| Summary: | Developing early warning indicators to accurately detect ecosystem disturbances is vital for enhancing ecosystem management. The seasonal and interannual variability of the phenolic content of <i>Nanozostera noltei</i> from Arcachon Bay, France, was explored over 47 consecutive months to identify suitable early indicators of the state of seagrass beds. Five phenolic acid derivatives and eight flavonoids were fully characterized using chromatographic and spectroscopic techniques; a caffeic tetramer was described for the first time as a metabolite of <i>N. noltei</i> and of the genus <i>Nanozostera.</i> The individual phenolic concentrations in each of the 47 collections were determined by quantitative HPLC and analyzed as a function of year and season. The variability of the phenolic content in the rhizomes of <i>N. noltei</i> from Arcachon Bay was also determined over one year, as well as rhizomes of <i>N. noltei</i> from three other locations in the Atlantic and Mediterranean for comparison purposes. In addition, the phenolic fingerprints of <i>Z. marina</i> rhizomes were also characterized for the first time. The results show that leaf phenolic chemistry could be used to signify changes in the ecological health of <i>N. noltei.</i> In particular, it appears that diosmetin 7-sulfate, rosmarinic acid and zosteranoic acid could be reliable and easy-to-use indicators for monitoring <i>N. noltei</i> meadows. From a phytochemical point of view, this work is the first report of zosteranoic acid in the leaves and the rhizomes of <i>N. noltei</i> and in the rhizomes of <i>Z. marina</i>.
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