The mean circulation of the southwestern Mediterranean Sea: Algerian Gyres

This is a study about the general circulation of the southwestern Mediterranean Sea based on observations of currents carried out in the southwestern Mediterranean Sea in the framework of the Mass Transfer and Ecosystem Response (MATER) program (EEC/MAST3 program). From July 1997 to August 2002, profiling floats (MEDPROF experiment), isobaric floats (LIWEX experiment), and moored current meters (ELISA experiment) give evidence of two large-scale barotropic cyclonic circulations, the here-called Western and Eastern Algerian Gyres, centered around [3730′N, 230′E] and [3830′N, 600′E], respectively. These gyres have typical horizontal scales of 100–300 km and are characterized by orbital velocities of about 5 cm/s corresponding to rotational periods of about 4 months. They are strongly related to the bottom topography of the basin and to the planetary vorticity gradient: closed f/H isocontours (f is the planetary vorticity, H the water depth) correspond to the locations of the gyres and favor such circulations as free geostrophic modes. A linear and barotropic model is used to investigate the possibility of wind driving, but the results suggest that the wind stress is not responsible for establishing such circulations. The boundary currents flowing along the continental slope of Africa, Sardinia, and the Balearic Islands are proposed to be the main drivers of these gyres

Geographic and seasonal patterns and limits on the adaptive response to temperature of European Mytilus spp. and Macoma balthica populations

Seasonal variations in seawater temperature require extensive metabolic acclimatization in cold-blooded organisms inhabiting the coastal waters of Europe. Given the energetic costs of acclimatization, differences in adaptive capacity to climatic conditions are to be expected among distinct populations of species that are distributed over a wide geographic range. We studied seasonal variations in the metabolic adjustments of two very common bivalve taxa at European scale. To this end we sampled 16 populations of Mytilus spp. and 10 Macoma balthica populations distributed from 39° to 69°N. The results from this large-scale comprehensive comparison demonstrated seasonal cycles in metabolic rates which were maximized during winter and springtime, and often reduced in the summer and autumn. Studying the sensitivity of metabolic rates to thermal variations, we found that a broad range of Q10 values occurred under relatively cold conditions. As habitat temperatures increased the range of Q10 narrowed, reaching a bottleneck in southern marginal populations during summer. For Mytilus spp., genetic-group-specific clines and limits on Q10 values were observed at temperatures corresponding to the maximum climatic conditions these geographic populations presently experience. Such specific limitations indicate differential thermal adaptation among these divergent groups. They may explain currently observed migrations in mussel distributions and invasions. Our results provide a practical framework for the thermal ecophysiology of bivalves, the assessment of environmental changes due to climate change and its impact on (and consequences for) aquaculture

Biological response to mesoscale eddies in the Algerian Basin

International audienceThe Eddies and Leddies Interdisciplinary Study in the Algerian Basin (ELISA) experiment (1997–1998, MAST‐3/MTP2/MATER program) was a multidisciplinary and multiplatform experiment designed to study the anticyclonic Algerian Eddies (AEs) generated by the instability of the Algerian Current and their influence on the general circulation and biological phenomena. This paper presents preliminary results of the data obtained over the four year‐round cruises ELISA‐1 to 4. Two AEs (called 96‐1 and 97‐1) were tracked with satellite images over their counterclockwise circuit in the eastern Algerian Basin, from the Algerian to the Sardinian slope, and then to the open sea. They have been sampled over different periods and positions. Associated biological response was analyzed considering the hydrodynamical structure and the distribution of chlorophyll and nitrate concentration in the upper 300 m. In summer both AEs (96‐1 located along slope and 97‐1 located offshore) corresponded to highly oligotrophic areas. The deep chlorophyll maximum was ∼90–110 m, with concentrations up to ∼1 mg m−3, the nitrate concentrations were low down to ∼250 m in the AEs' central zone. The downward entrainment of chlorophyll along isopycnals in the AEs' peripheral zone was well observed down to ∼250 m. In spring the maximum integrated chlorophyll concentrations were found offshore, in 96‐1, where the upper ∼150 m were quasi‐homogeneous. Lower integrated chlorophyll concentrations were found inshore in 97‐1, which was embedded in an Algerian Current meander and remained stratified throughout wintertime. AEs generate secondary phenomena such as small‐scale cyclonic shear eddies, where the highest chlorophyll concentrations (∼4 mg m−3) were found. We show that through the AEs it generates, the Algerian Current can be responsible for producing areas in the coastal zone that are at least as oligotrophic as the eastern Mediterranean and, alternately, for productive areas offshore. As AEs generally follow a counterclockwise circuit in the Algerian Basin, it is inferred that they play an important role in the redistribution of matter on a basin scale. However, the biological response associated with AEs varies according to their history, a combination of trajectory, location, and season, in ways that are not yet clear

Linking French Atlantic rivers to low salinity intrusions in the western English Channel: highly resolved monitoring from the EU FerryBox project

