Impact of salinity on early life history traits of three estuarine fish species in Senegal

The adaptive mechanisms on the early life stages of fishes to hypersaline stress are still poorly understood and probably determine the resistance of a population to disruption, compared with other less plastic species. The Casamance River in Senegal is an ideal location to test the adaptation to salinity as a dam was built in 1998 to exclude saline water intrusion. This lowered the salinity from 70 to 5 upstream and 60 downstream. The salinity influence on the growth in the early life of three West African fish species (Ethmalosa fimbriata, Sarotherodon melanotheron, and Tilapia guineensis) was studied using the width of microstructures in the otoliths and the individual migratory behaviour analysed from strontium (Sr) to calcium (Ca) ratios in the otoliths. The Sr:Ca ratio was quantified along individual transects measured from the posterior edge of the otolith to the core. The fishes were sampled on both sides of the dam that separated water with low salinity upstream from metahaline and hyperhaline water downstream.The results showed that salinity has different influence on the growth of each species. Ethmalosa fimbriata has the highest growth during the first 180 days in the freshwaters, indicating growth inhibition in the hyperhaline areas. For the two other species no growth difference were found. The Sr/Ca ratio varied widely, in Tilapia and Sarotherodon from below the dam. Individual life histories were more heterogeneous than upstream and showed a crossing of the dam for some individuals which could reach half of the fishes analysed. On the contrary in E. fimbriata, despite the large range of salinity, identical Sr/Ca profiles were found both upstream and downstream. This indicated that Sr/Ca ratio was not appropriate to evaluate life history patterns linked to salinity for this specie.     

Post-glacial persistence of turbiditic activity within the Rhône deep-sea turbidite system (Gulf of Lions,Western Mediterranean): Linking the outer shelf and the basin sedimentary records

Emplacement of post-glacial turbidites is commonly controlled by rapid changes in sea level or by seismicity. On the continental rise of the Gulf of Lions (Western Mediterranean), an aseismic area, we identified turbiditic beds deposited during the rising stage and highstand of sea level. Swath bathymetry, sediment cores, in situ Cone Penetrating Tests (CPTU), heavy mineral associations and radiocarbon dating determined the source, composition, distribution and age of the turbiditic beds. Turbidites are composed of homogeneous to positively graded silts to medium sand with quartz (up to 90%), shell debris and shelfal benthic faunas. Their distribution on the sea floor is very patchy and controlled by abundant inherited erosional bedforms. Their source is found in relict regressive sands at the outershelf. Their deposition occurred just after the onset of the post-glacial sea level rise and the concomitant sediment starvation of the Rhône deep sea turbiditic system until recently. Whilst canyons are fed with sand by strong seasonal hydro-sedimentary dynamics on the outershelf, the emplacement of post-glacial turbidites is not controlled by sea level changes but probably by the periodic flushing of the canyons. Our study revealed that this low energy aseismic margin undergoes significant transport of sand, down to the base of slope, during the sea-level rise and the Holocene highstand.     

The rate of water loss from olivine-hosted melt inclusions

Diffusive water loss from olivine-hosted melt inclusions has been reported previously. This process must be considered when interpreting melt inclusion data. This study measured the rate of water loss from olivine-hosted melt inclusions during heating-stage experiments to test a previous diffusive reequilibration model and the hydrogen diffusion mechanism that controls the rate. Olivine-hosted melt inclusions were heated to a constant temperature in reduced Ar gas in a heating stage for a few hours, and unpolarized Fourier transform infrared spectra were repeatedly measured through the inclusions. Water loss occurred rapidly in the experiments. Within a few hours, the water absorbance at 3,500 cm⁻¹ wavenumber decreased by half. The observed water loss rate can be explained by the diffusive reequilibration model and hydrogen diffusion in olivine coupled with metal vacancy. The beginning of water loss was different in the low- and high-temperature experiments. At low temperatures (1,423 and 1,437 K), water loss did not occur in the initial 1 or 2 h. At high temperatures (1,471–1,561 K), water loss began immediately. The initial time period without water loss at low temperatures may be explained by a hydrogen fugacity barrier in the host olivine. At low temperatures, the internal pressure may be lower than the equilibrium pressure of melt inclusion and olivine, causing lower hydrogen fugacity in the melt inclusion than in the olivine, which will delay the water loss from the melt inclusion. The tested model and diffusivity were used to estimate the rate of water loss during homogenization experiments and magma eruption and cooling. For 1-h homogenization experiment, the model shows that large inclusions (50 μm radius) in large olivines (500 μm radius) are robust against water loss, while large or small inclusions (50–10 μm radius) in small olivines (150 μm radius) may suffer 30–100% water loss. For natural samples, the correlation between water concentration and melt inclusion and olivine sizes may be helpful to infer the initial water concentration, degree of diffusive reequilibration, and magma cooling rate.     

