Guy Bachelet

Open Atlantic Coast of Europe — A Century of Introduced Species into French Waters

This paper aims to establish a comprehensive list of exotic species introduced into Atlantic French waters over the last century as well as their origin, and the vectors involved in the process. At least 104 nonindigenous species have been recorded in the Atlantic and Channel coastal waters. Among those species, only a few (Bonamia ostreae, Crepidula fornicata) have resulted in highly significant negative impacts and side-effects on the coastal activity. In contrast, the introduction of the Japanese oyster Crassostrea gigas can be considered to be successful, sustaining a large industry counting more than 4,000 companies and 150,000 tons of yearly production. Over the last decade, French and European laws have strenghtened the controls over exotic species introductions, limiting their impacts. These new regulations and increased awareness from the scientific community with regard to coastal industry and biodiversity have reduced significantly the cases of deliberate introduction, while the risk analysis requirement has improved the overall management. However, in spite of these improvements, cases remain significant, mostly by accidental and/or increased vectors of introduction such as the ballast waters and sediment. Moreover, global changes should now be considered as a major issue, likely inducing future geographic distribution changes over a large European coastal range. Understanding the invasion population dynamics for exotic species remains a challenging research issue currently addressed by the French INVABIO multidisciplinary program.

The choice of a sieving mesh size in the quantitative assessment of marine macrobenthos: a necessary compromise between aims and constraints

Abstract The efficiency of several screen sizes in retaining macrofaunal organisms was tested in subtidal and estuarine intertidal communities. Although macrobenthos is currently defined as metazoans retained by a 1 mm mesh, sieving efficiency of this screen was found to vary between 20 and 70%, when compared with 0·5 mm meshes. Furthermore, 25–65% of specimens belonging to macrofaunal species passed through a 0·5 mm mesh, which shows that sieves finer than 1 or 0·5 mm clearly improve the quantitative collection of macrobenthos. Sieving efficiency displayed considerable seasonal variation due to settlement pulses, and differed according to phyla (generally, crustaceans > polychaetes > bivalves) and between species within a phylum. In some cases, species richness, the diversity index and evenness were influenced by sieve mesh size. Even though the subdivision of benthos in macro- and meiofauna reflects a real pattern, the boundary fixed between them at about 1·0–0·5 mm seems too clear-cut, due to the protracted stay of juvenile macrofauna in the meiofaunalsized class and a number of methodological factors and morphological features. The mesh selected for a particular task should be chosen with a clear idea of the study aims. It is thus suggested that a 1·0 mm mesh is sufficient only for biomass estimates or bionomics studies, whereas sieves with 0·1–0·2 mm mesh openings should be used in studies involving population dynamics of macrofauna, to provide an adequate estimate of abundances of individuals in small size classes.

Uniform variation in genetic traits of a marine bivalve related to starvation, pollution and geographic clines

Consistent patterns of genetic variation in the marine bivalve Macoma balthica (L.) were found after exposure to low levels of copper, starvation, and along geographic clines. The geographic clines were related to temperature and salinity. Genetic differences were primarily found in the LAP (Leucine aminopeptidase) locus; under stress the frequency of one specific allele, and thereby the heterozygosity, decreased strongly. The degree of changes depended on age and condition of the animals; adult animals showing stronger changes than juveniles. Low concentrations of copper yielded stronger genetic changes than high concentrations. It is concluded that genetic diversity and stress-sensitivity of species change predictably along a gradient in response to environmental harshness. Animals near the southern limit of their range were more sensitive to additional stress.

Recruitment and year-to-year variability in a population of Macoma balthica (L.)

Because of methodological problems, macrobenthic studies usually neglect the juvenile stages of invertebrate communities, due to the fact that appearance of recruits in samples is only detected some weeks or even months after their true recruitment. During this period, the temporary meiobenthos undergoes high rates of mortality. From year to year, juvenile survival rate is thus responsible for temporal patterns observed in adult population densities.The results presented here relate to the population dynamics of the tellinid bivalve Macoma balthica (L.). A study of temporary meiobenthos was conducted over two consecutive years in an intertidal Macoma-community located at the mouth of the Gironde Estuary in southwest France. Sampling of juvenile stages required short intervals (2 weeks) between successive samplings and a fine sieving mesh size (63 µm). Other population parameters, such as temporal patterns in density, reproductive cycle, and individual growth, were recorded.Recruitment processes showed a year-to-year variability, with regard to settlement density, settlement period, and survival rate. In 1983, recruitment was moderate and protracted over several months. Only one main recruitment period was detected in 1984, resulting in a high juvenile density. In a previous study (1977), by contrast, recruitment was almost non-existent.This variability is discussed as a function of climatic and sedimentological conditions which prevailed in the estuary throughout the study period. However, none of these physical factors appeared to underlie the recruitment fluctuation in Macoma balthica. It is suggested that biological interactions are of prime importance in regulating population densities in this community.

