1. Pête D., Quattrochi L., Velimirov B., Lepoint G. and Gobert S.
An overview of the microbenthic loop in Posidonia oceanica meadows: The good, the bad and the ugly
Pête D., Quattrochi L., Velimirov B., Lepoint G. and Gobert S.

2. The microbenthic loop Organic matter Microphytobenthos Bacteria
1. Introduction
The microbenthic loop
Organic matter
Bacteria
Microphytobenthos
Meiofauna

3. Why studying the microbenthic loop in Posidonia oceanica meadows?
1. Introduction
Why studying the microbenthic loop in Posidonia oceanica meadows?
P. oceanica
Endemic species of the Mediterranean Sea
Indicator of healthy coastal environment
Disappears when important environmental perturbations
Aims:
Microbenthic loop = early indicator of environmental perturbations?
Costs/Benefits?
➡ Too late to react
➡ Using the microbenthic loop to detect pollution early?
Advantages:
Sediment = final container of pollution
Fast turnover
Life cycle inside sediment
Disadvantages:
Hard to process
Small organisms
… works well for beaches or muddy areas

4. Sampling sites 2. Material and methods = Reference site
10 m, 22 m
Small scales
Alteration
Shading
= Reference site
From STARESO SA
Perturbated site
Fish farm
22 m
Adapted from Vermeulen et al., 2011

5. Reference site vs. Fish farm (0-2 cm)
3. Results and discussion
Reference site vs. Fish farm (0-2 cm)
↗ Microphytobenthos = ↗ Nutrients
BUT nutrients content of the pore water or of the water column not higher
➔ Direct consumption by microalgae and epiphytes?

6. Reference site vs. Fish farm (0-2 cm)
3. Results and discussion
Reference site vs. Fish farm (0-2 cm)
Trend: ↘ Total meiofauna abundance consistent with fish farm perturbation but time consuming!
➔ Mirto et al. (2009): seagrass effect masks fish farm effects

7. Reference site vs. Fish farm (0-2 cm)
3. Results and discussion
Reference site vs. Fish farm (0-2 cm)
Important similarity between samples
Higher multivariate dispersion among replicates at fish farm site
➔ Signal of an ecosystem perturbation (Warwick & Clarke, 1993)?
➔ Too few replicates?
➔ Too « clean » perturbated site?

8. In situ experiments: sediment addition and shading
3. Results and discussion
In situ experiments: sediment addition and shading
Duration : 3 months
Sediment addition: depleted in OM, ↗ mud content
Shading: 50% light extinction
➔ No consistent effects on seagrass or bacteria, microphytobenthos
➔ Too short experiment?
➔ Too few replicates?
➔ Bad period (beginning of the senescent season, thank you Catherine McCormack!)?
➔ Seagrass masking effect?

9. Variation at small scale
3. Results and discussion
Variation at small scale
25 cm
Sampling : 12 points randomly chosen
125 cm

10. Variation at small scale (0-2 cm)
3. Results and discussion
Variation at small scale (0-2 cm)
Important spatial variability at small scale
No correlation with physico-chemical properties of the sediment (grain size, pore water nutrient content)
➔ Seagrass masking effect?
➔ Existence of microhabitats due to the seagrass presence?

11. Need for further investigations…
4. Conclusions
Could the microbenthic loop be a good early indicator to detect environmental perturbations in a P. oceanica meadow?
The Good:
Microphytobenthos and meiofauna abundance seems to react to a fish farm perturbation ➔ but only trends…
The Bad:
Determination of meiofauna = time consuming ➔ not an easy indicator
No consistent impact shown by in situ experiments (sediment addition and shading)
The Ugly:
Important variability at small spatial scale
No correlation with measured sediment physico-chemical properties
➔ Seagrass masking effect?
➔ Microhabitats?
Need for further investigations…

12. Thank you for your attention!