Seasonal dynamics of trophic relationships among co-occurring suspension-feeders in two shellfish culture dominated ecosystems

The temporal dynamics of carbon and nitrogen isotope values of co-occurring suspension-feeders in two shellfish culture areas (Normandy, France) were investigated over two years to evaluate the inter-specific trophic partitioning and relative contributions of organic matter sources to benthic suspension-feeders' diet. Oysters (Crassostrea gigas), mussels (Mytilus edulis), cockles (Cerastoderma edule), slipper limpets (Crepidula fornicata), and sand-mason worms (Lanice conchilega) were sampled in an estuarine environment (Baie des Veys, east Cotentin, Normandy), while oysters, mussels, slipper limpets, and honeycomb worms (Sabellaria alveolata) were sampled in an open-marine environment (Lingreville-sur-mer, west Cotentin, Normandy). Whatever the sampling period, the bivalves, C. gigas and M. edulis, exhibited the lowest values of δ¹³C and δ¹⁵N compared with the other species. Feeding relationships among suspension-feeders in both C. gigas culture areas exhibited temporal variations due to the marine/estuarine influence and seasonal changes in food supply. In the open-marine ecosystem, the contribution of phytoplankton remained the most important for all species except S. alveolata, while in the estuarine ecosystem, microphytobenthos and/or macroalgae detritus contributed a larger extent to the organisms' diets. During phytoplankton bloom periods (e.g. May and July) suspension-feeders, except for S. alveolata, relied strongly on phytoplankton; however, the majority of suspension-feeders exhibited different opportunistic behaviour in winter when phytoplankton biomass might be a limiting factor. We hypothesized that differences in particle capture and selection by the suspension-feeders influenced their isotopic values. Feeding ecology of suspension-feeders partly explained why competition was limited and why ecosystems can often support unexpectedly large numbers of suspension-feeders. We also showed that understanding ecosystem characteristics of the organic matter sources is of primary importance to determine the extent to which members of the suspension-feeding guild potentially compete for food.     

Structure of the Rambler Rhyolite, Baie Verte Peninsula, Newfoundland: Inversions using UBC-GIF Grav3D and Mag3D

Hosted within the Pacquet Harbour Group (PHG) on the Baie Verte Peninsula of north-central Newfoundland, the Rambler rhyolite is a 487 Ma unit of felsic tuffs, flows and subvolcanic intrusive rocks. The PHG has been affected by multiple phases of deformation with the youngest D₄ deformation event producing broad northeast plunging upright cross folds in the Rambler rhyolite. Fold culminations on the upper bounding surface of the rhyolite host Cu +/− Au volcanogenic massive sulfide deposits (e.g. Rambler and Ming mines). Geophysical inversions of recently acquired high resolution gravity and magnetic data have been implemented to determine the extent of the fold axis (dome) at depth. To direct the outcome of the inversion process towards a more geologically reasonable solution this study outlines a procedure which permits the inclusion of known geological and geophysical constraints into the input (reference) model for inversion using the MAG3D and GRAV3D algorithms provided by the University of British Columbia Geophysical Inversion Facility. Reference model constraints included surficial geological contacts as defined by aeromagnetic data, and subsurface distribution of physical property variations from a series of drill-hole logs. The output (computed) model images the surface of the rhyolite dome as dipping roughly 40° to the northeast as a series of voxels with density values ranging from 2.71 to 2.75 g/cm³. While previously published ore deposit models parallel this structure in the near surface, results from these inversions suggest deeper exploration may be favorable. Magnetic inversion modeling has not provided any insight into dome morphology however it outlines the distribution of gabbroic dykes surrounding the dome.     

On the interest of using field primary production data to calibrate phytoplankton rate processes in ecosystem models

In many ecosystem models based on empirical formulations, parameters generally are calibrated in order to achieve the best fit between measured and simulated chlorophyll a standing stocks. An accurate calibration of rate processes as primary production rarely is taken into account. In this paper, we test the usefulness of calibration of phytoplankton photosynthetic processes in an ecosystem model using field primary production data. We used 18 months of photosynthetic process data from the Baie des Veys ecosystem (Normandy, France). Five empirical formulations of photosynthesis–irradiance curve models amongst the most widely used were tested. In each formulation, the variability of photosynthetic parameters (i.e. the light-saturated rate of photosynthesis (P_max^B) and the initial slope of the photosynthesis–light curve (α^B)) was considered depending on environmental factors (temperature and nutrient availability). The fit of the five equations as well as the calibration of parameters on field measurements (i.e. the light-saturated rate of photosynthesis (P_ref^B), the initial slope of the photosynthesis–light curve (α_ref^B), the half-saturation constant for nitrogen (K_N) and silicates uptake (K_Si), and the coefficient in the exponential thermal effect (K_T)) was performed using the whole available data set of P vs. E curves (n = 143, P vs. E curves). Then, the Smith formulation allowing the best simulation of the Baie des Veys primary production and corresponding parameters were introduced in an ecosystem box model. This formulation led directly to a satisfactory representation of the Baie des Veys phytoplankton dynamics without additional calibration. Results obtained were compared with a more classical approach in which ecosystem models were calibrated using published values of parameters. This comparison showed that for the two years studied, annual primary production estimated through the ecosystem model was 13% and 26% higher with our approach than with the more classical approach. This work emphasizes the importance of accurately representing rate processes in ecosystem models in order to adequately simulate production as well as standing stocks.     

