We describe shoot density, aboveground and belowground biomass, form and leaf production per shoot over a one-year cycle for Cymodocea nodosa meadows in the Ria Formosa lagoon (South Portugal). Habitat conditions in Ria Formosa allow the development of lush meadows (up to 1752 shoots m(-2)), supporting high shoot biomass (up to 945 g DW m(-2)), high leaf productivity (up to 14 g DW m(-2) d(-1)) and large shoots (up to 113 cm leaf length) despite being located at the northern limit of the distributional range in the Atlantic Ocean. Biomass in the meadows examined is higher than other Atlantic sites with C. nodosa, and comparable to the most productive seagrass meadows yet reported. Shoot morphometry and biomass distribution were variable within Ria Formosa, with the stands growing in muddy sediments having a lower ratio of belowground/aboveground biomass than those in sandy sites. C. nodosa had a clear unimodal growth cycle, reaching maximum leaf development in summer. Mean leaf length and number of leaves per shoot, the rate of appearance of new leaves and the rate of leaf fall, average leaf growth and leaf turnover rates (P/B) were minimal in February and March, and maximal in July and September. The high production of the meadows examined may be related to the high nutrient availability in Ria Formosa, evidenced by the high leaf nutrient content (3.4% of DW and 0.38% of DW for nitrogen and phosphorus, respectively).