Experimental shading effects on the photosynthetic activity of the seagrass Posidonia oceanica

The Mediterranean seagrass Posidonia oceanica (L.) Delile forms dense coastal meadows from the surface to 40m depth. As benthic primary producer, its growth and development rely on the light intensity and quality of Mediterranean oligotrophic coastal waters. A major environmental stressor is the deterioration of the water column transparency resulting from human activities, e.g., eutrophication, sediment resuspension, installation of marine structures. There is a lack of knowledge regarding low to medium light reduction effects on P. oceanica. The aim of this study was to follow its photosynthetic response to such light reduction over the growing period.
The experiment was performed in the healthy P. oceanica meadow of the Calvi Bay (Corsica, France), in front of the STARESO research station. Three 9 m2 shading nets were deployed at 15 m depth above the meadow canopy. Light reductions were 15, 30 and 60% compared to the nearby unshaded control meadow. Structures were deployed from April until August 2018. Posidonia oceanica leaves were monitored monthly for photosynthesis with a DIVING-PAM-II fluorometer. We recorded in situ rapid light curves (RLCs) and chlorophyll fluorescence effective and maximum quantum yield of PSII (ΔF/Fm' or Fv/Fm) at dawn or zenith. Photosynthesis-irradiance curves (PI-curves) were recorded on dark adapted samples in the laboratory. Light and temperature were continuously monitored. Water samples were collected for CH4, N2O and H2S measurements in pore-water and nutrient analyses in pore- and canopy-water.
RLC parameters, i.e., maximum relative electron transfer rate (rETRm), alpha (α) and irradiance at the onset of light saturation (Ik) responded distinctly to the 4 treatments. rETRm decreased with shading and showed up to one month of time lag for summer maxima between control and under 60 % shading. Regarding α and Ik, they respectively increased or decreased with shading. Spring to summer decreases of ΔF/Fm' in control and under 15% shading were higher than under 30 and 60 % shading. Spring to summer seasonal trends of PI-curve parameters, i.e., maximum production (Pmax), α and Ik did not differ among treatments. A similar trend among treatments was also determined for Fv/Fm, which showed a slight decrease over time. Of all environmental parameters (nutrient data not available yet), only light availability changed according to treatment.
Our results on RLCs and ΔF/Fm' suggest that P. oceanica has adapted its photosynthetic activity to light availability: the seagrass shows great photosynthetic plasticity to changing ambient light conditions. Further studies and complementary analyses on seagrass samples (biometry, growth, sugars, C, pigments, DMSP and DMSO) are still necessary to strengthen these conclusions and verify that the low to medium light reduction did not affect the plant photosynthetic apparatus over the five months of experiment.