Seasonal response of Posidonia oceanica to light disturbances

The effects of changes in the irradiance reaching a Posidonia oceanica meadow were investigated seasonally in Portlligat Bay, NW Mediterranean, by means of in situ shading experiments. The results showed that reduced irradiance during the spring-summer period (SS) had a severe impact on the physiology, morphology and structure of P. oceanica meadows, whereas the same disturbance during autumn-winter (AW) had no significant effect. Changes in all the variables investigated (net leaf production, delta C-13, shoot density and cover) clearly indicate that P. oceanica meadows are significantly more vulnerable to light deprivation between March and July than between September and January. During SS, leaf production in the plots under screens filtering 99% of the incident irradiance dropped by more than 80% with respect to the control plots, and plant sheath delta C-13 values dropped below -16%, with a total decline greater than 4% with respect to the control plots. Shoot density and cover were significantly reduced in the 99% shaded plots after ca. 120 d of shading in SS (61 and 54%, respectively). However, in AW none of the shading treatments showed a significant reduction in leaf production, delta C-13 values, shoot density or cover. Shading in SS led to a significant decrease in epiphyte biomass, which may have increased the light reaching the leaves and smoothed the effects of shading. The results from this study show that coastal management activities that lead to changes in water transparency (i.e. dredging, beach restoration, sediment load, waste discharge, etc.) in the vicinity of climax seagrass ecosystems should be minimized in SS and performed during the shortest possible period in AW.