Thau Lagoon Research Articles

A preliminary model of nutrient cycling in sediments of a Mediterranean lagoon

A model of nitrogen and phosphorus cycles in the sediment of a lagoon has been developed. This model was applied to the Thau lagoon (southern France). Sediment was sliced in three layers to reproduce the oxygen profile, which is simulated within the model. Following an equilibrium hypothesis, the model was calibrated against field data. State variables and fluxes were estimated in the sediment and across the sediment-water interface. A Monte Carlo sensitivity analysis was performed to determine the most sensitive parameters and sediment state variables. A dynamic simulation with varying oxygen concentrations then showed the influence of anoxia on the phosphorus and nitrogen fluxes between water and sediment.

Morphosedimentary multiyear changes on a barred coast (Gulf of Lions, Mediterranean Sea, France)

An overall estimate of the relationship between the changes in the nearshore morphology, the evolution of the sedimentary budget, the beach morphodynamics and the variation of grain-size characteristics along the beach barrier of Thau Lagoon, Languedoc, France, was attempted by synthesizing the data from different surveys: a basic survey of two separate grids of 11 closely-spaced (50 m) transverse profiles during 1989–1991, a long-baseline (11 km) survey of 11 beach profiles during 1988–1991 and an earlier, long-run (1984–1990) survey of 6 widely-spaced bathymetric profiles covering discretely the whole stretch of the nearshore zone. Time intervals between profile monitorings within each type of survey differed, but many measurements during the recent 1989–1991 period were coincident.The stable two-bar system that was generally developed all over this nearshore zone has been gradually changing at the northeastern end, south of the town of Sète, since 1989, under the effect of a series of new defense structures upstream in the dominant NE-SW wave-induced longshore current, with a tendency toward a single-bar profile. The average nearshore slope was steeper (0.91%) in the Sète close-survey area than in the southwestern area (0.83%). During the same period, acceleration in beach erosion has been observed at the northeast, in contrast with a stable dynamic equilibrium over the southwestern beach and nearshore profiles.Over a longer period (1984–1990), scouring of the nearshore sand-prism down to the 10-m isobath at the northeastern end between the extreme bathymetric profiles, could be estimated at about 37,000 m3, while the overall sedimentary budget was positive.Bar migration and morphologic changes in the study areas were not related to any seasonal cycle in the wave climate, but were more significant when this was more energetic, as during 1989. An overall attenuation in nearshore morphodynamics was observed during the following two years, although some rapid local changes between consecutive measurements could be detected.Temporal variation of grain-size characteristics were therefore small and/or not significant. Conversely, on the subaerial beach, alongshore differentiation in mean diameter and in sorting and skewness parameters is consistent with the evolution of the erosion-accumulation processes following a long-period wave-like sand-transfer from northeast to southwest.

Distribution and dynamics of bacterial production in relation to wind perturbations in a Mediterranean lagoon

We examine the relationship between spatial and temporal variations of bacterial production (3H‐thymidine incorporation into DNA) and wind‐induced physical perturbations in a marine coastal ecosystem. Spatial and temporal patterns of bacterial production are described on different scales in Thau Lagoon (France). The spatial patterns generally consist of gradients whose highest values are located in areas of permanent input from a watershed. On the seasonal scale, these patterns are relatively constant, and only the mean values of thymidine incorporation change. On the daily scale, the spatial patterns can be less constant. The variability arises from the intensity of wind forces acting on the surface, which produce hydrodynamic changes. A strong relationship (r = 0.91, P ≤ 0.0001) is demonstrated between windspeed and the spatial variance of bacterioplankton production. The frequency of windspeed > 5 m s−1 also affects daily change in thymidine incorporation into DNA. Physical processes must be taken into account to explain spatial and temporal patterns of bacterial variables in marine coastal environments.

