Effects of shading and sediment alterations on Posidonia oceanica

meadows:in situ experiments

Dorothée Pête1, Branko Velimirov2, Gilles Lepoint1 & Sylvie Gobert1

1University of Liege, Belgium, 2Medicine University of Vienna, AustriaCorresponding author: Dorothee.Pete@ulg.ac.be

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 mGobert 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:- 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 %- Cleaning once a week to avoid fouling

Sampling

Impacts on P. oceanica: Chlorophyll a

T0 T7

123456789

Time

0123456789 Control

Alteration

Shading

Time

• 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

T0 T71.0

1.5

2.0

2.5

Time

May 09 June 09 July 09 August 091.0

1.5

2.0

2.5Control

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 The plant is still trying to cope with the perturbation…

Impacts on P. oceanica: Chlorophyll a/b

T0 T7

1.6

1.7

1.8

1.9

2.0

2.1

Time

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 lightIs there something in the sediment to explain that????

In the sediment: Grain size

0

25

50

75

100Gravel

Sand

Mud

0

50

100

0

50

100 • 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

May 09 June 09 July 09 August 091000

2000

3000

4000Control

Alteration

Shading

TimeT0 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

disponibility in nutrients (bacteria have nothing to recycle)

In the sediment: Biomass of bacteria

T0 T70.0

0.5

1.0

1.5

2.0

TimeMay 09 June 09 July 09 August 09

0.0

0.5

1.0

1.5

2.0Control

Alteration

Shading

Time

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

T0 T70

1

2

3

4

5

Time

May 09 June 09 July 09 August 090

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,…