Fluxes of selected trace metals from the Seine estuary to the eastern English Channel during the period August 1994 to July 1995

Two sampling cruises conducted in the Seine estuary (France) under low-water and flood conditions produced high resolution profiles for dissolved cadmium, lead, copper, zinc and nickel concentrations versus salinity. The distribution of dissolved trace metals differed depending on hydrologic conditions, partly because of the dilution of upstream inputs during flood periods. Daily fluxes of these dissolved trace metals were estimated for the two sampling periods (September 1994 and February 1995) by extrapolating the dilution lines observed in higher salinity waters to salinity=0 and then multiplying the effective freshwater concentrations thus obtained by the corresponding freshwater flow. Several procedures were subsequently applied to deduce each daily flux for the year studied from data for these two periods. A consensus was found among these procedures, allowing the determination of net fluxes of dissolved trace metals with a precision of 20–35%. The net fluxes thus estimated were 4 T yr⁻¹ for Cd, 4 T yr⁻¹ for Pb, 40 T yr⁻¹ for Cu, 130 T yr⁻¹ for Zn and 50 T yr⁻¹ for Ni.     

Modelling the influence of environmental factors on the physiological status of the Pacific oyster Crassostrea gigas in an estuarine embayment; The Baie des Veys (France)

It is well known that temporal changes in bivalve body mass are strongly correlated with temporal variations in water temperature and food supply. In order to study the influence of the year-to-year variability of environmental factors on oyster growth, we coupled a biogeochemical sub-model, which simulates trophic resources of oysters (i.e. phytoplankton biomass via chlorophyll a), and an ecophysiological sub-model, which simulates growth and reproduction (i.e. gametogenesis and spawning), using mechanistic bases. The biogeochemical sub-model successfully simulated phytoplankton dynamics using river nutrient inputs and meteorological factors as forcing functions. Adequate simulation of oyster growth dynamics requires a relevant food quantifier compatible with outputs of the biogeochemical sub-model (i.e. chlorophyll a concentration). We decided to use the phytoplankton carbon concentration as quantifier for food, as it is a better estimator of the energy really available to oysters. The transformation of chlorophyll a concentration into carbon concentration using a variable chlorophyll a to carbon ratio enabled us to improve the simulation of oyster growth especially during the starvation period (i.e. autumn and winter). Once validated, the coupled model was a suitable tool to study the influence of the year-to-year variability of phytoplankton dynamics and water temperature on the gonado-somatic growth of the Pacific oyster. Four years with highly contrasted meteorological conditions (river inputs, water temperature and light) 2000, 2001, 2002 and 2003, were simulated. The years were split into two groups, wet years (2000 and 2001) and dry years (2002 and 2003). Significant variability of the response of oysters to environmental conditions was highlighted between the four scenarios. In the wet years, an increase in loadings of river nutrients and suspended particulate matter led to a shift in the initiation and the magnitude of the phytoplanktonic spring bloom, and consequently to a shift in oyster growth patterns. In contrast, in the dry years, an increase in water temperature—especially during summer—resulted in early spawning. Thus, the gonado-somatic growth pattern of oysters was shown to be sensitive to variations in river loadings and water temperature. In this context, the physiological status of oysters is discussed using a relevant indicator of energy needs.     

The importance of local mean time in remote sensing for mapping megatidal zones

The local mean time is rarely debated in applied remote sensing, although it is a key parameter, especially for the geological mapping of coastal areas with high tidal ranges. French test sites are used to illustrate the role of this parameter. Depending from location and for an average time of acquisition of 11:00 UTC, the exposed tidal flat ranges from 55% (Baie des Veys), 80% (Mont Saint-Michel) to 100% (Cap-Ferret). Eight multisource satellite images were used in the Mont Saint-Michel Bay to draw the limit of the sea and evaluate the corresponding tidal flat's surfaces for various times of acquisition. The relation emphasizes some geometrical properties of the lower part of the intertidal zone. A NW–SE profile allows one to identify a distal part with a slope of 0.2% and a proximal part with a slope of only 0.06%. JERS 1-OPS and ALOS-AVNIR-2 data have been also compared to evaluate the progression of the halophytic vegetation, covering more than 6 km² between 1992 and 2007. From a methodological point of view, the ALOS-AVNIR-2 acquired on October 2007 is the first satellite data covering the Mont Saint-Michel Bay with a water elevation of only 2.56 m, i.e. a tidal flat free of water over 80% of its surface.     

Contrasting trace element geochemistry in two American and French salt marshes

Trace metals were analyzed in the bulk fine phase of two contrasting salt marsh cores on the North Atlantic east (Baie de l'Aiguillon, LaRochelle, France) and west (Lewes, DE, USA) coasts. The purpose is to gauge how the different clay mineral sources and surface sedimentary diagenetic conditions affect the respective trace element geochemistries. It has been established that there is considerably more clay mineral diagenesis at the zone of redox transition in the Delaware core than in the l'Aiguillon core. The Delaware core shows unweathered glacial sedimentary material directly deposited from the continental shelf whereas the l'Aiguillon core is formed from soil clays deposited after river and estuarine transport. Although there is strong Mn redox depletion in the Delaware salt marsh core, and enrichment in the French one at l'Aiguillon, transition elements seem to follow iron content in both cores. The trace pollutant elements Pb, Sn and Zn show pronounced depth profiles in both North Atlantic cores. The Pb, Sn and Zn appear to be carried by clay sources into the French marshes, while appearing to be largely deposited by atmospheric processes as well into the Delaware marsh.Upon deposition, the Zn appears to become decoupled from lead, which is probably more strongly fixed on clays. The Sn seems to be both air transported, and carried as well on non-clay phases contaminated by industrial organo-tins, including antifouling bottom paints. Copper while uniformly enriched in France, probably from use in regional vineyards, appears enriched only near the top at Delaware, suggesting diagenetic recycling in the more abundant peat in the surface. The Cs and Li appear to be tracers for specific soil-derived clay sources in the French core source materials. These observations suggest that clay is a very important carrier in fixing trace elements for historical records as salt marsh sediments.