Influence of the Drainage Basin input on a Pathogenic Bacteria (Salmonella) Contamination of a Mediterranean Lagoon (The Thau Lagoon - France) and the Survival of This Bacteria in Brackish Water

Research has been carried out over a 10 month period with the aim of identifying potential sources of Salmonella contamination in a Mediterranean lagoon (the Thau Lagoon). Two types of source have been sampled, permanent ones which are checked monthly, and incidental ones. The highest occurrence of Salmonella is linked to episodic events brought on by rainfall. After summer low water levels, the rising of river water brings back into circulation water from pools previously formed by waste water from sewage treatment plants and sediments, thus creating a significant source of bacteria. The sewer system can also supply the Thau Lagoon with varying quantities of Salmonella when the system breaks down, and also when it is overloaded by an excess of rainwater following a storm. Monthly checks reveal that discharges of Salmonella occurred occasionally in permanent inputs, whereas the outflow from the Mèze pond provides a quasi-permanent, but negligible, source of contamination. The major risk of shellfish breeding contamination is thus associated with rainfall, and the occasional overflow of raw wastewater.The survival of Salmonella in the lagoon waters varies in function with the seasons, and the risk is greatest at those times of the year when autopurification can be affected.

Phytoplanktonic size spectrum modifications induced by shell-fish in thau lagoon (France)

Phytoplanktonic size spectrum modifications induced by shell-fish in thau lagoon (France)

Wind‐induced hydrodynamic control of the phytoplankton biomass in a lagoon ecosystem

To provide a closer insight into the relationship between wind‐induced hydrodynamic motion and the biological dynamics of shallow bodies of water, an alternating conditional expectation (ACE) algorithm has been used to examine the nonlinear relationship between the phytoplankton biomass in the Thau lagoon (southern France) and wind intensity. Chlorophyll content (mg m‒3), periodically sampled at diff.erent stations, was used as the dependent variable, and windspeeds (m s‒3) averaged over both short and long intervals preceding each sampling day, were used as predictor variables. Results showed a dome‐shaped relationship between chlorophyll content and mean windspeed. The optimum value of ∼4.0 m s‒1 was strongly associated with an alternation between the dominant hydrodynamic processes involved, i.e. the beneficial effect of vertical turbulent diffusion, and the detrimental influence of horizontal advective dispersion. Spatiotemporal development of this nonlinear relationship is proposed, based on environmental thresholds, i.e. a depth of <4 m, and a wind‐stress memory of 1–4, 5–8, and 9–13 d. An environmental optimum for phytoplankton enhancement within these time‐space limits is found.

Flow cytometry approach of the phytoplankton structure of the thau lagoon

Flow cytometry approach of the phytoplankton structure of the thau lagoon

V.I.T.E.: Short-range modelling of a pollutant input

The VITE software uses a random-walk method to model the dispersive transport of pollutant plumes. Convective currents are calculated by a bidimensional hydrodynamic model and dispersive effects are described by a random velocity that is related to time and turbulent level. The evolution rates of the pollutant are taken into account by a stochastic approach. The VITE software runs on a PC computer and integrates a graphic tool that displays the simulated pollutant patch on the screen. The model is exemplified on the Thau lagoon (French Mediterranean coast).

Structure spatiale des sels nutritifs au sein d'un ecosysteme lagunaire: l'etang de thau

