Author: uset

Experimental assessment of light decrease
on the biology of Posidonia oceanica

J. Richir1,2,*, Luyckx A.3, Champenois W.1, Chou L.4, Corato A.5, Descy J.-P.1, Gobert S.2,6, Leduc M.6, Lutts S.3, Pfündel E.7, Walz S.7 and Borges A.V.1 (* jonathan.richir@uliege.be)
1 Unit of Chemical Oceanography, Uliege, Belgium. 2 Laboratory of Oceanology, ULiege, Belgium. 3 Groupe de Recherche en Physiologie végétale, UCLouvain, Belgium. 4 Service de Biogéochimie et Modélisation du Système Terre, ULB, Belgium.
5 Laboratory of Bioenergetics, ULiege, Belgium. 6 STARESO, France. 7 Heinz Walz GmbH, Germany.

I - Introduction

Seagrasses are coastal higher plants. They build structurally complex (engineer species) and highly productive systems that provide many goods and services (high financial value).
As primary producers, they require light to grow. Light deprivation due to eutrophication and suspended particles, an issue expected to worsen in the future, is therefore a major stress.
=> The main objective of this study was to experimentally assess the effects of environmentally relevant shading on a keystone seagrass endemic to the Mediterranean Sea, Posidonia oceanica.

II - Material and methods

Experiment performed
in a well-preserved,
oligotrophic Corsican (France) bay.

Monitored parameters
Environment:
light and sediment porewater chemistry (H2S, CH4, N2O, nutrients).
Seagrass physiology:
leaf pigments, photosynthesis (RLC, P/I curve and quantum yield), carbohydrates, shoot biometry and leaf biomass productivity.

In situ deployment
of 9 m2 shading screens,
2 m above sediment.

III - Results

The chemistry of sediment porewater, in particular toxic H2S was not altered by shading treatments.
P. oceanica adapted its photosynthetic activity (RLC, graphs of maximum electron transfer rate) and efficiency (effective quantum yield) to cope with light reduction; neither the pigment contents nor the P/I curves differed between light treatments.
P. oceanica shoots maintained their growth and leaf biomass productivity despite the decrease in light, but at the expense of storing carbohydrates (graphs of total soluble carbohydrates in rhizomes).

IV - Take home message

High resistance and resilience of P. oceanica to five months light deprivation stress.
Because of the measured decrease of storage carbohydrates, seagrass meadow perennity when exposed to longer, recurrent shading is of concern.
Carbohydrates and photosynthetic activity and efficiency as early warning indicators of light reduction stress ?

Funding