Does Manila clam cultivation affect habitats of the engineer species Lanice conchilega (Pallas, 1766)?

The major French site of Manila clam Ruditapes philippinarum (Adams and Reeves, 1850) cultivation is located in the Chausey Archipelago where the associated practices are highly mechanized: every steps of production are made with tractor-driven machinery. The Manila clam concessions are concentrated on Lanice conchilega (Pallas, 1766) bioherms, which are known to increase alpha-diversity and to locally modify sediment dynamics. This study focus on the impacts of Manila clam cultivation on (i) the natural populations of L. conchilega and on (ii) the structure of the associated benthic assemblages during the different steps of the farming production cycle. We found that the L. conchilega populations are significantly affected within the concessions where their total abundances drastically decrease, their spatial patterns are modified and the associated benthic assemblages are significantly altered. Our results are discussed in a context of a sustainable management of the Manila clam cultivation in coastal areas.     

In situ oxygen uptake rates by coastal sediments under the influence of the Rhône River (NW Mediterranean Sea)

The influence of riverine inputs on biogeochemical cycling and organic matter recycling in sediments on the continental shelf off the Rhône River mouth (NW Mediterranean Sea) was investigated by measuring sediment oxygen uptake rates using a combination of in situ and laboratory techniques. Four stations were investigated during two cruises in June 2001 and June 2002, with depths ranging from 9 to 192 m and over a distance to the Rhône River mouth ranging from 4 to 36 km. Diffusive oxygen uptake (DOU) rates were determined using an in situ sediment microprofiler and total oxygen uptake (TOU) rates were measured using sediment core incubations. There was good agreement between these two techniques which indicates that the non-diffusive fraction of the oxygen flux was minimal at the investigated stations. DOU rates ranged from 3.7±0.4 mmol O₂ m⁻² d⁻¹ at the continental shelf break to 19.3±0.5 mmol O₂ m⁻² d⁻¹ in front of the Rhône River mouth. Sediment oxygen uptake rates mostly decreased with increasing depth and with distance from the Rhône mouth. The highest oxygen uptake rate was observed at 63 m on the Rhône prodelta, corresponding to intense remineralization of organic matter. This oxygen uptake rate was much larger than expected for the increasing bathymetry, which indicates that biogeochemical cycles and benthic deposition are largely influenced by the Rhône River inputs. This functioning was also supported by the detailed spatial distribution of total organic carbon (TOC), total nitrogen (TN) and C/N atomic ratio in surficial sediments. Sediments of the Rhône prodelta are enriched in organic carbon (2–2.2%) relative to the continental shelf sediments (<1%) and showed C/N ratios exceeding Redfield stoichiometry for fresh marine organic matter. A positive exponential correlation was found between DOU and TOC contents (r²=0.98, n=4). South-westward of the Rhône River mouth, sediments contained highly degraded organic matter of both terrestrial and marine origin, due to direct inputs from the Rhône River, sedimentation of marine organic matter and organic material redeposition after resuspension events.     

Sedimentary patterns and geometries of the Bahamian outer carbonate ramp (Miocene–Lower Pliocene, Great Bahama Bank)

Core, logging and high-resolution seismic data from ODP Leg 166 were used to analyse deposits of the Neogene (Miocene–Lower Pliocene) Bahamian outer carbonate ramp. Ramp sediments are cyclic alternations of light- and dark-grey wackestones/packstones with interbedded calciturbidite packages and minor slumps. Cyclicity was driven by high-frequency sea-level changes. Light-grey layers containing shallow-water bioclasts were formed when the ramp exported material, whereas the dark-grey layers are dominantly pelagic. Calciturbidites are arranged into mounded lobes with feeder channels. Internal bedding of the lobes shows a north-directed shingling as a result of the asymmetrical growth of these bodies. Calciturbidite packages occur below and above sequence boundaries, indicating that turbidite shedding occurred during third-order sea-level highstands and lowstands. Highstand turbidites contain shallow-water components, such as green algal debris and epiphytic foraminifera, whereas lowstand turbidites are dominated by abraded bioclastic detritus. Gravity flow depocentres shifted from an outer ramp position during the early Miocene to a basin floor setting during the late Miocene to early Pliocene. This change was triggered by an intensification of the strength of bottom currents during the Tortonian, which was also responsible for shaping the convex morphology of the outer ramp. The Miocene and Lower Pliocene of the leeward flank of Great Bahama Bank provides an example of the poorly known depositional setting of the outer part of distally steepened carbonate ramps. The contrast between its sedimentary patterns and the well-known Upper Pliocene–Quaternary slope facies associations of the flat-topped Great Bahama Bank shows the strong control that the morphology of a carbonate platform exerts on the depositional architecture of the adjacent slope and base-of-slope successions.     

