Nabila Mazouni

Modelling nitrogen, primary production and oxygen in a Mediterranean lagoon. Impact of oysters farming and inputs from the watershed

An ecosystem model based on nitrogen cycling and oxygen has been developed for the Thau lagoon. It takes into account the specific features of this Mediterranean lagoon, a semi-confined system with watershed inputs and oyster farming. The ecosystem model uses currents calculated by a two-dimensional hydrodynamic model and integrated into a box model. This model is compared with a year survey data and used to estimate nitrogen and oxygen fluxes between the different ecosystem compartments. The yearly simulation shows that the ecosystem behavior is driven by meteorological forcing, especially rain which causes watershed inputs. These inputs trigger microphytoplankton growth, which is responsible for new primary production. During dry periods, nitrogen is recycled into the lagoon thanks to oysters excretion, sediment release, microzooplankton excretion and mineralization. Ammonium produced in this way is consumed by a population of pico- and nanophytoplankton causing regenerated primary production. Consequently, the ecosystem remains highly productive in summer even without external inputs. Shellfish farming also plays an important role in the whole lagoon through biodeposition. Driven by biodeposition, sediment release is the major source of nitrogen in the water column and causes oxygen reduction. The oysters contribute to the recycling activity by excretion, which supports the regenerated primary production. They are also involved in oxygen consumption by respiration which can cause local hypoxia. Further improvements are proposed before this model may become a functional environmental model for a lagoon ecosystem.

Nutrient and oxygen exchanges at the water–sediment interface in a shellfish farming lagoon (Thau, France)

The Etang de Thau (France) is a shallow lagoon characterised by the semi-intensive farming of oysters (Crassostrea gigas, Thunberg) cultured in suspension on frames. Analysis of the benthic fluxes of inorganic nutrients and oxygen over a period of a year has provided a basis for describing the dynamics of the water–sediment interface in the lagoon. Monthly measurements of fluxes at the water–sediment interface at two stations have been compared. One station (UC) is located under a culture table, and is subject to intensive accumulation of organic matter (biodeposition); the other (OC) is located outside the area directly under the impact of the culture activities. Oxygen consumption ranged from 288.24 to 1026.85 μmol·m−2·h−1 according to the season and the station. Ammonium production was maximal at station UC in Summer (600 μmol·m−2·h−1) and minimal at station OC in the Autumn (30 μmol·m−2·h−1). In general, the fluxes recorded at station UC were 1.8–3 times higher than those recorded at station OC for oxygen and 1–5 times higher for ammonium. Nevertheless, the variability between stations was lower than the seasonal variability. Using a Multiple Correspondence Analysis (MCA), it was possible to point out the occurrence of an atypical event that was responsible for the disruption of the seasonal cycle. This event was a state of hypoxia known locally under the generic name of malaigue. The dystrophic crisis consists of a major perturbation of the ecosystem, responsible for a massive mortality affecting both the benthos and the reared stocks.

Relations milieu-ressources · impact de la conchyliculture sur un environnement lagunaire méditerranéen (Thau)

Abstract Shellfish farming leaves its mark on the environment in which it has developed, and the men who depend upon it. These changes have altogether balanced the lagoon cycle and have caused disastrous episodic events. Increased water clarity caused by the uptake of particulate material by shellfish farming allows seagrass to grow in deeper areas of the lagoon (down to five metres). Shellfish farming nutrient transformations increase ecosystem productivity, even if the filtration pressure keeps phytoplankton biomass at a low level. Storage of phosphorus and nitrogen in animal tissue limits eutrophication in this ecosystem. Transfer of oysters from growout facilities increases animal and vegetal specific diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enrichment from biodeposition changes the specific composition of soft-bottom benthos. In the deeper areas, (less than six metres), where summer thermoclines limit oxygen transfer from surface water, the organically enriched substrate induces oxygen depletion and ammonium and nitrogen sulfide accumulation in the water column. This ecosystem dysfunction kills benthic populations, and sometimes reaches pelagic populations and affects the shellfish farming economy.

