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The three main estuaries in the French Atlantic coast – the Seine, Loire and Gironde, all with high tidal regimes – are interfaces between the continental and the coastal ecosystems. The Seine and Gironde are highly contaminated, whereas the Loire remains in a more natural state. Both the Seine and Gironde have suffered from harbour construction, and as a result, their biological units are extremely compartmentalized. Benthic species and communities have adapted to tolerate temporal physical and chemical changes ( e.g. salinity, substrata, depth, and levels of fine particles and oxygen) and human activities ( e.g. dredging, shipping traffic, and habitat reduction). Although numerous bio-indicators and indices are used to define the ecological quality status (EcoQS) of coastal waters, very few of them were developed specifically for environments with a mosaic of conditions and salinity levels, including freshwater. The main problem appears to be that all the indices for determining anthropogenic stress examine the abundances of stress-tolerant species, which may also be able to tolerate natural stressors such as those occurring in estuaries. This paper takes a look at the development status of the benthic indicators and index approaches used in the three main estuaries along the French Atlantic coast. In addition, it examines the adaptation of the different benthic indicators to the taxonomic sufficiency principle, and the adaptation of the Benthic Opportunistic Polychaetes Amphipods index (BOPA) and the Benthic Opportunistic Annelida Amphipod index (BO2A) for use in the freshwater zones of transitional waters ( i.e. up to the upper limit of the tidal range). Several perspectives are discussed in light of the diverse disturbances and the heterogeneity of such stressed zones, such as the use of multimetric and scoring approaches. [ABSTRACT FROM AUTHOR]

Abstract The Gironde mud fields are thin blankets (< 4 m) that extend from the inner to mid-shelf over a restricted area (600 km2 ) and overlie palimpsest sands and gravels seaward of the modern Gironde estuary, south-western France. The bulk of the mud has accumulated over the last two millennia and reflects the complex interactions of: (1) an increasing estuarine supply of suspended particulate matter; (2) resuspension of substrate material by storm events; and (3) biological mixing. The combination of these processes has produced various features within the deposits, related to: (1) the water depth and distance from source of the fine-grained material, which are manifested as a proximal–distal cross-shelf trend, from an area of alternating mud sedimentation (temporary after large river flows) with resuspension by storm events to an area of mud accumulation in deeper water where bioturbation predominates; (2) expansions of the mud depocentres over time, such that they become more remote from sources of coarser shelf sediment, resulting in an upward-fining trend. The lithofacies of the Gironde shelf mud fields exhibit a number of similarities to those of larger scale modern muddy shelves and ancient shallow-water mudrocks; this work provides a detailed description of a facies model for fine-grained shelf sediments deposited under relatively constant sea level. [ABSTRACT FROM AUTHOR]

The distribution of suspended particle size and concentration were measured along the Gironde Estuary, France, from the river seaward to the ocean. The suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage. Sediments discharged by the rivers coagulate upon encountering the very low salinities (0.2%) of the upper estuary (confirmed with laboratory experiments), and are then transported and deposited by currents in the remainder of the estuary. This coagulation, coupled with estuarine circulation, procudes a turbidity maximum which is offset between the surface and bottom waters. The floe size maximum is oceanward of the turbidity maximum and is, likewise, offset along the estuary by about 30 km. The estuary can be subdivided into the following zones: (1) coagulation; (2) hydrodynamic, landward of the null point; and (3) hydrodynamic, seaward of the null point. Initial coagulation appears to be completed in coagulation zone (1), and particles are transported and settled (with very little floc breakage and recoagulation) in zones (2) and (3) only. The floc settling velocities, coupled with estuarine circulation, control the concentration and size distributions of flocs in the water column, and eventually control the deposition of sediments. [ABSTRACT FROM AUTHOR]

The article looks into the patterns of intertidal zonation for a series of rocky shores on the Atlantic coast, ranging from Brittany to the Basque in France. The River Gironde localities include the Littorina, Balanoid and Laminarian zones. The Basque coast is divided into four zones including the Littorina, Balanoid, Corallina, and Cystoseira zones. The 'critical levels' concept is examined and is shown to be applicable to the Breton and Basque coasts. M.S. Doty's tide factor hypothesis is explored in relation to the vertical distribution of littoral organisms of St. Jean-de-Luz. The study considers that while tidal action does not produce zonation, it may account for the appearance of 'critical levels' on the shore through the action of 'tide factors' at certain levels.

An R-mode factor analysis was made on the grain-size classes of the sediments in the Gironde estuary in order to group the sieve sizes into dynamically significant factors. It was found that three main groups of sand populations exist within the distributional province investigated. Comparison with previous works suggests that each group reflects a specific transport mechanism. The main group boundaries are the same as those observed on the log-probability cumulative size curves of the sediments. The three main groups consist of the following sizes: finer than 3.0 Φ, 3.0 Φ–0.6 Φ, and coarser than 0.6 Φ. The three groups are thought to represent respectively ‘pure’ (or uniform) suspension transport, mixed suspension bed-load, and ‘pure’ bed-load (surface creep) transport. Furthermore, a subdivision exists in the mixed-suspension-bed-load group: the sediments between 3.0 Φ and 2.0 Φ seem to represent a ‘graded suspension’ transport while the sizes between 2.0 Φ and 0.6 Φ are thought to represent ‘saltation’ (graded suspension plus bed-load). All the samples containing more than 5% of the surface creep population plot either as ‘graded-suspension plus rolling’ or as ‘rolling’ in the CM pattern of the sediments. [ABSTRACT FROM AUTHOR]