6533b7d9fe1ef96bd126d5f5

RESEARCH PRODUCT

and variability in Posidonia oceanica associated with seasonality and plant fraction

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Abstract 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 13 C in summer (average δ 13 C ∼ −10‰) than in winter (average δ 13 C ∼ −13‰). In contrast, greater δ 15 N enrichment was observed in winter (average ∼ 5‰) than in summer (average ∼ 3‰). The possibility of a link between δ 13 C and δ 15 N 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 in P. 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 δ 13 C and δ 15 N , respectively, a range that largely exceeds that associated with the transition between consecutive trophic levels (1 and 3.5‰ for δ 13 C and δ 15 N , 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.