Factors influencing primary production of seagrass beds (Zostera noltii Hornem.) in the Thau lagoon (French Mediterranean coast)

The primary production and the respiration of Zostera noltii beds in the Th au lagoon were studied by means of the benthic bell jar technique, Concurre ntly, environmental data (temperature, light and nutrients) as well as morp hological data of seagrass meadows (leaf width and height, density of shoot s, above/below-ground biomass ratio) were collected with the purpose of exp laining most of the observed variations in metabolism, Seagrass plus epiphy te respiration rates were influenced mainly by the water temperature, showi ng a typical exponential response to an increase in temperature, Surprising ly, measurements of production rates were not related to incoming light int ensities recorded at the seagrass canopy level, An equation frequently used for terrestrial standing crops, involving the leaf area index (LAI) and th e characteristics of the canopy architecture (parameter K, depending on lea ves optical and geometrical properties), was applied to the seagrass ecosys tem in order to estimate the light energy actually available for the plants , i.e. the light intercepted by the seagrass canopy (Q(abs)). Linear relati onships were then validated between gross production rates and calculated Q (abs) for Z. noltii beds, and the best fits were obtained with K values nea ring 0.6, confirming the similarities between terrestrial graminaceae and s eagrasses, A linear regression model for primary production is proposed, in volving the calculated Q(abs), the water temperature and the leaf nutrient content. (C) 2001 Published by Elsevier Science B.V.