denitrification kinetics as a function of nitrate concentration in carefully controlled flow through reactor experiments on sediments taken from six shallow coastal sites in Port Phillip Bay, Victoria, Australia. The results showed that denitrification commenced rapidly (within 30 min) after the onset of anoxia and the kinetics could be well described by Michaelis–Menten kinetics with half saturation constants (apparent K_m) ranging between 1.5 and 19.8 μM, and maximum denitrification rate (V_max) were in the range of 0.9–7.5 nmol mL^?1 h^?1. The production of N₂ through anaerobic ammonium oxidation (anammox) was generally found to be less than 10 % of denitrification. V_max were in the same range as previously reported in cohesive sediments despite organic carbon contents one order of magnitude lower for the sediments studied here. The ratio of sediment O₂ consumption to V_max was in the range of 0.02–0.09, and was on average much lower than the theoretical ratio of 0.8. As a consequence, models implemented with the theoretical ratio of 0.8 are likely to overestimate denitrification by a factor of ~3. The most likely explanation for this is that the microbial community is not able to instantaneously shift or optimally use a particular electron acceptor in the highly dynamic redox environment experienced in permeable sediments. In contrast to previous studies, we did not observe any significant rates of oxic denitrification.     

Microbially — related redox changes in a subtropical lake I.In situ monitoring of the annual redox cycle

Using a recently developedin situ multiprobe the redox development in the water column of warm-monomictic Lake Kinneret was investigated during three annual cycles. During the time when sulfide release into the meta- and hypolimnion is initiated, our measurements show a linear relationship, close to the thermodynamic function, between the platinum electrode potential and the amount of sulfide produced by the sulfate reducing bacteria. A change of this relationship during summer stratification coincides with the bloom of the phototrophic sulfur bacteriumChlorobium phaeobacteroides.     

Day and night trophic variations of dominant fish species in a lagoon influenced by freshwater seeps

The aim of this study was to determine the trophic structure and nycthemeral variations in the diet of dominant fish species (Ariopsis felis, Bairdiella chrysoura, Micropogonias undulatus, Eucinostomus gula, Eucinostomus argenteus, Lagodon rhomboides and Sphoeroides testudineus) in Celestun Lagoon, a biosphere reserve located in the southern Gulf of Mexico, and influenced by freshwater seeps. A total of 1473 stomachs were analysed and nine trophic groups were recorded. Bray–Curtis analyses with analyses of similarity (ANOSIM) statistical tests were used to determine two groups of feeding guilds: zoobenthivores and omnivores, with significant differences between time and habitat. The relationships between fish feeding habits, size class and environmental variables were investigated using canonical correspondence analysis (CCA). Most of the species showed a low niche breadth with high specialization towards amphipod consumption, with the exception of L. rhomboides (0·60), which indicated generalist feeding. This study in a protected area is an important source of information for drawing up conservation policies in relation to the management of aquatic resources, and will aid in the establishment of priority areas for conservation.     

Plathelminths in tropical intertidal sediments of northeastern Australia

Assemblages of free-living interstitial plathelminths are described from the tropical coast of northeastern Australia. About one hundred species were recorded from sediments of mangroves, tidal flats, and seagrass beds. The majority (88%) of the plathelminths occurred in only one havitat. Richest in species were Kalyptorhynchia (39), followed by Acoela (17) and Typhloplanoida (16). Over 50% of the species found were represented by one to three individuals, but a few species were frequently encountered. The preferred habitat was sand with 86% of all species, while abundances were higher in muddy sediments. The mean species density was 7 in 10 cm^–2 of sediment, and abundances ranged from 8 to 52 individuals 10 cm^–2 with an average plathelminth abundance of 25 individuals 10 cm^–2. Predatory plathelminths were predominant with up to 77% of the individuals. A comparison is made to plathelminth assemblages in temperate tidal flats.     

Short-term changes of meiofaunal abundance in intertidal sediments

Some species of various meiofaunal taxa may actively emerge from the sediment and swim in the water column, preferably at night. In the water column they get dispersed by tital currents. The hypothesis that this drift may cause significant short-term changes in the abundance of these species was tested. Such changes were verified for harpacticoid copepods and plathelminths. Depending on the hydrographic conditions, abundance changes may be unpredictable or show regularly alternating patterns. In the light of these results, small-scale spatial estimates of abundance become generally very difficult to make in the taxa containing emergent species. It is suggested that reliable averages can only be obtained by repeated or large-scale spatial sampling.     

