Abstract
Nitrogen (δ15N) and carbon (δ13C) isotopic compositions of Posidonia oceanica were determined during three seasons along a bathymetric gradient (4–38m depth).The δ15N values are low (2.2±0.9 ‰ ) and variable. They do not show any relation to depth or sampling dates. There is a significant difference between the δ15N values of the youngest and the oldest leaves, probably as a result of N resorption and senescing during leaf ageing. The δ13C values of young Posidonia leaves vary with depth, showing the relationship between δ13C values and primary productivity rate, and the use of a bicarbonate/CO2 mixture as an inorganic carbon source. The δ13C values of the oldest P. oceanica leaves are depleted in 13C compared to those of young leaves. This modification of the 13C signatures in relation to leaf age is particularly important between 20 and 29 m depth. This modification could be related to photosynthetic rate change during ageing, but also to change in carbohydrate composition and content.
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