Relations entre apports terrigènes et conchyliculture dans les Pertuis Charentais

This work presents interactions between quantitative and qualitative river freshwater inputs and the shellfish farming (oyster and mussel) in the Pertuis Charentais. The quantity of freshwater (i.e. salinity) seems to have a weak influence on the shellfish farming contrarily to its quality determined by particulate and dissolved matters contained in the water. In autumn and winter, large precipitations have a "globally positive" effect amending the coastal ecosystem. Associated dissolved nutriments and the organic matter largely determine the quality of the coming spring growth for bred shellfish, itself controlling in turn the annual yield efficiencies. However, in winter their effects are postponed because of strong mineral load, low luminosity and temperature, then limiting the primary production. The spring contributions, directly linked to territorial practices, agriculture and tourism are more variable in quantity and quality from one year to another. They often correspond to high-risk inflows since numerous substances from anthropogenic watersheds can be found diluted in the coastal zone as in the Pertuis Charentais. Their impacts on in situ estuarine ecosystems are still poorly known since these substances are mainly studied and estimated in laboratory in controlled conditions. Several studies showed anthropogenic contaminations (i.e. cadmium, pesticides) could have significant direct or indirect effects on shellfish farming. For instance, the "summer" mortalities between 1990 and 2000 in the South of the Marennes-Oléron bay (MOB), that induced environmental and physiological oyster disorders, could be linked to pesticide effects, measured during consecutive years on the oyster bed of Ronce Perquis in the South of the MOB. The weak results from the spring larval rearing of the IFREMER experimental hatchery in the South of the bay, and chromosomal abnormalities measured on the stocks of wild oysters of the Pertuis could confirm a high-risk spring environment for the shellfish farming. In summer terrestrial inputs are reduced by low precipitations, anthropogenic water removals (drinking water, irrigation) and by plant evapotranspiration. Consequently certain years, a significant salinity increase in water masses of the Pertuis Charentais is observed. However, based on long-term observations, the significant interannual variability noticed in freshwater contributions constitutes one of the most important facts of these last years. When contributions are weak (i.e. 1991 and 2011), the mean annual salinity is 34.5 in the MOB. To the contrary, other years (i.e. 1977, 1981, 1983 and 1988), the mean salinity reduced to 30.5 shows the significant freshwater contributions to the bay. Elsewhere, particularly in the mediterranean region, oyster breeding water conditions characterized by high salinity values show the freshwater does not seem to be necessary for biological functions of the Pacific oyster Crassostrea gigas. Indeed, the oyster embryonic life in particular is well adapted to high salinity values as long as trophic resources are substantial and temperatures remain high. These two factors firstly condition the embryonic survival before the water salinity. Besides, in the Pertuis Charentais, wind conditions and the geographical bloodstock position rather determine the success of the larvae capture than seawater physic-chemical conditions. Finally, a misunderstanding still remains on summer freshwater contributions to the oyster larvae food supply.

Résultats de la mise en oeuvre des réseaux REPHY et SRN – Zones côtières de la Manche orientale et de la baie sud de la Mer du Nord – Bilan de l’année 2015

The coastal zone of the Nord – Pas de Calais / Picardie showed dysfonctioning patterns of the ecosystem considered to be link to human activities along shores. These results in regular massive development of species, such as the phytoplanktonic seaweed, Phaeocystis sp. which life cycle was partly linked to nutrients availability and consequently to anthropogenic inputs. As part of the evaluation of the influence of continental inputs on the marine environment (nitrates, phosphates,…) and on potential eutrophication processes, of the estimation of the efficiency of the sewage treatments plants in the possible elimination of dumpings and in order to establish a long-term survey to follow up the change in coastal waters quality, the regional nutrients monitoring network was implemented by Ifremer in collaboration with the Agence de l'Eau Artois-Picardie in 1992 in order to complete the REPHY (Phytoplankton and Phycotoxins) monitoring programme. This study reports the main results for the year 2015 in terms of temporal change of the main physico-chemical and biological parameters characteristic of water masses sampled along three transects offshore Dunkerque, Boulogne-sur-Mer and the Bay of Somme.

Observations of turbulence within the surf and swash zone of a field-scale sandy laboratory beach

Current coastal-evolution models generally lack the ability to accurately predict bed level change in shallow (<~2 m) water, which is, at least partly, due to the preclusion of the effect of surface-induced turbulence on sand suspension and transport. As a first step to remedy this situation, we investigated the vertical structure of turbulence in the surf and swash zone using measurements collected under random shoaling and plunging waves on a steep (initially 1:15) field-scale sandy laboratory beach. Seaward of the swash zone, turbulence was measured with a vertical array of three Acoustic Doppler Velocimeters (ADVs), while in the swash zone two vertically spaced acoustic doppler velocimeter profilers (Vectrino profilers) were applied. The vertical turbulence structure evolves from bottom-dominated to approximately vertically uniform with an increase in the fraction of breaking waves to ~ 50%. In the swash zone, the turbulence is predominantly bottom-induced during the backwash and shows a homogeneous turbulence profile during uprush. We further find that the instantaneous turbulence kinetic energy is phase-coupled with the short-wave orbital motion under the plunging breakers, with higher levels shortly after the reversal from offshore to onshore motion (i.e. wavefront).