Influence of oyster culture on water column characteristics in a coastal lagoon (Thau, France)

The development of shellfish farming activities causes great changes in ecosystems functioning. In the Thau lagoon, oysters are reared on long constantly submerged lines, and these become fouled by several epifaunal species. The assemblage (oysters and epifauna) is defined as Oyster Culture Unit (OCU). The aim of our study was to estimate (i) how much the composition and the abundance of the epifaunal species can influence the nutrient and oxygen fluxes recorded at the shellfish—water interface and (ii) how these fluxes modify water column characteristics. We used Principal Component Analysis with Instrumental Variables (PCAIV). Two analyses were carried out, using sets of data on fluxes, the specific composition of the cultivated communities, and on oxygen, nutrient and chlorophyll a concentrations in the water column. The highest fluxes at the OCU—water interface were measured when epifaunal species richness was maximum. However, at our measurement scale (i. e. the oyster frame) no influence of this filter-feeders assemblage was observed on the chlorophyll a level. Conversely, we found a significant influence of oyster culture on the oxygen and dissolved nitrogen concentrations in the water column. The use of this recent factorial analysis was helpful to estimate the influence of the biofouling species composition on the fluxes at the OCU—water interface, and to estimate the potential impact of oyster cultures on the conditions prevailing in the water column.

Oxygen fluxes involving the benthic micro- and macrophytic components in the Thau Lagoon under pre-anoxic conditions

Measurements of oxygen fluxes at the water-sediment interface (transparent and opaque enclosures) were made on bare sediments inhabited by microphytes on the one hand, and on sediments diversely colonized by macrophytes (macroalgae and seagrasses) on the other hand. Five stations, typical of different biotopes of a Mediterranean shallow lagoon were studied from May to July 1993 in order to observe an anoxic event which usually occurs at that time of the year. Average diurnal respiration of benthic communities ranged from 40 mg (or 1.25 mmol) O2 m2 h−1 in bare sediments (31 % of the lagoon area) to 76 mg (or 2.37 mmol) O2 m−2 h−1 in sediments with a medium coverage of macroalgae (37 % of the total area) and, finally, to 100 mg (or 3.12 mmol) O2 m−2h−1 in the denser macrophytic area (32 % of the total area). The highest diurnal gross production was observed in the zone colonized by macroalgae and seagrasses, and especially in corridors between shellfish-cultivation tables (300 mg O2 m−2 h−1, or 9.37 mmol O2m−2 h−1, equivalent to 113 mg C m−2 h−1). Overall, during this period, net bottom oxygen production was close to nil in ca. 30 %, and positive in 70 % of the lagoon area. The average net oxygen production for the whole lagoon in summer is thus in the order of 100 mg O2 m−2h−1. In 1993, at the end of July, an anoxic event was avoided due to a period of strong wind.

Oyster Farming Impact on the Environment of a Mediterranean Lagoon (THAU)(Preliminary results of the OXYTHAU programme)

The lagoon of Thau is a shallow microtidal system exposed to terrestrial and marine influences. The residence time and the vertical mixing of water-mass are controlled by winds. Periodical absence of wind in Summer can lead to strong anoxia in sediments and bottom waters. Moreover, low inorganic nitrogen and chlorophyll-a levels confer seasonal oligotrophic features to the ecosystem. Nevertheless, a large productive stock of oysters (Crassostrea gigas) cultivated on suspended lines and colonized by epibiota (invertebrates, algae, etc...) covers 20 % of the total area. The aim of the present programme (OXYTHAU 1991–1995) is to study effects of oyster farming on the environment of the lagoon of Thau. Preliminary results obtained are presented here.

A Metadata Service for Managing Spatial Resources of Coastal Areas

Within the framework of the SYSCOLAG pluridisciplinary research programme on ICZM in the Languedoc-Roussillon region of France, we propose an ISO-19115-compliant metadata service to share informational resources. Each resource is described using a metadata form. Each form consists of textually and spatially controlled terms which are stored in a single DBMS (Postgres and Postgis). The graphical user-interfaces contain the expert knowledge (thematic and spatial ontologies) and include a mapping service (SVG and OGC-compliant WMS-WFS) that allows spatial and thematic queries to be addressed to the metadata service.