In situ stimulation of bacterial sulfate reduction in sulfate-limited freshwater lake sediments

Abstract By adding sulfate in the form of solid gypsum, it was possible to transform in situ a predominantly methanogenic sediment ecosystem into a sulfate-reducing one. The concentrations of sulfate, sulfide, methane, acetate, propionate, soluble iron, and manganese were determined in the porewater before and after the transition. Although sulfate was no longer limiting, acetate and propionate continued to accumulate and reached much higher concentrations than under sulfate-limited conditions. Metabolic activities of fermenting bacteria and of sulfate reducers, which belong to the group that incompletely oxidizes organic material, might be responsible for the increased production of volatile fatty acids. The elevated concentrations of soluble Fe(II)²⁺ and Mn(II)²⁺ observed in the porewater stem from iron and manganese compounds which may be reduced chemically by hydrogen sulfide and other microbially produced reducing agents or directly through increased activities of the iron and manganese reducing bacteria. In the horizon with high sulfate-reducing activities the methane concentrations in the porewater were lower than in non-stimulated sediment regions. The shape of the concentration depth profile indicates methane consumption through sulfate reducing processes. The in situ experiment demonstrates the response of a natural microbial ecosystem to fluctuations in the environmental conditions.     

In situ hybridization and chromosome banding in mammalian species

Chromosome banding is often required in conjunction with fluorescent in situ hybridization of labelled probes for chromosome painting, satellite DNA and low-copy sequences to allow identification of chromosomes and simultaneous probe localization. Here, we present a method that reveals both patterns with only one observation step. The band pattern is produced by restriction-enzyme digestion of chromosomes, followed by fixation with paraformaldehyde in PBS, a short chromosome denaturation step in hybridization solution, and then standard in situ hybridization, washing and detection protocols. Using a range of different mammalian species, chromosome-banding patterns were immediately recognizable, although synchronisation procedures normally required for high- resolution G-banding were not needed. Unlike other methods available, only one round of observation is required using a conventional fluorescence microscope, the method works without modification in many species, and in situ hybridization is not used for chromosome identification (allowing multiple targets and minimizing background). The banding pattern is probably generated by a combination of DNA dissolution and heterochromatin reorganisation after enzyme digestion, followed by paraformaldehyde fixation of the new chromatin structure and incomplete denaturation. The method is of widespread utility in comparative genomics and genome organization programmes.     

In situ enzymology of DNA replication and ultraviolet-induced DNA repair synthesis in permeable human cells

Using permeable diploid human fibroblasts, we have studied the deoxyribonucleoside triphosphate concentration dependences of ultraviolet- (UV-) induced DNA repair synthesis and semiconservative DNA replication. In both cell types (AG1518 and IMR-90) examined, the apparent Km values for dCTP, dGTP, and dTTP for DNA replication were between 1.2 and 2.9 microM. For UV-induced DNA repair synthesis, the apparent Km values were substantially lower, ranging from 0.11 to 0.44 microM for AG1518 cells and from 0.06 to 0.24 microM for IMR-90 cells. Control experiments established that these values were not significantly influenced by nucleotide degradation during the permeable cell incubations or by the presence of residual endogenous nucleotides within the permeable cells. Recent data implicate DNA polymerase delta in UV-induced repair synthesis and suggest that DNA polymerases alpha and delta are both involved in semiconservative replication. We measured Km values for dGTP and dTTP for polymerases alpha and delta, for comparison with the values for replication and repair synthesis. Km values for polymerase alpha were 2.0 microM for dGTP and 5.0 microM for dTTP. For polymerase delta, the Km values were 2.0 microM for dGTP and 3.5 microM for dTTP. The deoxyribonucleotide Km values for DNA polymerase delta are much greater than the Km values for UV-induced repair synthesis, suggesting that when polymerase delta functions in DNA repair, its characteristics are altered substantially either by association with accessory proteins or by direct posttranslational modification. In contrast, the deoxyribonucleotide binding characteristics of the DNA replication machinery differ little from those of the isolated DNA polymerases.(ABSTRACT TRUNCATED AT 250 WORDS)     

Potential toxicity of heavy metals in beach and intertidal sediments: A comparative study

The spatial distributions of heavy metals, magnetic susceptibility and granulometric contents have been assessed for beach and intertidal sectors of North-East coast of Tamilnadu, India. Metal concentrations and magnetic susceptibility are high, moderate and low in WER, MER and LER respectively. The measured concentrations are compared with background and toxicological reference values, results insist that in both sectors adverse biological effect is possible in some sites due to the excess Ni and Pb. Calculated three different indexes (PLI, PER and mERM-Q) indicate that in beach and intertidal samples shows moderate and low degree of potential ecological risk respectively. The studied metals might have 21% probability of being toxic in both sectors. Studied metal concentrations, calculated indexes, magnetic susceptibility and clay content are low in intertidal sector. Dominant factors for calculated indexes have been assessed in both sectors. In both cases, all the calculated indexes are positively (low) correlated with magnetic susceptibility and clay.     