The present studies were undertaken to assess the geographical distribution of the nutritive richness in the Thau lagoon (Hérault, France). Three descriptors of nutrients in this ecosystem were studied: ammonium, nitrates and phosphates. A systematic sampling strategy was adopted (Frontier, 1983): 63 sampling points described the lagoon; the distance between two contiguous sampling points was 1 km (Fig. 1). The variables were evaluated at each point, during 1 year, at four sampling periods. Analytical methods for nutrient determination were described by Aminot and Chaussepied (1983) and Parsons et al. (1984). The automatic procedure for nitrates and phosphates determination proposed by Tréguer and Lecorre (1975) was used. The following statistical methods were applied: 1. (1) descriptive statistics for each period 2. (2) contiguous grouping linked to: ◦ - hierarchical classification, the similarity between the two points was calculated by the Gower coefficient (Legendre and Legendre, 1984); ◦ - k is a classification method using the later partition as an initial one. Both programs using these computations were part of an R package (Legendre, 1985) 3. (3) discriminant analysis for testing geographical zones obtained by grouping methods (Romeder, 1973). The stepwise discriminant analysis gives the most discriminant period in the different groups. The neighbor discriminant procedure indicates the percent of well-classified sampling points. Statistical characterization of periods for nitrates is shown in Table 1. The two winter periods have high nitrate concentrations. Grouping methods give seven geographical zones (Fig. 2). Statistical characterization of these zones is indicated in Table 2. The most discriminant period for this zonation is February (Table 3), June was not included. February explains 70% of well-classified sampling points (Table 4). Combining February with other periods explains specific groups, i.e. group 4 is confirmed by adding the May period and group 1 by adding the October period. The zonation results from two gradients due one to winter nitrates inflow, and one to summer nitrates consumption. Four sampling points were never well-classified, because of the geographical constraint. Statistical characterization of periods for ammonium ion is given in Table 5. As for nitrates, the two winter periods have high ammonium ion concentrations and summer periods have low ammonium ion concentrations. Grouping methods give six geographical zones (Fig. 3). Statistical characterization of these zones is given in Table 6. The correlation coefficients of the four periods are approximately equal (Table 7), therefore the combination of the four periods give a very good percent of well-classified sampling points (Table 8). October period explains group 2, May period group 4 and February period confirms the percent of groups 1 and 3 given by the June period. For ammonium ion, discriminant analysis shows the contribution of the four periods, each period has a different gradient concentration due to different ecological phenomena (sediment release, inflow, consumption, regeneration). Eleven sampling points are never well classified (Fig. 3), they are on groups borders. Statistical characterization of periods for phosphates is given in Table 9. Grouping methods give five geographical zones (Fig. 4). Statistical characterization of these zones is given in Table 10. The correlation coefficients of the four periods are high (Table 11). Table 12 shows that only group I has always a high percent of well-classified sampling points. The geographical zonation of phosphates seems inadequate. The data collected during 1 year, analysed by classification methods, gave the spatial structure of the lagoon appropriate to any period. Our studies showed different spatial distribution for the three nutrients during 1 year. An attempt was made to link these spatial patterns to environmental changes. In addition the discriminant analysis revealed three kinds of singular sampling points: • - located on the lagoon border • - located between two adjacent groups • - located near waste water treatment plant.

Interpretation of the seasonal variations of nutrients in a mediterranean lagoon: �tang de Thau

A network of 63 stations was used on four occasions (June 1986, October 1986, February 1987, and May 1987) to study the spatio-temporal distribution of inorganic nutrients in Thau Lagoon (‘l'etang de Thau’), which covers 7500 hectares on the French Mediterranean coast. Three environmental factors, revealed by multiple regression models, govern the distributions observed. Allochthonous inputs from the watershed enrich the environment with nitrogen and phosphorus compounds in the winter and autumn. Internal sources are essentially localized in the shellfish breeding zone of the lagoon. In the summer, shellfish excretions and the rapid remineralization of organic deposits produce ammonium ions. Uptake by phytoplankton has a much larger impact on the seasonal variation of inorganic nitrogen than on that of phosphorus; the latter is present in excess in the lagoon waters. Thus, nitrogen appears to be the primary limiting nutrient for the development of the chorophyllous biomass.

Sargassum muticum begin to invade the mediterranean

In the Mediterranean, the Japanese brown alga Sargassum muticum (Yendo) Fensholt is no longer confined to the relatively closed Thau lagoon (France), but is spreading along the Languedoc open-sea coasts: 11 new localities, from Grau du Roi (Gard) to Port-la-Nouvelle (Aude), are mentioned.