Depth-acclimation of photosynthesis, morphology and demography of Posidonia oceanica and Cymodocea nodosa in the Spanish Mediterranean Sea

Depth-related changes in population structure, biomass partitioning and photosynthesis were studied in populations of Cymodocea nodosa and Posidonia oceanica on the NE Spanish coast. The population structure of both species changed much more with depth than leaf morphology and physiology. Leaf biomass declined 5- to 7-fold along the depth gradient reducing self-shading within the canopy, whereas the leaf area per unit leaf biomass and the photosynthesis-light response varied less than 1.5-fold among depths. Moreover, C. nodosa developed a greater proportion of leaves relative to rhizomes and roots at greater depths, thereby promoting the balance between photosyn- thesis and respiration in the shoots. C. nodosa, being a potentially fast-growing species compared to P. oceanica, had higher maximum photosynthetic and respiration rates as well as light compensation points for photosynthesis. Photosynthetic efficiency at low light, however, was almost the same for the 2 species as suggested by the relatively small differences in mass-specific light absorption. Only C. nodosa acclimated physiologically to depth as light-use efficiency increased, and light compensation point declined significantly from shallow to deep water. P. oceanica, however, possessed low respira- tion rates and slightly lower light compensation points values than C. nodosa throughout the depth range. Shoot mortality and recruitment rates were unaffected by rooting depth. C. nodosa stand experienced fast shoot turnover compared to P. oceanica, and shoot longevity of the former species decreased significantly with depth, suggesting higher risk of patch mortality at the depth limit. In contrast, P. oceanica shoot longevity was highest at great depths. Overall, these species differences in leaf metabolism and shoot dynamics suggest that C. nodosa responds faster to changing light con- ditions, whereas P. oceanica is able to survive longer at low irradiance due to low growth and respi- ratory maintenance rates.

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