Effects of in situ experimental shading on the Mediterranean seagrass Posidonia oceanica

Abstract

The response of the Mediterranean seagrass Posidonia oceanica (L.) Delile to in situ light reduction was examined over a 4 mo period (May to August) in 1993 and 1995. Shoot density, leaf growth rate, shoot size, photosynthetic activity, carbohydrate reserves, epiphyte load and tissue nutrient concentrations were measured in replicated plots under the following light conditions: ambi- ent (36% of surface irradiance (SI) control plots), moderately shaded plots (16.7% SI, MS plots) and severely shaded plots (10.4% SI, SS plots). By the end of the experimental period, all shoots under SS treatments had died, while 70 to 80% had survived under MS treatment; in control plots, shoot den- sities had remained almost constant. Leaf growth rate and biomass decrease with shading were evi- dent 30 d after the beginning of the experiment; shoot density and number of leaves per shoot also fell with shading, but this was apparent only 60 to 90 d after the onset of the shading treatments. Epiphyte load was also considerably reduced by shading. Photosynthetic activity (Pmax) and dark res- piration rates (R) of control plants showed a characteristic increase from spring (May) to the end of summer (August), resulting in higher saturation (Isat) and compensation (Ic) points and lower photo- synthetic efficiencies (α ) at the end of the experimental period; however, photosynthetic parameters in shaded plants (MS treatment) first increased (as in control plots) from May to June, but decreased afterwards (from June to August), indicating photoacclimation of P. oceanica to low light conditions. Shading depressed carbohydrate storage in the rhizomes of shaded plants, in which starch concen- trations were quickly reduced to 60% of those of control plants. The leaves of shaded shoots showed a significant increase in nutrient (specially nitrogen) concentrations. This was probably due to translocation from shed leaves and also to low utilisation rates for growth. We infer that minimum light requirements of P. oceanica are between 16 and 10%, of surface irradiance, values which are consistent with light availability at the deep limits of nearby meadows. One year after shades had been removed, shoot densities remained unchanged (relative to the moment of screen removal), indi- cating both the absence of cascading effects and a very low recovery capacity of this seagrass.