Influence of experimental in situ shading on Posidonia oceanica (L.) Delile biology

Posidonia oceanica is a keystone seagrass species indigenous to the Mediterranean Sea. It forms dense monospecific meadows from the surface to depths of 40-45 m. The meadows are highly productive and provide many valuable goods and services. However, P. oceanica mead- ows are threatened by several environmental and anthropogenic stressors. Among these, light deprivation is a major problem that is expected to worsen in the future, particularly because of eutrophication and sediment resuspension. This work aims to understand how P. oceanica responds to environmentally relevant shading during its spring to summer important period of primary productivity. Screens of different transparency (nominal reduction of 15, 30 and 60% compared to control) were deployed on a healthy meadow at a depth of 15 m, in Corsica. The experiment took place between April and August 2018. Pigments content (chlorophylls and xanthophylls) in leaves was analyzed with HPLC; photosynthetic activity (rapid light curves, photosynthesis/irradiance curves and quantum yield) was monitored with a Diving-PAM; biometry and primary production were estimated after dissection of shoots; starch and soluble carbohydrates content in rhizomes was investigated by spectrophotometry. Results show a remarkable capacity of P. oceanica to maintain its pigment content, as well as to adapt dynamically its photosynthetic activity to the light reduction. However, an adaptation over the experiment period of the photosynthetic apparatus is not observed. Carbohydrates accu- mulation in rhizomes over time is negatively impacted by shading treatments, resulting in a lower content at the end of summer. In spite of that, light deprivation does not reduce shoots growth. The high resistance of healthy P. oceanica shoots to a five months experimental in situ light deprivation stress is underlined. Furthermore, the potential consequences for the plant survival under a longer shading period, in light of the decrease in reserve carbohydrates content, is discussed as well. These results are compared to bathymetric studies and other in situ shading experiments, emphasizing the genetic differentiation along the depth gradient and the resulting adaptation of P. oceanica shoots to a given depth, i.e., to a given light environment.