Low salinity surface waters at the southern entrance (Ushant) to the western English Channel have been monitored from a near-continuous record of data collected on a ferry operating between Portsmouth (UK) and Bilbao (Spain) since April 2002. These fresh waters originate from poleward flowing plumes from French Atlantic rivers (e.g. Loire). Winter river outflows were above average in 2004 and 2003 and average in 2002 which is consistent with the minimum salinities observed near Ushant in late winter (33.68, 33.90, 34.53 in 2004, 2003, 2002, respectively). These surface water masses intrude into the western English Channel in all three years of monitoring, suggesting a frequent phenomena. The extent of the intrusion is linked to prevailing winds, with southerly winds favourable to intrusion. In contrast, northerly winds force the plume waters offshore. It is hypothesised that the more intense of these low salinity intrusions (in 2003) can enhance summer blooms of the red tide forming dinoflagellate Karenia mikimotoi through both increased stability of the upper water column and nutrient supply

Coastal submesoscale processes and their effect on phytoplankton distribution in the southeastern Bay of Biscay

Submesoscale processes have a determinant role in the dynamics of oceans by transporting momentum, heat, mass, and particles. Furthermore, they can define niches where different phytoplankton species flourish and accumulate not only by nutrient provisioning but also by modifying the water column structure or active gathering through advection. In coastal areas, however, submesoscale oceanic processes act together with coastal ones, and their effect on phytoplankton distribution is not straightforward. The present study brings the relevance of hydrodynamic variables, such as vorticity, into consideration in the study of phytoplankton distribution, via the analysis of in situ and remote multidisciplinary data. In situ data were obtained during the ETOILE oceanographic cruise, which surveyed the Capbreton Canyon area in the southeastern part of the Bay of Biscay in early August 2017. The main objective of this cruise was to describe the link between the occurrence and distribution of phytoplankton spectral groups and mesoscale to submesoscale ocean processes. In situ discrete hydrographic measurements and multi-spectral chlorophyll a (chl a) fluorescence profiles were obtained in selected stations, while temperature, conductivity, and in vivo chl a fluorescence were also continuously recorded at the surface. On top of these data, remote sensing data available for this area, such as high-frequency radar and satellite data, were also processed and analysed. From the joint analysis of these observations, we discuss the relative importance and effects of several environmental factors on phytoplankton spectral group distribution above and below the pycnocline and at the deep chlorophyll maximum (DCM) by performing a set of generalized additive models (GAMs). Overall, salinity is the most important parameter modulating not only total chl a but also the contribution of the two dominant spectral groups of phytoplankton, brown and green algae groups. However, at the DCM, among the measured variables, vorticity is the main modulating environmental factor for phytoplankton distribution and explains 19.30 % of the variance. Since the observed distribution of chl a within the DCM cannot be statistically explained without the vorticity, this research sheds light on the impact of the dynamic variables in the distribution of spectral groups at high spatial resolution

Low salinity intrusions in the western English Channel

Low salinity (<35) surface waters (LSSW) at the southern entrance to the western English Channel (48.5°N, 5.1°W, near Ushant) were observed in late winter (March–April) in three successive years (2002–04) during near continuous ship of opportunity operations. The source of the LSSW is the northward spreading plumes from the Loire (47.5°N, 2.5°W) and Gironde (45.6°N, 1.2°W) along the French Atlantic coast. Fastest plume travel times were associated with northeasterly winds, consistent with Ekman theory. Differences between years in the mean winter (January–March) combined river discharges (D) was consistent with the minimum salinities (Smin) of the LSSW (2004: D=4211 m3 s?1, Smin=33.68; 2003: D=3630 m3 s?1, Smin=33.90; 2002: D=1579 m3 s?1, Smin=34.53). Winter mean (1905–74) salinity is otherwise 35.33 near Ushant.The LSSW intruded into the western English Channel in each year, suggesting a common phenomenon. The low salinity intrusion was freshest (mean=35.11±0.21) and most penetrative (reaching 50.7°N, 1.0°W by the end of the year) in 2003 on account of (1) entering on a spring tide giving greater tidal excursion into the western English Channel and (2) intrusion favourable winds (southwesterly/southeasterly) acting on the longer term residual flow. Less penetration occurred in 2004 when the arrival of the LSSW coincided with a neap tide followed by intrusion-resistant northwesterly winds, resulting in a less saline (mean=35.20±0.23) intrusion. In 2004, transport tended to be offshore to at least 100 km from the French Atlantic coast (47°N, 4.8°W–48°N, 4.7°W). In 2002, the lower volume of plume water relative to the other years produced a more saline intrusion (mean=35.25±0.12). Prevailing westerly winds may have pushed this intrusion northwards beyond the route of the ferry, making it difficult to assess the true extent of the intrusion in 2002. A link of the LSSW to phases of the winter North Atlantic Oscillation index from a literature search of the last 84 years was inconclusive

Tracking dynamic mesoscale structures in the Algerian Basin during operation ELISA (1997-1998) by satellite thermal imagery (NOAA/AVHRR): Potential obstacles to automatic recognition

The amplitude of the variability associated with dynamic mesoscale phenomena is most often greater than that associated with seasonal variability. The role of these phenomena is of the utmost importance as they modify the general circulation of the water masses, and thus, potentially, the circulation in the coastal zone too. But in situ observations at mesoscale require fine spatiotemporal sampling, requiring much effort. Under these conditions, however, NOAA/AVHRR thermal satellite imagery is an extremely efficient tool, as it routinely provides information on a spatial domain over thousands of kilometres, and can generate high-resolution long-term time series. Providing somne precautions are taken, thermal signatures can be interpreted in terms of dynamical structures and associated currents. The use of this satellite imagery within the operation ELISA (1997-1998) provides an opportunity to review the potential limitations to an automatic recognition (detection and tracking) of such mesoscale structures