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 oceanica (L.) Delile in the Costa Brava littoral (NW Mediterranean) in 5 meadows was investigated by means of repeated nutrient additions along a year cycle. Nutrient additions had a modest effect on leaf growth. The response to fertilization varied considerably among meadows, with those meadows consistently maintaining low (300 μM dissolved inorganic nitrogen and 9 μM soluble reactive phosphorus) dissolved inorganic nutrient concentrations in sediment pore waters showing the greatest response to fertilization. The seasonality of the growth response to fertilization was consistent with calculated nutrient deficiencies, which were greatest in late spring and summer. Nutrient deficiency was alleviated in the fall, partially due to reduced growth, but also to nutrient reclamation from old leaves, which represented 20% and 18% of the annual nitrogen and phosphorus incorporation, respectively. The relative 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 differed greatly in magnitude and nature (nitrogen vs phosphorus) depending on local nutrient regimes.