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

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

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

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

The salinity responses of tropical estuaries to changes in freshwater discharge, tidal mixing and geomorphology: case study of the man-affected Senegal River Estuary (West Africa)

Salinity in estuaries is influenced by a variety of processes including tidal advection and diffusion and river discharge. The effect of hydrodynamic features on salinity was studied in the Senegal River Estuary (SRE). This estuary is of strategic importance for large populations, but it has been greatly affected by human action (regulation of the freshwater inflow, change in the location of the river mouth ...), which has caused major changes in salinity and ecological functioning. To analyse the impacts of these changes and to determine the spatial and temporal variations in salinity in the SRE, we used a combination of three-dimensional modelling and field measurements. The overall salinity depends on freshwater inflow and tidal fluctuations. Salinity variation is mainly driven by ebb and flood near the mouth and by fortnightly cycles upstream. The enlargement of the mouth increases salinity, whereas its shifting location changes the location of the salinity front and creates a slack water zone downstream. Connection and disconnection of the tributaries also affects the salinity. The present study explains how the recent modifications have increased spatial variation and reduced seasonal differences of salinity and provides a tool for managing the water in the estuary

Impact of Tropical Cyclones on Pelagic Sargassum

International audienceThe strong proliferation of holopelagic Sargassum in the Tropical Atlantic from 2011, raises many questions on the environmental factors controlling their growth and decay at interannual, seasonal, and intraseasonal scales. In this work, we specifically investigate the response of the Sargassum aggregations to high wind events such as those found in Tropical Cyclones (TCs). The evolution of Sargassum coverage obtained from MODIS observations in the north tropical Atlantic Ocean (from 0 to 30°N and from 0 to 100°W) was analyzed under 86 historical paths of TCs and tropical storms from 2011 to 2020 that crossed Sargassum aggregations. Our results show on average a 40% drop in Sargassum coverage under TC trajectories, which can exceed 60% for the most intense TCs. We associate this drop with a sinking of Sargassum toward the deep ocean. Our estimates suggest that TCs contribute to the seasonal decay of Sargassum biomass from August to November

Microplastics in surface waters of the Gulf of Gabes, southern Mediterranean sea : distribution, composition and influence of hydrodynamics

The Mediterranean Sea has been described as one of the most affected areas by marine litter in the world. Although microplastics and their effects have been investigated in this area, most of the currently available studies have been limited to the northwestern part of the basin. This study constitutes a first attempt to determine the abundance, characteristics and composition of microplastics in near surface waters of the Gulf of Gabes (southern Mediterranean Sea, Tunisia). Samples were collected using a 200 mu m-mesh size trawl net along two transects. The study revealed an average concentration of 63,739 items/km(2) where fragments and films were the most frequent microplastics. Polyethylene, reformulated polyethylene and polypropylene were the most abundant plastics identified among the samples (86-100%). The influence of hydrodynamics on microplastics in the Gulf of Gabes was investigated through the use of a Lagrangian tracking model to simulate the dispersion of particles in water. Modelling results seem to be in agreement with the reported distribution and characteristics of microplastics in this area

Coupled effects of tide and swell on water renewal in a meso-tidal channel lagoon: Case of the Toliara Lagoon (Madagascar)

International audienceIn this article, based on the example of the Toliara lagoon in Madagascar, we identified the major impacts of the two main drivers of the water renewal in a typical channel lagoon: the tide and the swell breaking on the reef. For this purpose, we applied the numerical model Croco which is based on ROMS-AGRIF and uses a realistic and high-resolution configuration. The validation was performed with in situ ADCP data, then, realistic and sensitivity simulations were run to infer tides and waves role in ocean-lagoon exchanges and to estimate water age and water origin. The modelling approach adopted enabled an understanding of the effects of swell and tide on the water renewal. During low wave conditions, tide and waves counteract each other on cross-reef flow: the cross-reef inflow tends to increase with waves breaking on the reef, whereas cross-reef flow tends to decrease or to be outflowing with tidal action. Hence, the lagoon-ocean exchange is reduced and is at a minimum when wave height reaches 0.85 m and the tide has minimum amplitude. During high wave conditions, both tide and waves increase ocean-lagoon exchanges by increasing the cross-reef inflow, whereas during low wave conditions, the cross-reef flow is mainly outward with increase with tidal amplitude and decrease with wave height until the threshold of 0.85 m

Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the Ouano Lagoon, New Caledonia

This manuscript presents a combined experimental and numerical study of the impact of cross-reef fluxes on coral reef lagoon dynamics. The selected field site is the Ouano Lagoon (New Caledonia Island, France) in the South Western Pacific Ocean. Measurements of wave transformation above the reef and current profiles through passages and reef openings have been carried out during a 3-month survey. Data analysis reveals the preponderant roles played by both tides and waves on the lagoon dynamics. Based on field data, a simple parameterization of cross-reef fluxes is implemented in a coastal lagoon circulation model and a satisfactory agreement is found between parameterized model and field results. The model is thus used as a numerical experimental tool in order to analyse the cross-reef flows' possible influence on a narrow lagoon dynamics. The results highlight the importance of cross-reef fluxes induced by wave breaking over the reef barrier on the whole lagoon circulation and water properties