A numerical model (MARS-2D) was developed, with the aim of describing the hydrodynamics that prevail in Arcachon Bay. Direct model results as well as derived mixing and transport timescales (tidal prism, local and integrated flushing times, age of water masses), were used to understand the behaviour of water masses and exchanges between the Bay and its frontiers. Particular attention was paid to the processes that drive the hydrodynamics (tides, wind and rivers), in order to understand their respective influence. The Arcachon Bay hydrodynamic system appears primarily to be highly influenced by tides; secondarily, by winds. About two third of the lagoon total volume is flushed in and out at each tidal cycle, which represent a mean tidal prism of 384 millions of cubic meters. The percentage of seawater flushed out during the ebb, that returns into the lagoon during the following flood flow is very high (return flow factor=0.95). This pattern leads to calculated integrated flushing times (IFT) ranging from 12.8 to 15.9 days, respectively, for the winter 2001 and summer 2005 simulations (two contrasting climatological situations: in summer, light northwesterly winds and low discharges in the rivers and, in winter, stronger southwesterly winds and higher river flows). Moreover, it has been found that northerly and westerly winds tend to reduce the flushing time, whilst southerly and easterly winds tend to hinder the renewal of the water in the Bay. The behaviour of the waters originating from the two main rivers of the lagoon, was studied also by means of the mean age assessment, under varying conditions of river flow and wind regime.
Mussels were translocated from a shell-fish breeding area (Sète, on the French Mediterranean coast) to sites exposed to trace element inputs in April 2000. They were recovered 3 months later. Whole soft tissues from all of the sites (n = 97) were analysed for arsenic, cadmium, chromium, copper, mercury, nickel, lead and zinc. Metallothioneins (MTs) were also measured in the digestive gland and in the remaining tissues (allowing calculation of whole soft tissue concentrations) at 22 of the 97 sites. MT concentrations in the digestive gland and the whole soft tissues were strongly correlated. The condition index varied with food availability at different sites. This did not influenced MT concentrations in the whole soft tissues, whereas the condition index was negatively correlated to trace element concentrations. A model is proposed to minimize this influence of condition. Metal concentrations adjusted using this model showed significant correlations with MT levels for those metals (cadmium, copper, nickel and zinc) that are known to bind to this protein, with the exception of mercury. Even in moderately contaminated sites, measurement of the MT level in the soft tissues of mussels was generally able to discriminate between different levels of contamination, allowing the use of a simplified procedure compared with dissection of the digestive gland. It is recommended to avoid translocation and sampling during the reproductive period, which is well documented for commercial species such as Mytilus sp.
Highlights ► A significant French production area lost 9000 t of blue mussels in winter 2014. ► We studied potential environmental drivers linked to those unprecedented mortalities. ► This winter 2014 had the sixth highest NAO + index since 1865. ► Such conditions characterized other winters without comparable mortalities in the area. ► Hydrodynamic connectivity participated to specific mortality vector diffusion.
The intertidal zone is characterized by a sharp vertical gradient of environmental stress, which structures species distribution and their interactions. Few studies, however, have examined the influence of tidal height on host–pathogen interactions. Here, we investigated how the tidal height influence outbreak of the Ostreid herpesvirus type 1 (OsHV-1) affecting the Pacific oyster. A volunteer network composed of 20 oyster growers monitored the survival of 28 batches of oysters during an epizootic event in Southern Brittany, France. Oysters were spat from wild collection or hatchery production. The sampling sites were spread over a 150-km2 area with a tidal height ranging from 0.98 to 2.90 m. Concomitantly, we followed survival of oyster spats in relation with OsHV-1 DNA detection at two sites and conducted risk analysis. We found that tidal height was associated with a lower risk of mortality. This effect was higher for hatchery than for wild oysters probably reflecting differences in health status. Our study opens perspectives for mitigation strategies based on tidal height and emphasizes the value of volunteer science in marine epidemiological studies.
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