Carbon and nitrogen isotopic ratios of the seagrass Posidonia oceanica: Depth-related variations

Nitrogen (delta(15)N) and carbon (delta(13)C) isotopic compositions of Posidonia oceanica were determined during three seasons along a bathymetric gradient (438 m depth).The delta(15)N 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 delta(15)N values of the youngest and the oldest leaves, probably as a result of N resorption and senescing during leaf ageing. The delta(13)C values of young Posidonia leaves vary with depth, showing the relationship between delta(13)C values and primary productivity rate, and the use of a bicarbonate/CO2 mixture as an inorganic carbon source. The delta(13)C values of the oldest P. oceanica leaves are depleted in C-13 compared to those of young leaves. This modification of the C-13 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.