Cymodocea nodosa metrics as bioindicators of anthropogenic stress in N. Aegean, Greek coastal waters

•Cymodocea leaf N and maximum fluorescence are promising bioindicators of anthropogenic stress.•Morphological metrics, chlorophyll and C content in leaf are driven by seasonality.•PAM measurements under laboratory conditions and a hierarchical sampling design can reduce bias. Anthropogenic derived stressors are known to affect seagrasses. Cymodocea nodosa, a widespread seagrass in the Mediterranean Sea with high phenotypic plasticity, is known to acclimatize rapidly to prevailing environmental conditions. To use this species in the biomonitoring of anthropogenic stress, physiological (effective quantum yield of photosystem II [ΔF/Fm′], maximum quantum yield [Fv/Fm], and maximum fluorescence [Fm]), phenological (leaf length and width, number of leaves per shoot), and biochemical (Chl-a, and Carbon, Nitrogen, Phosphorus contents of leaves) metrics were measured between two meadows under different levels of anthropogenic influence in the Kavala Gulf, North Aegean Sea. To reduce bias and separate seasonality from anthropogenic stress responses the physiological parameters were measured under constant laboratory conditions and a hierarchical sampling design was employed. Two well-described meadows, one pristine (Brasidas) and one under significant anthropogenic stress (Nea Karvali), were sampled on six occasions between June 2007 and March 2009 at three spatial scales ranging from hundreds of meters (area) to kilometer (site) to 10s of kilometers (meadow). Of the twelve metrics measured, N-content and Fm were the most effective at discriminating between the two C. nodosa meadows and, therefore, should be considered as promising bioindicators. Statistically significant differences were identified between the cold and hot periods for almost all metrics measured, suggesting that seasonality is a key driver of the observed responses.