Seasonal dynamics of Zostera noltii Hornem in the Bay of Arcachon (France)

Seasonal dynamics of Zostera noltii was studied during 1984 in Arcachon Bay, France. In this Bay, 2. noltii colonizes 70 km(2), i.e. approximately 50% of the total area, while 2. marina occupies only 4 km(2). Densities and length of vegetative and generative shoots and above-ground and below-ground biomasses were monitored in four meadows which differed according to their location in the Bay, tidal level and sediment composition. Three of these meadows were homogeneous, well-established beds whilst the fourth was under colonization and patchy. Shoot densities and maximal below-ground biomass were lower in the inner silty seagrass bed than in the sandy meadows located in the centre of the Bay. Maximal aboveground biomasses were similar in the two population types. In the well-established beds, vegetative shoot densities, above-ground and below-ground biomasses showed a unimodal pattern with minima in winter (4000 to 9000 shoots . m(-2), 40 to 80 g DW . m(-2), and 40 to 60 g DW . m(-2), respectively) and maxima in summer (11000 to 22000 shoots . m(-2), 110 to 150 g DW . m(-2), and 140 to 200 g DW . m(-2), respectively). Reproductive shoots were observed from the beginning of June until the end of September, except in the colonizing bed where they persisted until December. Furthermore, in the latter, maximal reproductive shoot density was higher (2600 shoots . m(-2)) than in the established beds (650 to 960 shoots . m(-2)). The total production of Z. noltii in Arcachon Bay was estimated to approximately 35.6 . 10(6) kg DW . y(-1) (19.4 . 10(6) kg DW . y(-1) for above-ground parts and 16.2 . 10(6) kg DW . y(-1) for below-ground parts).