Production ecology of Posidonia oceanica (L.) Delile meadows in Nueva Tabarca Marine ReserveGrowth, biomass and nutrient stocks along a bathymetric gradient

Résumé

The effects of the interaction between depth and nutrients on the primary production of the seagrass Posidonia oceanica in a meadow off Nueva Tabarca island (E Spain) were investigated by assessing C, N and P concentration in the plant, the resource allocation of the different nutrients and the response of the plant to nutrient additions along a bathymetric gradient (1-24 m). The results point to a nutrient limitation in the shallow zone (1- 5 m). The N and P concentrations were low (0.7-1.8 %N; 0.05-0.1%P relative to dry weight) and the N/P atomic ratio high (30-40). Clear tissue enrichment was found after both N and P addition, with lowering of the N/P ratio (to values of 25 by atoms). However, such a possible limitation was not supported by an increment in leaf growth after nutrient addition, although some of the growth features were modified (e.g., relative growth contribution of different leaves to total shoot growth). In a plant with a complex growth mechanism such as Posidonia oceanica, a short-term nutrient addition (1 month) do not suffice to induce a significant growth response. Thus, we conclude that P. oceanica growth off Nueva Tabarca island is probably nutrient limited in shallow areas (1-5 m) in early summer, and that this limitation is due to different concurrent factors: general oligotrophy of the area, massive leaf litter export and leaf losses due to both hydrodynamism and grazing.