Effect of macrofaunal bioturbation on bacterial distribution in marine sandy sediments, with special reference to sulphur-oxidising bacteria

Abstract We have studied the impact of the bioturbating macrofauna, in particular the lugworm Arenicola marina and the bivalve Cerastoderma edule , on abundances and distribution patterns of total bacteria and of bacteria of selected functional groups in sandy intertidal sediments. The selected groups comprised the colourless sulphur-oxidising bacteria and the anoxygenic phototrophic bacteria, which are expected to occupy small zones at the oxygen–sulphide interface in stable (non-bioturbated) sediments. The presence of a wooden wreck buried in the sediment at 10 cm depth within a large area of intertidal sand flat colonised by lugworms provided a unique opportunity to confront field observations with laboratory simulations. The site with the wooden wreck, which was used as control site, was devoid of both A. marina and C. edule , while the composition of the rest of the zoobenthic community was rather similar to that of the surrounding area. In the field, the density of total bacteria was approximately one order of magnitude higher in the control site than in the natural (bioturbated) site. This can be explained by the higher contents of silt and clay particles (higher surface-area/volume ratio) and higher total organic-carbon contents found at the control site. It appears that the presence of macrofauna affects sedimentation processes, which indirectly influence bacterial dynamics. Samples from the control site have been incubated in the laboratory with A. marina and C. edule added (bioturbated core), while an unamended core served as a control. The laboratory experiments contrasted with the field observations, because it was found that total bacteria were actually higher in the deeper layers of the bioturbated core. Moreover, the populations were more homogeneous (less stratified) and colourless sulphur bacteria were on average less numerous in the bioturbated core. In general, laboratory incubations resulted in a decrease of total bacteria with a concomitant increase of colourless and phototrophic sulphur-oxidising bacteria and thus in modifications of the bacterial community structure. Hence, our results demonstrate that care must be taken in extrapolating results from laboratory experiments (e.g. mesocosm research) to field situations.

A comparative study on the relation between copper and condition in marine bivalves and the relation with copper in the sediment

The relation between condition and copper concentration was assessed in three bivalve species (the Baltic clam Macoma balthica, the cockle Cerastoderma edule and the mussel Mytilus edulis) from several Dutch and French estuaries. In general, the copper concentration in the bivalves was negatively related to condition. The slope of this relation was strongest in the Baltic clam M. balthica, with an almost inversely proportional change of Cu concentration at a change of condition, indicating an almost constant Cu content (body burden) per individual, i.e. no elimination or accumulation of Cu. In the mussel M. edulis, the slope of this relation was absent or the weakest, with an almost uniform Cu concentration at a change of condition, indicating an almost proportional elimination (or accumulation) of Cu at a decrease (or increase) of the condition. Thus in Baltic clams the copper body burden is kept at an almost constant level, whereas in mussels the copper concentration is almost constant. In Baltic clams, but not in mussels and cockles, spatial differences were related to copper concentrations in the sediment. It is discussed that mussels maintain a homeostatic Cu concentration within a short time-period (days; copper highly exchangeable by complexation to metallothioneins), whereas the Cu concentration in Baltic clams is more slowly regulated (several weeks; fossilization of copper in mineral granules) and remains on an average in (partitioning) equilibrium with the Cu concentration in the sediment. Therefore, the Baltic clam is a better indicator of sediment copper pollution than the commonly used mussel. In addition, the observed differences between species were compared with differences in their life-strategies. The Cu concentration in the sediment is strongly related to the silt fraction (< 16 μm) of the sediment.

