The effects of salinity and temperature on the photosynthetic and growth performance of the seagrasses Cymodocea nodosa and Ruppia cirrhosa were studied to understand their local seasonality and distribution. Cymodocea nodosa shoots were collected from Cape Vrasidas, and R. cirrhosa shoots from the coastal lagoon Fanari, all from the Eastern Macedonian and Thrace Region, Greece. Effective quantum yield (ΔF/Fm′), leaf chlorophyll-a content (mg g−1 wet mass) and growth (% of maximum) were tested at different temperatures (10–40°C) and salinities (5–60). The results showed that: (a) R. cirrhosa was more euryhaline (5–55/60) than C. nodosa (10–50), (b) the upper thermal tolerance of C. nodosa (34–35°C) was higher than that of R. cirrhosa (32–34°C), (c) C. nodosa could not tolerate 10°C, whereas R. cirrhosa could, and (d) the growth optimum of C. nodosa was 30°C and that of R. cirrhosa 20–30°C. The thermal optima and tolerances of growth and photosynthesis confirm the seasonal patterns of R. cirrhosa but not of C. nodosa. However, the sensitivity of C. nodosa to low salinities and temperatures may explain its absence from shallow coastal lagoons. Ruppia cirrhosa could be vulnerable to future climate change.
Excess macro-nutrients, metal contamination and light limitation are three of the most commonly encountered anthropogenic stressors affecting seagrass meadows. In this study, the effects of different combinations of nutrients (N-NO 3 , P-PO 4 ), copper and irradiance were investigated in shoots of Cymodocea nodosa collected from three meadows in the N. Aegean Sea, one (Nea Karvali) impacted by anthropogenically-derived environmental stressors and two in more pristine condition (Thasos, Brasidas). In a series of laboratory experiments, shoots were exposed to varying nutrient and heavy metal concentrations, as well as varying irradiance levels, for 8 days and the effective quantum yield (ΔF/Fm′) and leaf elongation were quantified. Results showed that C. nodosa increased ΔF/Fm′ under high nutrient concentrations (30 μΜ N-NO 3− -2 μΜ P-PO 4 3− ) but significant differences were only apparent in shoots collected from the oligotrophic-less stressed meadows. Irradiance affected ΔF/Fm′ significantly in all shoots irrespective of source and PO 4 -P concentration, while higher values were measured under low light conditions and it was identified as the main pathway of eutrophication stress in N. Aegean Cymodocea meadows. Shoots, independently of acclimation were tolerant to copper enrichment, with only the highest copper concentrations (4.7 and 7.9 μM) having significant negative effects on ΔF/Fm′. Shoots from the more pristine meadows were less affected by Cu than those from the highly stressed meadow.
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