Effects of shading and sediment alterations on Posidonia oceanica meadows: in situ experiments Dorothée Pête 1, Branko Velimirov 2, Gilles Lepoint 1 & Sylvie Gobert 1 1 University of Liege, Belgium, 2 Medicine University of Vienna, Austria Corresponding author:

Introduction  Perturbation of coastal ecosystems (anthropogenic)  Sediment of P. oceanica and organisms linving in.  In situ experiments to mimic: 1. Nutrients and OM enrichment 2. Shading effect caused by nutrients enrichment Link reactions of the plant and in the sediment

Sampling sites Healthy & very dense meadow = Reference site Aquaculture 22 m Gobert et al., 2003

Experimental design: organic enrichment Loss of OM Loss of nitrogen Loss of carbon  Very poor sediment!!!

Experimental design: Alteration of the sediment  Sites: STARESO, 10 m depth.  Duration: 3 months (from end of May to end of August 2009).  Alteration: ml of sediment were added once a week on 21 marked points in a 3x3 m frame.

Experimental design: Shading  Shading: - 3 nets (3x1 m, mesh size: 0,5 mm 2 ) about 50 cm from the canopy. - Light extinction: 52 ± 1,6 % - Cleaning once a week to avoid fouling

Sampling  When? - after 1 week - once a month  What? - light (continuously on the sediment) with a sensor  No difference - density of P. oceanica  No difference - shoots (biometry, chlorophylls, N, C, stable isotopes) - sediment cores (vertical profile for bacteria, OM, microphytobenthos, meiofauna, RPD, grain size, TOC, TON) - pore water (nutrients)

Impacts on P. oceanica: Chlorophyll a Alteration: - Same pattern as control site Shading: - Decrease – Increase – Decrease In the review of Leoni et al., 2008: Increase.  Short experiment  The plant is still trying to cope with the perturbation…

Impacts on P. oceanica: Nitrogen content Alteration: - Same pattern as control site after 1 month. Shading: - Decrease – Increase In the review of Leoni et al., 2008: Increase.  Short experiment  The plant is still trying to cope with the perturbation…

Impacts on P. oceanica: Chlorophyll a/b Alteration: - Fast decrease! Shading: - Same pattern as control site In the review of Leoni et al., 2008: Decrease.   capacity to absorb the light  Is there something in the sediment to explain that????

In the sediment: Grain size Finer sediment  pore water circulation  RPD less deep (from ≈ 5 cm to <2 cm)  Might affect the roots and rhizomes system 0-2 cm 2-5 cm 5-10 cm

In the sediment: OM Alteration: - Decrease after 1 month. Shading: - Same pattern as control site  OM because of the poor content in OM of the added sediment No measurable organic nitrogen in the 0-5 cm part of cores after 3 months  disponibility in nutrients (bacteria have nothing to recycle)

In the sediment: Biomass of bacteria 0-2 cm  biomass of bacteria after 3 months in the 0-2 cm layer.  « organic » food.  Support the « low nutrients disponibility » hypothesis

And now… How about the shading?????

Shading: Biomass of bacteria  biomass of bacteria after 3 months in the 5-10 cm layer. RDP slightly less deep.  Modification of the sediment biogeochemistry by the plant?  Too short experiment.

Conclusions  Shading -No clear impact on the photosynthetic capacity -No clear impact on the sediment BUT  total biomass of bacteria after 3 months.  Too short experiment to have a big impact.  Alteration (sediment loading) -Modification of the photosynthetic capacity  Finer sediment  Impact on the oxygenation of the sediment  Low biomass of OM  Poor quality of OM Impact on the nutrients recycling by bacteria by  total biomass Shading alone takes time to impact a healthy P. oceanica meadows and its sediment. A healthy P. oceanica meadow seems sensitive to sediment loads and trends to react quite fast to adapt.

Thank you! Tanks to Loïc Michel, Renzo Biondo, Gilles Lepoint, Sylvie Gobert, Branko Velimirov, people of the STARESO, students, cleaning team, spreading team, repairing team,…