Jean Blanchot

Man-induced hydrological changes, metazooplankton communities and invasive species in the Berre Lagoon (Mediterranean Sea, France).

The Berre Lagoon has been under strong anthropogenic pressure since the early 1950s. The opening of the hydroelectric EDF power plant in 1966 led to large salinity drops. The zooplankton community was mainly composed of two common brackish species: Acartia tonsa and Brachionus plicatilis. Since 2006, European litigation has strongly constrained the input of freshwater, maintaining the salinity above 15. A study was performed between 2008 and 2010 to evaluate how these modifications have impacted the zooplankton community. Our results show that the community is more diverse and contains several coastal marine species (i.e., Centropages typicus, Paracalanus parvus and Acartia clausi). A. tonsa is still present but is less abundant, whereas B. plicatilis has completely disappeared. Strong predatory marine species, such as chaetognaths, the large conspicuous autochtonous jellyfish Aurelia aurita and the invasive ctenophore Mnemiopsis leidyi, are now very common as either seasonal or permanent features of the lagoon.

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.

Identifying the drivers of abundance and size of the invasive ctenophore Mnemiopsis leidyi in Northwestern Mediterranean lagoons

Acknowledged as among the worst invasive marine species, Mnemiopsis leidyi has spread through European Seas since the mid-1980s. Here we report a bimonthly survey conducted in 2010-11 in three lagoons (Bages-Sigean, Thau and Berre) and at two adjacent coastal stations (Sète and SOMLIT-Marseille) along the French Mediterranean coast. M.xa0leidyi was present only in Berre and Bages-Sigean with maximum abundances observed in late summer. M.xa0leidyi adults were present year round in Berre with the largest organisms (∼6xa0cm) observed in April. In Bages-Sigean, they occurred in sufficient abundance to be recorded by fishermen between August and November. Multiple linear regressions highlighted that abundance in both lagoons was mainly influenced by direct effects of salinity and chlorophyll-a, and temperature to a lesser extent. While M.xa0leidyi has not yet been recorded in Thau, the lagoon is continually monitored to detect the potential establishment of M.xa0leidyi.

Hydrodynamics of the Toliara Reef Lagoon (Madagascar): Example of a Lagoon Influenced by Waves and Tides

ABSTRACT Chevalier; C.; Devenon, J.-L.; Rougier, G., and Blanchot, J., 2015. Hydrodynamics of the Toliara reef lagoon (Madagascar): Example of a lagoon influenced by waves and tides. In meso-tidal lagoons, the coral reef barrier can be temporarily submerged at high tide and partially exposed at low tide, which causes highly specific lagoon dynamics. To understand those dynamics, three field-measurement campaigns were conducted in the Toliara Lagoon (Madagascar), where the tide and the waves increase that effect. The method combined measurements taken from fixed moorings and measurements from a small, moving, inflatable boat along the transects through the reef passages. A tidal analysis methodology adapted to this lagoon was used to reconstitute the currents through the passages and to determine the spatial and temporal current variability. Then, the lagoon water dynamics were studied. The tidal dynamics of the lagoon appear to depend significantly on flows through the passages, although they were also affected by water passing across the reef. Water entered the lagoon through the passages during the tidal flow and left it during the ebb. The tidal-prism flushing time (estimated using the tidal prism method) was 1 day during spring tide and 5 days during neap tide. At the same time, the average flow during a tidal cycle appears to be directly linked to the waves breaking over the reef. Indeed, the average cross-reef flow was inflowing and was mainly driven by the ocean swell. That inflow was balanced by an outflow through the passages. Hence, the average bulk flushing time was estimated at 13 days during the wet season and 4 days during the dry season. Résumé Dans les lagons méso-tidaux, la barrière de corail peut être alternativement immergée à marée haute et quasiment émergée à marée basse ce qui entraîne une dynamique des masses deau tout à fait particulière. Pour comprendre cette dynamique, 3 campagnes de terrain ont été réalisée dans le lagon de Tuléar. Lapproche expérimentale combine à la fois des mesures en mouillages et des mesures lors de transects réalisés à partir dune petite embarcation. A partir de ces mesures et grâce à une méthode spécifique danalyse de données, nous reconstituons les courants dans les passes et obtenons leur variabilité spatiale et temporelle. Nous étudions alors la dynamique du lagon. Leau entre dans le lagon par les passes durant le flot et en sort durant le jusant. Cette dynamique est toutefois modulée par les courants au-dessus du récif barrière En effet, ces courants qui dépendent à la fois de la marée, mais aussi de la houle déferlant au-dessus du récif, peuvent engendrer des distorsions dans les ellipses de marée au voisinage du récif, générés une onde quart-diurne au sein du lagon. Enfin, alors que la dynamique tidale du lagon semble fortement dépendre du flux au niveau des passes, les flux moyennés sur un cycle de marée dépendent plus particulièrement du flux moyen au-dessus de la barrière et semblent directement lié à la houle déferlant le long du récif : le flux cross-récifal moyen est entrant et son intensité varie en fonction de la houle au large. Cet apport est compensé par un flux sortant au niveau des passes. Ainsi, le temps de résidence moyen est de 13 jours en saison humide et 4 jours en saison sèche.

The same but different: stable isotopes reveal two distinguishable, yet similar, neighbouring food chains in a coral reef

Stable isotope compositions were studied in particulate organic matter (POM), zooplankton and different trophic groups of teleosts to compare food chains based on plankton at two sites (lagoon and outer slope) in a New Caledonian coral reef. For each trophic compartment, d 13 C values were always lower in the outer slope than in the lagoon. This result may be explained by potential differences in POM composition between the two environments, suggesting that the two food chains are based on different primary sources of carbon. In contrast, d 15 N values did not vary between the lagoon and the outer slope, indicating that these two food chains presented similar length and trophic levels, despite being distinguishable.