Seasonal biomass and nutrient dynamic of Zostera marina L. in two Mediterranean lagoons: Thau (France) and Venice (Italy)

Two populations of Zostera marina L. were monitored over one year (February 1994-February 1995) in two Mediterranean shallow coastal areas: the lagoons of Thau (southern France) and Venice (north-eastern Italy), which differ in morphological and hydrological (water depth, tides and water exchanges) characteristics. Significant differences of salinity (Thau: 36.0 +/- 3.2 PSU, Venice: 30.5 +/- 2.0 PSU) and nutrient concentrations in the water column (dissolved inorganic nitrogen, DIN: 69.0 +/- 24.5 and 14.4. +/- 8.6 mu M, reactive phosphorus, RP: 2.51 +/- 1.97 and 0.43 +/- 0.19 mu M at Thau and Venice, respectively) and pore water (DIN: 380 +/- 192 and 522 +/- 238 mu M, RP: 10.3 +/- 9.3 and 27.6 +/- 11.6 mu M) were found. On a yearly basis, Z. marina displayed moderate variations at Thau (shoot biomass: 241-1350 g FW m(-2), shoot number: 103-235 shoots m(-2)) and a marked growth trend at Venice (biomass: 504-3321 g FW m(-2), shoot number: 393-1093 shoots m(-2)). The comparison between the growth (Thau: April-September, Venice: May-October) and the dormancy periods in the two populations was computed by the principal component analysis on the basis of the main contribution of salinity and nitrogen compunds at Thau, temperature, biomass and nutrient availability at Venice. Both eelgrass beds were characterized by perennial populations (persistence of overwintering shoots) but the quick increase of biomass by vegetative and reproductive shoots induced a semi-annual feature for seagrasses at Venice. In spite of the remarkably different nutrient availabilities in the water of the two lagoons, nutrient contents of Z. marina (total nitrogen and phosphorus) exhibited a similar seasonal pattern characterized by higher levels in leaves (total nitrogen, TN: 1.72 +/- 0.67 and 2.34 +/- 0.45% DW; total phosphorus, TP: 0.24 +/- 0.05 and 0.23 +/- 0.08% DW at Thau and Venice, respectively) than in roots-rhizomes (TN:1.12 +/- 0.59 and 1.23 +/- 0.27% DW, TP: 0.17 +/- 0.04 and 0.10 +/- 0.02% DW at Thau and Venice respectively).