Drifting away from reality : A cheap way to get lagrangian measurements

&amp;lt;p&amp;gt;Beauty may sometimes lie in the eyes of the beholder, but in science it always lies in simplicity. We tested a very simple concept to get drifting platforms&amp;amp;#160; that we could track and equip with sensors. We equipped an available floating device with a commercial GPS tracking system.&amp;amp;#160; We tested this in several campaigns ( Italia, New-Caledonia, Tunisia and Guadeloupe) to study surface drifts. Later, we added chemical sensors to collect of lagrangian measurements. Here we present&amp;amp;#160; the general setting of the drifter and the results of the first tests, which proved its efficiency and robustness despite its cheapness and its simplicity to use. We also discuss possibility of adding various kinds of sensors.&amp;lt;/p&amp;gt; </jats:p

Tidal influence on the hydrodynamics of the French Guiana continental shelf

This study investigates the circulation on the French Guiana continental shelf under tidal influence. Indeed, hydrodynamics are characterised by a weak salinity tongue located in the middle of the shelf and induced by the Amazon River, a coastal current flowing from the southeast, and a tidal standing wave whose co-range lines are parallel to the coast. In addition to field observations, a numerical model also is used to evaluate the tidal influence on coastal circulation. The model makes use of the MOBEEHDYCS code, a three-dimensional free surface time-splitting model whose domain is bounded with a closed coastal boundary, two active boundaries (offshore and lateral) and a passive boundary. The boundary configuration and hydrodynamics require a careful choice of passive open boundary conditions. The initial and boundary conditions come from field data. The tidal currents are essentially cross-shore and do not have a great influence on the main current direction on the offshore part of the shelf. The offshore currents remain parallel to the coast. In the inner shelf, the tidal influence is found to be much more important and the tidal currents can reach 0.45 m/s. Vertically, the tidal currents are barotropic, in spite of the high stratification and they induce a horizontal cross-shore migration (about 3 km) of the weak salinity tongue and vertical oscillations of the halocline without complete mixing

Refugia area for the ctenophore Mnemiopsis leidyi A. Agassiz 1865 in the Berre Lagoon (southeast France) : the key to its persistence

The invasive ctenophore, Mnemiopsis leidyi has been proliferating in lagoons and coastal areas around Europe for almost 20 years but the role and the impact of its presence in sink ecosystems is still not completely understood. In France, Mnemiopsis leidyi is present in Mediterranean lagoons and estuaries along La Manche Sea. Mnemiopsis was first recorded in the Berre Lagoon in early 2000. This lagoon has been highly perturbated for years, with a large volume of freshwater inflow through natural rivers and a succession of large hydroelectric power plants, inducing important eutrophication. Legislation has been implemented to improve the health status of the lagoon since 1994. A long-term study was undertaken in 2010 to measure Mnemiopsis population dynamics, and to identify the main drivers of its persistence in this highly anthropogenic lagoon. In 2011 and 2012, during extreme winter conditions, populations of this ctenophore were not observed for months. Its re-appearance later in year could be linked to either a new introduction from the Mediterranean Sea or the existence of retention areas where individuals sought refugia. Following measurement of biochemical conditions (i.e. chlorophyll a), plankton biomass and Mnemiopsis populations structure (eggs, cydippid larvae/transitional phase and adult) in different areas of the lagoon, as well as the lagrangian modelling of "particles' distribution (i.e. Ichthyop), we highlighted the seasonal patterns in the population structure, the level of available carbon always above the minimal for the survival of Mnemiopsis (24 mu gC L-1) and the potential refugia area role the Vaine sub-basin could play. Populations from the Vaine sub-basin probably serve as source populations for the rest of the lagoon by advective transport in spring

Effects of the sandbar breaching on hydrobiological parameters and zooplankton communities in the Senegal River Estuary (West Africa)

This study describes the changes in hydrology, zooplankton communities and abundance in the Senegal River Estuary (SRE) before and after the breaching of the sandbar in October 2003. Samples were taken in 2003 at 3 stations located upstream (DI), in mid estuary (HY) and downstream (RM), and in 2005 at the same stations (RM becoming Old River Mouth: ORM), plus the new river mouth (NRM) resulting from the morphological evolution of the SRE. The study showed marked seasonal variations that affected the structure and distribution of zooplankton as well as major changes caused by the sandbar opening: - increased marine influence throughout the whole SRE, - changes in the horizontal gradients, - arrival of euryhaline species and increase in meroplankton, in particular decapod larvae, - transformation of the ORM area into a slackwater area with limited exchanges and the highest zooplankton numbers during high waters

Numerical analysis of the combined action of littoral current, tide and waves on the suspended mud transport and on turbid plumes around French Guiana mudbanks

Large mudbanks migrate westwards in the nearshore zone from the Cabo Cassipore in the Amapa state (Brazil) to the Waini River in Guiana. These mudbanks are noticeable by their size (about 4 x 10(9) m(3) of sediment) and by the sediment dynamics they induce. Notably, visible remote sensing pictures present high turbid mud plume associated to mudbank erosion. The sediment transport is directly linked to the ambient forcing-littoral current, waves, and tide. In this paper, the turbid plume and the suspended mud transport around Guiana mudbanks are studied through a three-dimensional numerical study, under the three main different forcings. The study aims at describing the plume and the action of various physical processes in the suspended mud transport. The model results qualitatively agree with known observations issued from the literature. It is found that, the erosion-at the back of the bank-and the deposition-in front of the bank-could partly explain the migration process of these mudbanks. Waves are fundamental to create the erosion/deposition process, but littoral current and tide modulate it. Bottom flux and plume location vary with tide and these oscillations are accentuated during spring tide. In the same way, the wave incidence angle can explain the variability of erosion and deposition rate velocity along the Guiana coast