Demethylation and cleavage of dimethylsulfoniopropionate in marine intertidal sediments

Abstract Demethylation and cleavage of dimethylsulfoniopropionate (DMSP) was measured in three different types of intertidal marine sediments: a cyanobacterial mat, a diatom-covered tidal flat and a carbonate sediment. Consumption rates of added DMSP were highest in cyanobacterial mat slurries (59 μmol DMSP 1⁻¹) and lower in slurries from a diatom mat and a carbonate tidal sediment (24 and 9 μmol DMSP 1⁻¹ h⁻¹, respectively). Dimethyl sulfide (DMS) and 3-mercaptopropionate (MPA) were produced simultaneously during DMSP consumption, indicating that cleavage and demethylation occurred at the same time. Viable counts of DMSP-utilizing bacteria revealed a population of 2 × 10⁷ cells cm⁻³ sediment (90% of these cleaved DMSP to DMS, 10% demethylated DMSP to MPA) in the cyanobacterial mat, 7 × 10⁵ cells cm⁻³ in the diatom mat (23% cleavers, 77% demethylators), and 9 × 10⁴ cells cm⁻³ (20% cleavers and 80% demethylators) in the carbonate sediment. In slurries of the diatom mat, the rate of MPA production from added 3-methiolpropionate (MMPA) was 50% of the rate of MPA formation from DMSP. The presence of a large population of demethylating bacteria and the production of MPA from DMSP suggest that the demethylation pathway, in addition to cleavage, contributes significantly to DMSP consumption in coastal sediments.     

Nitrogen fixation by blue-green algal communities in the intertidal zone of the lagoon of Aldabra Atoll

Summary All eight types of blue-green algal community sampled from the intertidal sediments of the lagoon of Aldabra showed detectable rates of acetylene reduction, and six of them showed at least some examples of rates considered by previous authors to be rapid. When the reduction rates are related to the chlorophyll a contents of the communities, the maximum rates recorded come in the following order: Hyella balani, Scytonema sp., Hyella balani-Schizothrix sp., Calothrix crustacea, Rivularia sp., Microcoleus chthonoplastes, Hyella balani-purple sulphur bacteria, Pleurocapsa-Chroococcus. With the exception of the mixed Hyella balani-purple sulphur bacterial community, the rate of acetylene reduction was in all experiments greater in the light than in the dark, the difference being significant (P<0.05) in the majority of cases. The rates were similar in all experiments whether incubation was carried out with sea or brackish water. It seems probable that nitrogen fixation by blue-green algal communities makes an important addition to the lagoon ecosystem, and that not only heterocystous, but also some non-heterocystous, species are involved. Among the latter, the data for Hyella balani and Microcoleus chthonoplastes are especially convincing.     

Nereis diversicolor effect on the stability of cohesive intertidal sediments

The effect of the polychaete Nereis diversicolor on the stability of natural cohesive sediments was investigated in the laboratory. Three densities (450, 600 and 1200 ind m⁻²) of N. diversicolor were used. Sediment shear strength was measured using a cone penetrometer. Sediment erodability was assessed using an annular flume (current velocities from 5 to 55 cm s⁻¹) in which flow velocity was increased incrementally, and water sampled to quantify suspended material in order to derive critical erosion velocity and erosion rates. At low current velocities ( <25 cm s⁻¹), we found N. diversicolor to have a stabilising effect, reflected by an increase of up to 20% in the critical erosion velocity. This is related to an enhancement of ~50% in shear strength, due probably to gallery building activities, responsible for the promotion of lateral compaction, an increase in the area of the sediment–water interface, and enhanced microphytobenthos production. Once the sediment began to erode, the stabilising effect of N. diversicolor reverses, leading to an increase of up to 40% in eroded matter due to compaction, which resulted in the erosion of larger aggregates. The balance between the effect of N. diversicolor on herbivory and microphytobenthos production due to the presence of galleries is discussed. Our results indicate that neither chlorophyll a, nor shear strength nor critical erosion velocity are good indicators of erodability. This underlines the need to include biogeochemical processes in any realistic sediment transport model.     

In situ biochemical and bacterial variation of sediments enriched with mussel biodeposits

In order to estimate the qualitative variation of sedimented mussel deposits, biochemical and microbial measurements were undertaken after a sediment enrichment with fresh faeces and pseudo-faeces collected in a mussel farming area (Carteau, Gulf of Fos).During two months, cores sampled were investigated at three stations: a first station enriched with mussel deposits, a second considered as a reference station without mussel deposits and occasionally a third corresponding to a continuously enriched sediment under a rope culture.Carbohydrate, organic carbon and nitrogen contents in the sediments gave evidence of a short term variation after sedimentation.Bacterial production increased rapidly at the enriched station and returned to its initial level 8 days later. At this station, exoglucosidasic activity of bacteria, low at the beginning, presented a maximum two weeks after enrichment whereas exoproteolytic activity, which was high in the biodeposits, decreased in the course of the first week. These exoenzymatic activities were significantly higher at the enriched station than at the reference stations. Carbohydrate measurements were in agreement with these results.Degradation rate of biodeposits is discussed on the basis of exoenzymatic activity, organic carbon, nitrogen and free amino acid content at the three stations.