Effects of shading and sediment alterations on Posidonia oceanica meadows: in situ experiments

Effects of shading and sediment

alterations on Posidonia oceanica

meadows:

in situ experiments

Dorothée Pête

_{, Branko Velimirov}

_{, Gilles Lepoint}

_{& Sylvie}

Gobert

1_{University of Liege, Belgium,} 2_{Medicine University of Vienna, Austria}

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

Sampling sites

Healthy & very dense meadow = Reference site Aquacultur e 22 m Gobert et al., 2003

Experimental design: organic enrichment

Experimental design: Alteration of the

sediment

 Sites: STARESO, 10 m depth.

 Duration: 3 months (from end of May to end of August 2009).  Alteration:

- 500 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 mm2_{) about 50 cm from the} canopy.

- Light extinction: 52 ± 1,6 %

Impacts on P. oceanica: Chlorophyll a

- Same pattern as control site • Shading:- Decrease – Increase – Decrease In the review of Leoni et al., 2008: Increase.

Short experiment

Impacts on P. oceanica: Nitrogen content

May 09 June 09 July 09 August 09 1.0 1.5 2.0 2.5 Control Alteration Shading Time • Alteration:

- Same pattern as control site after 1 month.

• Shading:

- Decrease – Increase In the review of Leoni et al., 2008: Increase.

Short experiment

Impacts on P. oceanica: Chlorophyll a/b

May 09 June 09 July 09 August 09 1.6 1.7 1.8 1.9 2.0 2.1 Control Alteration Shading Time • Alteration:

- Fast decrease! • Shading:- Same pattern as control site In the review of Leoni et al., 2008: Decrease.

  capacity to absorb the light

In the sediment: Grain size

 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

May 09 June 09 July 09 August 09 1000 2000 3000 4000 Control_Alteration Shading Time T0 T7 1000 2000 3000 4000 Time • 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

In the sediment: Biomass of bacteria

May 09 June 09 July 09 August 09 0.0 0.5 1.0 1.5 2.0 Control Alteration Shading Time 0-2 cm

•  biomass of bacteria after 3 months in the 0-2 cm layer.  « organic » food.

And now…

Shading: Biomass of bacteria

May 09 June 09 July 09 August 09 0 1 2 3 4 5 Control Alteration Shading Time

•  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,…