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The primary production and the respiration of Zostera noltii beds in the Thau lagoon were studied by means of the benthic bell jar technique. Concurrently, environmental data (temperature, light and nutrients) as well as morphological data of seagrass meadows (leaf width and height, density of shoots, above/below-ground biomass ratio) were collected with the purpose of explaining most of the observed variations in metabolism. Seagrass plus epiphyte respiration rates were influenced mainly by the water temperature, showing a typical exponential response to an increase in temperature. Surprisingly, measurements of production rates were not related to incoming light intensities recorded at the seagrass canopy level. An equation frequently used for terrestrial standing crops, involving the leaf area index (LAI) and the characteristics of the canopy architecture (parameter K, depending on leaves optical and geometrical properties), was applied to the seagrass ecosystem in order to estimate the light energy actually available for the plants, i.e. the light intercepted by the seagrass canopy (Q(abs)). Linear relationships were then validated between gross production rates and calculated Q(abs) for Z. noltii beds, and the best fits were obtained with K values nearing 0.6, confirming the similarities between terrestrial graminaceae and seagrasses. A linear regression model for primary production is proposed, involving the calculated Q(abs), the water temperature and the leaf nutrient content.

Relationship between environment and resources: impact of shellfish farming on a Mediterranean lagoon (Thau, France). Shellfish farming leaves its mark on the environment in which il 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 c1arity caused by the uptake of particuJate material by shellfish farming allows seagrass to grow in deeper areas of the lagoon (down to five metres). Shellfish farming nutrient transformations inerease ecosystem productivity, even if the filtration pressure keeps phytoplankton biomass at a low leveI. Storage of phosphorus and nitrogen in animal tissue limits eutrophication in this ecosystem. Transfer of oysters from growout facilities increases animal and vegetal specifie diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enrichment from biodeposition changes the specifie composition of soft~bottom benthos, ln the deeper areas, (Iess than six metres), where summer thermoclines Iimit 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 pelagie populations and affects the shellfish farming economy. © Elsevier, Paris lagoon 1 shellfish farming 1 biogeochemistry 1 benthos 1 eutrophication Résumé L'impact de la conchyliculture dans les lagunes méditerranéennes (étang de Thau) se manifeste aussi bien dans les mentalités des populations qui y vivent que dans le fonctionnement de la lagune. Les transferts d'huîtres entre bassins ont contribué à l'augmentation de la diversité spécifique des organismes végétaux et animaux. Le développement des supports colonisables dû à la conchyliculture a permis une installation massive de benthos de substrat dur, alors que l'enrichissement organique dû à la biodéposition a transformé le benthos de substrat meuble. Fonctionnellement, les transformations se font sentir aussi bien sur la masse d'eau (frein aux courants, enrichissement en sels nutritifs, impact sur le phytoplancton et la production primaire) que sur le système benthique (enrichissement en matière organique, développement des herbiers à zostères, changement faunistique). La conchyliculture joue aussi un rôle dans la limitation de l'eutrophisation de l'écosystème soumis aux apports du bassin versant. © Elsevier, Paris Jagune 1 conchyliculture 1 biogéochimie 1 benthos 1 eutrophie Oceanologica Acta 0399 1784/98/06l© Elsevier, PariS 831 des modificatÎons dans la structure et Je fonctionnement général d'un écosystème, qu'il soit humain ou naturel. En effet, le développement d'un maillon biologique (comme r ostréiculture) dans l'écosystème peut, par réaction, changer le niveau des interactions directes et indirectes J.-M. DES LOUS-PAOLI et al. entre les peuplements et avec leur milieu. au moins à travers leur impact dans la chaîne trophique.Ai...