The respiratory performance and survival of the bivalve Macoma balthica (L.) at the southern limit of its distribution area: a translocation experiment

The hypothesis was tested that animals near their extreme Southern limits, due to high temperatures, have a high respiration rate, whereby they reach an extreme low weight-index and ultimately disappear. At estuarine stations the respiration rate of Macoma balthica (L.) (Baltic clam) did not show interseasonal changes, indicating seasonal acclimation, but within the season the respiration increased with increasing temperature, indicating the absence of short-term acclimation. In clams translocated from the Netherlands towards the Bidasoa estuary, 200 km South of their Southern distribution limit, their respiration rate was higher and weight-index lower than in specimens living in Dutch estuaries. Irrespective of an effect of the temperature, clams exposed in experiments to water from Bidasoa showed a higher respiration than clams exposed to water from the other stations. Moreover, at non-estuarine stations with a low food content, the clams showed reversed acclimation, i.e., the respiration rates in winter were much lower than summer rates, most probably a strategy to conserve energy by means of a depressed metabolism. A weight index of 5 mg DW/cm(3) and glycogen content of 2% DW are suggested as the minimal values below which the metabolic energy balance of Baltic clams becomes more negative and the clam population disappears. It was concluded that factors other than temperature influenced the respiration and weight-index of clams, and hence their presence or absence, e.g., food concentration, innate seasonal cycles, and possible pollutants in the water.

Sensitivity to stress in the bivalve Macoma balthica from the most northern (Arctic) to the most southern (French) populations: low sensitivity in Arctic populations because of genetic adaptations?

The stress sensitivity, determined in copper exposure experiments and in survival in air tests, and the genetic structure, measured by means of isoenzyme electrophoresis, were assessed in populations of the Baltic clam Macoma balthica (L.) from its southern to its northern distribution limit, in order to test the hypotheses that near the distribution limit the clams would be more stress sensitive and would have a lower genetic variability. The populations in west and north Europe show a strong genetic resemblance. The populations in the sub-Arctic White Sea are genetically slightly different, and show a low stress sensitivity. The populations in the Arctic Pechora Sea are genetically very distant from the other populations, and show the lowest stress sensitivity. Near the southern distribution limit, in agreement with the hypotheses, genetic variability is low and stress sensitivity high. On the other hand, in contrast to expectation, near the northern distribution limit, in the populations of the Pechora Sea, the genetic variability was higher, thus not reduced, and the stress sensitivity was low compared to all other populations. Yet, it remains a question if such is due to gradual physiological acclimatization (and ongoing differential selection) or to genetic adaptation.

Sensitivity to stress of the estuarine bivalve Macoma balthica from areas between the Netherlands and its southern limits (Gironde)

Abstract Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the speciess southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.

Non-selective settlement of Mercenaria mercenaria (L.) larvae in short-term, still-water, laboratory experiments

Abstract Sediment selection by settling larvae of the hard clam Mercenaria mercenaria (L.) was determined in 4-h, still-water, laboratory experiments where larvae were given a choice between two highly contrasting sediment treatments: a natural, organic-rich mud, and an abiotic, glass-bead mixture with a grain-size distribution similar to the mud. The experiments were designed to evaluate the effects of intrinsic (larval age) and extrinsic (sea water temperature) factors on sediment selectivity. These specific effects were tested because results of initial experiments designed to replicate those of Butman et. al, where M. mercenaria larvae had selected beads over mud in still water but not in flow, showed no significant selection. In the present study, only five of the 23 experiments conducted showed significant selection and, in all cases, for mud over beads. Larval age and water temperature had no significant effect on the outcome. In the earliest of these experiments, we discovered a problem with larval preservation in mud samples (dissolution of the larval shell due to low pH) that may have resulted in underestimates of the number of larvae settled in mud in Butman et al., thus confounding interpretation of those results. The conclusions of Butman et al. are therefore modified based on results of the experiments presented here: settling M. mercenaria larvae do not select between two extreme sediment treatments in 4-h, still-water, laboratory experiments. In addition, competency tests used here failed to establish a consistent, predictable, precipitous rise in competency for a given batch of M. mercenaria larvae. This may be due to natural, large variation in physiological development within a given larval pool such that a small proportion of competent larvae are available each day over an extended period. Furthermore, the between-experiment variability in selectivity and some results of the competency tests suggest that the duration of these experiments may be too short to document definitive, initial larval settlement. Occasional selection would then reflect continual redistribution of larvae prior to final settlement and metamorphosis.