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

Bathymodiolus growth dynamics in relation to environmental fluctuations in vent habitats

The deep-sea mussel Bathymodiolus thermophilus is a dominant species in the East Pacific Rise (EPR) hydrothermal vent fields. On the EPR volcanically unstable area, this late colonizer reaches high biomass within 4-5 years on new habitats created. by lava flows. The environmental conditions and growth rates characterizing the reestablishment of B. thermophilus populations are however largely unknown, leaving unconstrained the role of this foundation species in the ecosystem dynamics. A typical example from the vent field at 9 degrees 50'N that was affected by the last massive eruption was the Bio-9 hydrothermal vent site. Here, six years later, a large mussel population had reestablished. The von Bertalanffy growth model estimates the oldest B. thermophilus specimens to be 1.3 year-old in March 2012, consistent with the observation of scarce juveniles among tubeworms in 2010. Younger cohorts were also observed in 2012 but the low number of individuals, relatively to older cohorts, suggests limited survival or growth of new recruits at this site, that could reflect unsuitable habitat conditions. To further explore this asumption, we investigated the relationships between mussel growth dynamics and habitat properties. The approach combined sclerochronology analyses of daily shell growth with continuous habitat monitoring for two mussel assemblages; one from the Bio-9 new settlement and a second from the V-vent site unreached by the lava flow. At both vent sites, semi-diurnal fluctuations of abiotic conditions were recorded using sensors deployed in the mussel bed over 5 to 10 days. These data depict steep transitions from well oxygenated to oxygen-depleted conditions and from alkaline to acidic pH, combined with intermittent sulfide exposure. These semi-diurnal fluctuations exhibited marked changes in amplitude over time, exposing mussels to distinct regimes of abiotic constraints. The V-vent samples allowed growth patterns to be examined at the scale of individual life and compared to long-term records of habitat temperature and oceanographic mooring data in the years following the eruption. Both shell growth and habitat temperature at V-vent varied over the spring-neap tidal cycle and over longer periods of c.a. 60 days. The correlation of growth rate with temperature and, for some individuals, with current velocities supports the idea that tidal forcing impacts growth. Its influence on habitat conditions includes the spring-neap cycle, which is not reflected in current velocities but influences the venting rate. Additionally, it is expected that mesoscale eddies periodically passing across the ridge imprint shell growth through the influence of bottom current on the decimeter-thick mixing interface where mussels thrive. We conclude that diurnal-semidiurnal tidal fluctuations exert major abiotic constraints on B. thermophilus mussels and that low-frequency fluctuations act as significant determinants on growth. Finally, we postulate that the modulation of tidal fluctuations by large-scale hydrodynamic forcing ultimately constrains the capacity of this mussel species to form high biomass aggregations. This study indeed shows that the absence of these strong hydrodynamic drivers would limit the alternance of oxic and sulfidic conditions and significantly affect the growth rate of this species over time

Potential availability of trace metals in sediments in southeastern and southern Brazilian shipyard areas using the DGT technique and chemical extraction methods

Speciation and partitioning of trace metals, from solid to solution phases of sediments. control their bioavailability and thus their potential ecological risk to organisms. Therefore, in order to obtain a broad evaluation of their risk, it is necessary to couple methodologies that are able to assess metal mobility in sediment. In this study, the Diffusive Gradients in Thin Films (DGT) technique and the application of 0.1 M HCl acid extraction methods, together with solid-state voltammetric sensors, were used with the objective of assessing mobility and potential availability of Cr, Cu, Ni, Pb, V and Zn in sediment porewaters and solid sediments in southeastern and southern Brazilian shipyard areas. The highest labile metal concentrations were found in shipyards with the longest histories of operations. Trace metal distributions in porewater and in the solid phase of sediments (labile metals) and significant correlations among metals enabled to distinguish the contribution of anti-fouling paint components. The diffusive flux of every metal measured at the surface of the sediment indicated that CU DGT had the highest flux (3.66E-03 mmol.m(-2) d(-1)) in the shipyard with the longest operating time. Therefore, enrichment was observed for Cu, Pb and Zn in sediments, indicating a possible ecological risk level of 'Effects Range Median' to 'Apparent Effects Threshold' for oyster larvae (Mollusca) (Cu). bivalves (Pb) and the infaunal community (Zn). Probable Effect Concentrations (PEC) to sediment-dwelling biota can be expected as well, related to high concentrations of Cu and Zn in sediment. This study allowed a comprehensive evaluation of potential bioavailability and ecological risk of trace metals in aquatic systems where there is continuous and specific input of these elements. The use of the DGT technique with solid-state voltammetry in the sediment of distinct Brazilian estuarine systems demonstrated its potential to be applied in future environmental network programs

Aluminium in the North Atlantic Ocean and the Labrador Sea (GEOTRACES GA01 section): roles of continental inputs and biogenic particle removal

The distribution of dissolved aluminium (dAl) in the water column of the North Atlantic and Labrador Sea was studied along GEOTRACES section GA01 to unravel the sources and sinks of this element. Surface water dAl concentrations were low (median of 2.5 nM) due to low aerosol deposition and removal by phytoplankton. However, surface water dAl concentrations were enhanced on the Iberian and Greenland shelves (up to 30.9 nM) due to continental inputs (rivers, glacial flour and ice melt). A negative correlation was observed between dAl in surface waters and primary production, phytoplankton community structure and biogenic opal production. The abundance of diatoms exerted a significant (p  0.76) west of the Iberian Basin, suggesting net release of dAl at depth during remineralization of sinking biogenic opal containing particles. Enrichment of dAl at near-bottom depths was observed due to resuspension of sediments near the sediment-water interface. The highest dAl (up to 38.7 nM) concentrations were observed in Mediterranean Overflow Waters which act as a major source of dAl to mid depth waters of the eastern North Atlantic. This study clearly shows that the vertical and lateral distribution of dAl in the North Atlantic differs when compared to other regions of the North Atlantic and global ocean due to the large spatial differences both in the main source of Al, atmospheric deposition, and the main sink for Al, particle scavenging, between different oceanic regions

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