Mesoscale slope current variability in the Gulf of Lions. Interpretation of in-situ measurements using a three-dimensional model

The ECOLOPHY experiments aimed at investigating physical exchanges between coastal and open sea. They were carried out in June and December 2005 over the shelf-break in the North-eastern part of the Gulf of Lions (Northwestern Mediterranean Sea). This area is considered to be the generation zone for the eddy and meandering structures of the Northern Current (NC). The objective of the present work is to examine mesoscale variability of this coastal slope current in the light of available data. Numerical modeling is used to support the field data analysis. ADCP current measurements over a one-year period show that mesoscale activity is maximal in late winter, correlating with the seasonal variability of the NC and, also, partly with local winds. Measured currents exhibit mesoscale fluctuations with periods ranging from 3 to 30 days, in agreement with previous analyses. Fluctuations of periods longer than 10 days are found to be mainly oriented in the direction of the mean current, whereas more frequently observed high frequency fluctuations tend to be oriented cross-slope, suggesting a relationship with the NC mesoscale meandering. Moreover, trajectories of surdrift buoys launched in the NC vein exhibit mesoscale phenomena, such as current meanders or eddies and on-shelf intrusions. Numerical modeling provides a synoptic point of view and is used hereafter to support physical interpretation of punctual eulerian or lagrangian measurements. Therefore, modeled hydrodynamic fields are used to analyze surdrift buoy trajectories and computed vertically averaged current and Ertel potential vorticity provide a better understanding of these behaviors.     

Monitoring Sea Level in the Coastal Zone with Satellite Altimetry and Tide Gauges

We examine the issue of sustained measurements of sea level in the coastal zone, first by summarizing the long-term observations from tide gauges, then showing how those are now complemented by improved satellite altimetry products in the coastal ocean. We present some of the progresses in coastal altimetry, both from dedicated reprocessing of the radar waveforms and from the development of improved corrections for the atmospheric effects. This trend towards better altimetric data at the coast comes also from technological innovations such as Ka-band altimetry and SAR altimetry, and we discuss the advantages deriving from the AltiKa Ka-band altimeter and the SIRAL altimeter on CryoSat-2 that can be operated in SAR mode. A case study along the UK coast demonstrates the good agreement between coastal altimetry and tide gauge observations, with root mean square differences as low as 4 cm at many stations, allowing the characterization of the annual cycle of sea level along the UK coasts. Finally, we examine the evolution of the sea level trend from the open to the coastal ocean along the western coast of Africa, comparing standard and coastally improved products. Different products give different sea level trend profiles, so the recommendation is that additional efforts are needed to study sea level trends in the coastal zone from past and present satellite altimeters. Further improvements are expected from more refined processing and screening of data, but in particular from the constant improvements in the geophysical corrections.     

Morphologie et sedimentologie de la pente externe de la partie nord du Grand Recif ed Tulear (SW de Madagascar). Nature et repartition des elements organogenes libres. (Morphology and sedimentology of the outer slope of the northern part of the “Grand Recif” of tulear (Madagascar). Nature and distribution of free organic elements.)

The Tuléar area lies on the southwest coast of Madagascar. The Tropic of Capricorn runs across the center of the Bay of Tuléar which is closed by a barrier reef called the “Grand Récif”. Other reef tracts can be found in this bay: fringing reefs, inner reefs, coral islets, coral banks.This paper is the result of a study of the outer side of the area called the “shipwreck sector”, which extends just in front of the city of Tuléar, in the northern part of the Grand Récif. The investigations were made by diving as deep as 50 m and below this depth by dredging down to 150 m. The outer slope consists of several zones, the succession of which forms the elements of a morphological model.The numerical values of sedimentological indices and sedimentary facies were obtained by means of an I.B.M. 360/44 or a C.I.I. computer; they do not vary independently but derive from each other. A sedimentary sequence is defined as a precise succession of facies which continues to be modified during the transfer of sediments inside the sedimentary sequence. The numerical values of the indices of a given facies show coherent variations which point to the evolution of the sediments during their transfer. The successions and relations of different sequences form the sedimentological model. The combination of terms in the morphological and sedimentological models allows the morpho-sedimentary model to be defined. This model can be integrated into one elaborated for the Tuléar area as a whole.Generally, two sequences extend on the outer slope. The first one is the spur and groove sequence that starts with an accumulated facies of gravel in the surf zone. The shifting facies of sands in which a very evolved material is included, mark the transition to the stable facies of accumulation. In this sequence wave action plays a central part. The second sequence is formed by sedimentary deposits of the coral flagstone (deep coral flagstone forward the reef). They are characterized by the presence of free or loose organic elements. These elements derive from calcareous algea. They are chiefly made of rhodolite nodules, rhodolite pseudonodules and Lithothamnion particles mixed with sand very rich in Foraminifera.The nature, distribution and percentage of these elements were studied as a function of the depth where they were found and, therefore, of the distance from the reef front. These free organogenic elements are typical of the outer slope of coral-reef tracts beaten by heavy swells.     

Direction de croissance des eperons sur le front d''un recif barriere — L''exemple du Grand Recif de Tulear (Madagascar)

A study has been made of the orientation of the spurs of the outer front of a barrier coral-reef tract in the Bay of Tuléar on the southwest coast of Madagascar. All types of coral reef tracts are present: outer or barrier reefs, lagoon reefs and coral banks. The orientation of the spurs was studied as a function of the direction of the swell crests which are either parallel or oblique to the alignment of the reef front of the “Grand Récif” which extends over 18 km. When the swell is oblique, the spurs are not, as is generally held, perpendicular to the crests of the swell. The orientation of the spurs is a resultant direction between the direction of the swell orthogonals and the direction of the slope of the outer reef flat. From aerial photographs one can determine the directions of the spurs and of the slope of the outer reef flat. It is easy to obtain the direction of the crests of the swell and then to construct the wave diagram. The morphological aspects and the distribution of the deposits on the inner flat of the coral reef tract depend on the orientation of the swell.

Résumé

L'étude de l'orientation des éperons d'un front récifal externe a été réalisée dans la baie de Tuléar sur la côte sud-ouest de Madagascar. Dans ce secteur, tous les types d'édifices récifaux sont présents: récifs barrières externes, récifs de lagon et bancs coralliens. Nous avons étudié l'orientation des éperons en fonction de la direction de la houle qui est soit parallèle, soit oblique par rapport au front récifal du Grand Récif qui s'étend sur 18 km. Quand la houle est oblique, les éperons ne sont pas — comme on le dit généralement — perpendiculaires à la houle, mais leur orientation est une direction résultante entre la direction de propagation de la houle et celle de la pente du platier récifal au point considéré. Sur les photographies aériennes, on détermine la direction des éperons et celle de la pente. Une construction simple permet d'obtenir la direction de la crête des vagues et de construire le plan de vagues. Toute la morphologie et la répartition des dépôts du platier d'un édifice récifal sont sous la dépendance de l'orientation de la houle.     

Fitting a predator–prey model to zooplankton time-series data in the Gironde estuary (France): Ecological significance of the parameters

The relationships between the seasonal fluctuations of the copepod Eurytemora affinis and the mysid Neomysis integer were studied from observed data and experimental results, using a predator–prey model in the oligo-mesohaline area of the Gironde estuary. Mean seasonal fluctuations of abundances were derived from long term data series collected from 1978 to 2003 for both species. In situ predator–prey experiments over a seasonal cycle were used to estimate the seasonal variation of the consumption rate of N. integer on E. affinis and to verify the order of magnitude of the biological parameters given by the model.Predator–prey experiments revealed a high seasonal variation in maximum consumption rates with a mean of 56 ± 9 ind. pred⁻¹ d⁻¹. Maximum consumption rates were always higher for adults than for juveniles of Neomysis integer. Recorded selectivities were higher on nauplii than on copepodids + adults of Eurytemora affinis, both for the juveniles and the adults of N. integer. Neomysis integer mainly fed on meroplanktonic larvae, when they were available in higher abundances, than E. affinis in their environment.Spring increases of abundance for Eurytemora affinis copepodids + adults seemed to be mainly controlled by temperature whereas its decreasing abundance in summer was more related to Neomysis integer predation, suggesting that summer fluctuations of E. affinis abundance are probably controlled by mysid predation at summer times. Using a Lotka–Volterra predator–prey model, the seasonal peak of abundance of the mysid N. integer was well reproduced considering a predation on copepodids + adults of E. affinis, and suggested a dependence between mysid and copepod seasonal variations. However, the seasonal peak amplitude could not be explained solely by a predation on copepodids + adults or on nauplii of the copepod. Thus, N. integer is probably dependent on the seasonal fluctuations of the copepod's abundance, complementing its diet with macrophytal detritus during periods of scarce food.     

