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Phenols, photosynthetic pigments and leaf morphology as biomarkers of environmental influences on Posidonia oceanica (L.) Delile

The endemic Mediterranean seagrass Posidonia oceanica (L.) Delile is a very important benthic primary producer. Even though its meadows have multiple ecological functions they have been modified and destroyed due to a changing environment and an increased human impact. As a slow-growing and long-living species sensitive to environmental changes, P. oceanica is a good bioindicator. In this work, phenolic compounds (total phenolics, flavonoids and anthocyanins), photosynthetic pigments and leaf morphology were evaluated in intermediate leaves of P. oceanica. The samples were collected on 2 (or 3) and 10 m of depth, in summer and winter on three locations of the Zadar coastal area: Zadarski kanal (ZK), Brbišćica Bay (B) and Zaglav Port (ZA) on island Dugi otok. The aim was to find correlation among measured parameters, environmental conditions and the primary production of the seagrass meadow and to estimate the suitability of parameters to be used as bioindicators. Plants collected in ZK and B at 10 m depth had higher portion of flavonoids in total phenols than those taken at 2 (3) m. Also, higher values were noticed in winter. In samples from ZA the portion of flavonoids was almost the same regardless of depth and season. On the other hand, anthocyanin portion was the highest in summer at 2 (3) m of depth. The ratio of chlorophyll a and chlorophyll b was higher in winter in plants grown at both depths and all locations. The ratio of carotenoids and total chlorophylls was mainly 0.20-0.23, except in ZK in summer at 2 m with significantly lower value (0.16). Similar results were obtained for ZA samples. In morphological study mesophyll width was between 74% and 88% of total leaf thickness. Plants at all three locations had thicker mesophyll in winter and at 2 (3) m depth but the differences were not always significant. Principal component analysis showed that samples grouped mostly due to season and depth. Two deviations were noticed, namely samples in ZA at 3 m and in winter were closer to ZA samples collected at 10 m. Also, samples from ZK at 2 m and in summer differed from other samples due to higher primary production which could be connected with human impact.

Posidonia oceanica; seagrass; bioindicator; intermediate leaves

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