RESUMO
In shallow coastal waters, seagrass and macroalgae occur together but under eutrophic conditions, bloom-forming algae can take over seagrasses causing an irreversible regime shift. Understanding the effect of macroalgae loads on seagrass meadows at an early stage can help prevent the loss of these ecosystems and the services they provide. In the present study, in situ experiments were conducted for 90 days in Bekalta (eastern coast of Tunisia) to assess the response of the seagrass Cymodocea nodosa when challenged with shading induced by filamentous macroalgae Chaetomorpha linum. Structural, morphological and physiological variables were regularly measured during the experiment. Shaded plants showed a sharp decline in shoot density, growth rate, and above-ground biomass, the impact being more pronounced on the physiological traits. Besides, shading by C. linum induced a significant increase in the contents of leaf photosynthetic pigments and phenolic compounds, whereas causing a decrease in soluble protein and sugar concentrations. Thus, shading imposed by C. linum loads appeared to induce a phoadpatative response in C. nodosa concomitant with carbon mobilization.