Effects of simulated benthic fluxes on phytoplankton dynamic and photosynthetic parameters in a mesocosm experiment (Bay of Brest,France)

Benthic faunal activity and density play an important role in determining the rates of benthic nutrient fluxes, which enrich the water column and contribute to phytoplankton growth. The intensity of nutrient fluxes in the Bay of Brest depends on the density of the invasive gastropod, Crepidula fornicata. In order to study the impact of benthic fluxes on phytoplankton dynamics, realistic daily nutrient inputs simulating various densities of C. fornicata were added to six enclosures during three weeks. The increase in fertilization intensity influenced the phytoplankton biomass. A succession from Chaetoceros spp. to Pseudo-nitzschia spp. and Leptocylindrus danicus was observed in all enclosures, but the dynamics of successions were different. Pseudo-nitzschia spp. was favored in the three more fertilized enclosures, while Chaetoceros spp. persisted longer in less enriched enclosures. Despite an apparent nitrogen limitation, the quantum efficiency of PSII (F_v/F_m) was high (>0.5) and stable in all enclosures. The maximal photosynthetic capacity (P^Bmax) was also invariable and oscillated around an average value of 2.23 mg C (mg Chl a)⁻¹ h⁻¹. The stability of F_v/F_m and P^Bmax observed at different nutrient input intensities demonstrates that the daily inputs maintained the physiological balance of the microalgae. The maximal light utilization efficiency (α) and the light saturation parameter (E_k) were also quite stable after day 8, which reveals that photosynthetic parameters were driven by growth constraints due to nutrient availability and not by incident light or species successions. We suggest that our results correspond to an “E_k independent variation” regulation. We propose that such regulation of photosynthetic parameters appears when there are frequent nutrient additions which do not allow replete nutrient conditions to be reached but lead to physiological equilibrium.     

An individual-based model study of anchovy early life history in the northern Humboldt Current system

We used an individual-based model of anchovy (Engraulis ringens) early life history coupled with hydrodynamic outputs from the regional oceanic modeling system (ROMS) to investigate the factors driving variability in egg and larval survival rates in the northern Humboldt upwelling region off Peru. Individuals were released within a coastal area and followed for a period of 30 days. Those that were still in the coastal area at that time were considered as retained. We investigated the spatial and temporal variability in the release locations of the individuals retained, and compared these to observed egg concentration patterns reconstructed from a 40-year period of monitoring. A first set of simulations using passive particles to represent anchovy eggs and larvae revealed a large sensitivity of the results to the initial vertical distribution of particles. We then conducted two additional sets of simulations that included the effect of egg buoyancy, larval vertical swimming behavior and lethal temperature. We obtained (1) maximal coastal retention close to the surface in winter and in deeper layers in summer, (2) a large influence of egg buoyancy and of larval vertical behavior on coastal retention in all seasons, (3) a partial match between dates and locations of enhanced retention and observed egg concentration patterns and (4) a low effect of lethal temperature on survival except when associated with high egg density. The model suggests that an optimal temporal spawning pattern for maximizing coastal retention would have two maximums, the most significant in austral winter and the second in summer. This pattern agrees roughly with observed spawning seasonality, but with temporal discrepancy of about two months in the peaks of both series. Spatially, we obtained higher retention from 10 S to 20 S, whereas the observed maximum egg concentration was located between 6°S and 14°S. Among the three sets of simulations, the one taking into account larval vertical swimming behavior lead to the best match with the data.