Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special_issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

A synthesis of the POMME physical data set: One year monitoring of the upper layer.

International audienceThe Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment was designed to describe and quantify the role of mesoscale processes in the subduction of mode waters in the northeast Atlantic. During 1 year (September 2000–October 2001), in situ measurements were conducted over a 8° square area centered on 18°W, 42°N. We present the synthesis of the physical data set collected during this experiment. To improve time and space coverages, these measurements are combined with satellite information from sea surface temperature and altimetry. Daily fields of temperature, salinity, and stream function are produced over a seasonal cycle with a simplified Kalman filter. We analyze the annual cycle of the upper layer. The 1 year mean circulation in the upper 400 m resembles the scheme proposed by Paillet and Mercier (1997). The meridional component of the flow carries cold water southward, thus contributing to cool the POMME area. The annual mean heat budget shows that this advection by the mean current nearly balances the warming by the surface heat fluxes. The mixed layer maximum depth is reached in March. It increases, in zonal averages, from 100 m south of 38°N to more than 270 m north of 45°N and, remarkably, is shaped by the mesoscale. The subpolar mode water formed in the north of the POMME area has a lower density (26.8–27.0) than the mode water formed in the years 1988–1990 that Paillet and Arhan (1996) found in the density range (27.0–27.1). This mode water is continuously advected southward across the 42°N latitude, leading to an annual mean transport of 1.4 106 m3 s−1

Genetic pattern of the recent recovery of European otters in southern France

We investigated how landscape affects the population genetic structure and the dispersal of the elusive European otter Lutra lutra in a contemporary colonization context, over several generations and at the level of hydrographic basins. Our study area included 10 basins located in the Cevennes National Park (CNP), at the southern front of the natural otter recovery in France. Each basin comprised 50 to 300 km of permanent rivers that were surveyed for otter presence from 1991 to 2005. Faecal samples collected in 2004 and 2005 in this area were genotyped at 9 microsatellite loci, resulting in the identification of 70 genetically distinct individuals. Bayesian clustering methods were used to infer genetic structure of the populations and to compare recent gene flow to the observed colonization. At the regional level, we identified 2 distinct genetic clusters (NE and SW; FST=0.102) partially separated by ridges, suggesting that the CNP was recolonized by 2 genetically distinct otter populations. At the basin level, the genetic distance between groups of individuals in different basins was positively correlated to the mean slope separating these basins. The probable origins and directions of individual movements (i.e. migration between clusters and basin colonization inside clusters) were inferred from assignment tests. This approach shows that steep and dry lands can stop, impede or divert the dispersal of a mobile carnivore such as the otter

An effect of pier pilings on nearshore submarine groundwater discharge from a (partially) confined aquifer

Concurrent bulk ground conductivity mapping and direct measurements of seepage rates were carried out near a pier at Shelter Island, New York, U.S.A. A shallow sediment layer was identified to provide confinement for lower aquifer units. The conductivity and seepage rate data indicate that pilings of the pier apparently pierce this shallow sediment layer, producing a comparatively high seepage rate driven by the hydraulic head of the (partially) confined aquifer, resulting in a substantial increase in submarine groundwater discharge (SGD) near the pier. Seepage rate measurements made close to the pier, which runs perpendicular to the shoreline, cannot be considered representative for the area. At the study site, the magnitude of SGD depends both on the distance from shore and on the distance from the pier, a rmding that confounds the commonly observed patterns of decreasing SGD with increasing distance from shore. This alteration of a groundwater flow pattern is a previously undescribed effect of anthropogenic perturbation in a coastal system

Transports across the 2002 Greenland-Portugal Ovide section and comparison with 1997.

International audienceThe first Ovide cruise occurred in June–July 2002 on R/V Thalassa between Greenland and Portugal. The absolute transports across the Ovide line are estimated using a box inverse model constrained by direct acoustic Doppler current profiler velocity measurements and by an overall mass balance (±3 Sv, where 1 Sv = 106 m3 s−1) across the section. Main currents are studied and compared to the results of the similar Fourex section performed in August 1997 and revisited here. The meridional overturning cell (MOC) is estimated in two different ways, both leading to a significantly lower value in June 2002 than in August 1997, consistent with the relative strength of the main components of the MOC (North Atlantic Current and deep western boundary current). It has been found that the MOC calculated on density levels is more robust and meaningful than when calculated on depth levels, and it is found to be 16.9 ± 1.0 Sv in 2002 versus 19.2 ± 0.9 Sv in 1997. The 2002 heat transport of 0.44 ± 0.04 × 1015 W is also significantly different from the 0.66 ± 0.05 × 1015 W found in 1997, but it is consistent with the much weaker integrated warm water transport across the section than in 1997

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this Special Issue, results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among seventeen articles. Here, the scientific context, project objectives and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this Special Issue, results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among seventeen articles. Here, the scientific context, project objectives and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue