δ13C and δ15N variability in Posidonia oceanica associated with seasonality and plant fraction

The carbon and nitrogen isotopic compositions of fractions of the seagrass Posidonia oceanica (L.) Delile in a Mediterranean shallow environment (Stagnone di Marsala, western Sicily) were investigated seasonally throughout 1998. The stable isotope ratios of seagrass leaves (intermediate and adult), rhizomes, leaf litter and aegagropiles were compared over 1 year in order to distinguish between seasonal and plant part variability. Significant differences in the isotopic composition tested using ANOVA were observed as a function of both plant fraction and season. There was an overall trend towards less discrimination against 13C in summer (average δ13C ∼ −10‰) than in winter (average δ13C ∼ −13‰). In contrast, greater δ15N enrichment was observed in winter (average ∼ 5‰) than in summer (average ∼ 3‰). The possibility of a link between δ13C and δ15N seasonal variability and seagrass physiology such as plant carbon balance and reserve dynamics is proposed to explain the observed patterns. The demonstration of seasonal and plant fraction variability inP. oceanica isotopic composition provides a broader perspective to seagrass ecological and physiological studies. Overall, the observed variability can be up to 4 and 5‰ for δ13C and δ15N, respectively, a range that largely exceeds that associated with the transition between consecutive trophic levels (1 and 3.5‰ for δ13C and δ15N, respectively).Our results corroborate the need for, at least, seasonal sampling designs to obtain a good understanding of organic matter fluxes into food webs and highlight the limitations in drawing general conclusions about food webs where a low seasonal sampling effort has been applied. They also suggest that stable carbon and nitrogen isotope ratios deserve attention as a possible valuable approach in seagrass ecophysiological studies. © 2003 Elsevier Science B.V. All rights reserved.