Reaktionen benthischer Tiefsee-Lebensgemeinschaften auf mechanische Störungen der Sedimente

Ocean Dynamics - Das Ökosystem der Tiefsee wird zunehmend durch industrielle Eingriffe beeinflusst. Über die Auswirkungen dieser Aktivitäten auf das Benthos wurden bisher nur...     

Formation temperatures of clays from the volcaniclastic series of Site 841 ODP: an oxygen isotopic record of a paleothermal flux into the Tonga forearc

Oxygen isotopic compositions of clay minerals were determined on representative samples of the volcano-sedimentary series from Site 841 ODP (Tonga forearc). This isotopic study has demonstrated an abnormally high crystallisation temperature of the clay minerals with respect to temperature expected in burial diagenesis. Formation temperatures obtained using ¹⁸O reach up to 200 °C in a Fe-chlorite-corrensite paragenesis found in the vicinity of basaltic andesite sills intruded into the Miocene tuffs. The paleothermal flux resulting from the cooling of the sills has produced very low grade contact metamorphism in the Miocene Tonga forearc deposits. The consequence of this was the formation of a large amount of hydrous silicates characterised near the sills by a Fe-clays-analcime mineralogical association. Received: 26 September 1997 / Accepted: 22 September 1998     

Effects of increased inundation and wrack deposition on a high salt marsh plant community

Salt marshes respond to both slowly increasing tidal inundation with sea level rise and abrupt disturbances, such as storm-induced wrack deposition. The effects of inundation pattern and wrack deposition have been studied independently but not in combination. We manipulated inundation of tidal creek water and wrack presence individually and in combination, in two neighboring communities within a Virginia high salt marsh during 1994 and 1995. The effects of these manipulations were assessed by measurements of aboveground plant biomass. Altered inundation by itself produced little response in the various categories of plant biomass measured. Wrack deposition affected all species (i.e., Juncus roemerianus, Spartina patens, and Distichlis spicata) showing a significant reduction in aboveground biomass, as expected. Recovery after wrack deposition was dependent on the species. S. patens and D. spicata recovered from wrack deposition within one growing season, while J. roemerianus did not. Because the effects of wrack deposition greatly exceeded those of experimentally increased inundation, the possible interactions between the two were masked. Increased inundation may have inhibited the colonization of bare areas by some species after the removal of wrack from an area, although statistical significance at α=0.01 was not reached. Our results confirm that wrack deposition can cause the redistribution of species within the high marsh community. Altered inundation may have a greater effect on the re-establishment of the plant community after wrack deposition than it does without wrack deposition.     

Histological Identification of Macroalgae from Stomach Contents of the Antarctic Fish Notothenia coriiceps Using Semi-thin Sections

A histological method is described allowing the identification of macro-algal fragments from stomach contents of fish. The rhodophytes Porphyra endiviifolium and Neuroglossum ligulatum were determined from the stomach contents of the Antarctic fish Notothenia coriiceps . The results show P. endiviifolium to be a more important part in the diet of this fish than previously expected. On the basis of the depth distribution of the identified algae it is suggested that N. coriiceps can migrate to very shallow waters for feeding.