SPATIAL AND TEMPORAL VARIATIONS IN NUTRIENT LIMITATION OF SEAGRASS POSIDONIA-OCEANICA GROWTH IN THE NW MEDITERRANEAN

The extent and seasonality of nutrient limitation of Posidonia oceanic a (L.) Delile in the Costa Brava littoral (NW Mediterranean) in 5 mead ows was investigated by means of repeated nutrient additions along a y ear cycle. Nutrient additions had a modest effect on leaf growth. The response to fertilization varied considerably among meadows, with thos e-meadows consistently maintaining low (300 mu M dissolved inorganic n itrogen and 9 mu M soluble reactive phosphorus) dissolved inorganic nu trient concentrations in sediment pore waters showing the greatest res ponse to fertilization. The seasonality of the growth response to fert ilization was consistent with calculated nutrient deficiencies, which were greatest in late spring and summer. Nutrient deficiency was allev iated in the fall, partially due to reduced growth, but also to nutrie nt reclamation from old leaves; which represented 20% and 18% of the a nnual nitrogen and phosphorus incorporation, respectively. The relativ e deficiency of nitrogen and phosphorus differed among sites, with the highest response found in P-deficient meadows. Nutrient limitation of P. oceanica meadows was greatest in late spring and summer, but diffe red greatly in magnitude and nature (nitrogen vs phosphorus) depending on local nutrient regimes.