Atelier de restitution du REBENT Bretagne -Rennes, 16 mai 2006

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Atelier de restitution du REBENT Bretagne -Rennes, 16 mai 2006

WHY ARE MAERL BEDS SO SPECIES RICH ?

Scotland

(Clyde) Brittany

(Brest) Galicia

(Vigo) Alicante

(Spain) Malta

Latitude (°N) 56 48 42 38 36

Macroalgae 66 58 16 168 104

Porifera 10 12 3 5* 1

Cnidaria 35 14 9 11* 2

Annelida 120 169 174 78* 57*

Crustacea 104 137 107 103 83

Mollusca 138 135 96 99* 100

Echinodermata 26 20 19 24* 13

Bryozoa 22 6* 0* 9* 8*

Ascidiians 11 10 4 14 4

Fishes 32 12 8 15 17

Others 58 34 11 16 11

Total 622 601 447 316 335

MAERL DIVERSITY: THE IMPORTANCE OF ARCHITECTURAL COMPLEXITY AND EFFECTS OF DISTURBANCE

Number of macroorganisms species identified over a 2 years period during the BIOMAERL Program

Sampled area (m²) _13,6 _18,8 _10,4 _8,6 _9,4

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1 - Sedimentary heterogeneity*

*Defined here as the relative abundance of structural components of the habitat, following Mc Coy & Bell (1991), i.e. sedimentary particles and maerl thalli.

Depth in the sediment (cm)

Maerl on muddy sediment 0

2 4 6 8

Maerl on coarse sand 0

2 4 6 8

Sandy Mud Fine sand

0 2 4 6 8

0 2 4 6

Depth in the sediment (cm) 8

Sand 0

2 4 6 8

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A complex tri-dimensional structure: results from

the BIOMAERL

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Bay of Brest

Maerl beds on shallow-water muddy mixed sediments

(Eunis habitat code A4.61)

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Bay of Camaret

Maerl on coarse shallow-water sediments

(Eunis habitat code A4.61)

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Case 1: Maerl on mud vs Fine sand and Sandy mud in the bay of Brest

Mean Number of species

0 20 40 60 80 100 120

N. espèces.0,1m-2

Fine sand Sandy mud Maerl

n=15 n=20

n=30

*

Maerl

Sandy mud Fine sand

Pooled samples

Numberof species

0 20 40 60 80 100 120 140 160 180

0 5 10 15 20 25 30

Species richness Abundance Sandy mud 0.75 ^±^0.08 1.53 ^±^0.32 Muddy sand 0.85 ^±^0.15 0.95 ^±^0.13 Maerl on mud 1.35 ^± 0.15 ** 2.66 ^±^{0.59 **}

Epifauna : Endofauna ratio

Species accumulation curve

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Case 2: Maerl on coarse gravel vs Coarse sand and Fine sand in the Glenan

0 10 20 30 40 50 60 70

Coarse sand Fine sand Maerl

n=18 n=18

n=18

N species 0.1m-2 Numberof species

Coarse sand

0 20 40 60 80 100 120 140 160

0 5 10 15 20

Pooled samples

Maerl

Fine sand

Epifauna : Endofauna ratio Mean Number of species

Epifauna : Endofauna ratio Species richness Abundance Coarse sand 0.81 ^±^0.13 0.92 ^±^0.28

Fine sand 0.74 ^±^0.18 0.88 ^±^0.23 Maerl on coarse

sand 1.17 ^± 0.09 ** 1,78 ^± 0.38 **

Species accumulation curve

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2 - Architectural complexity*

*Defined here as the abundance of one structural components of the habitat per surface unit, following Mc Coy & Bell (1991), i.e. the varying abundance of maerl thalli per surface unit.

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The influence of maerl thalli density (complexity) on species richness

0 10 20 30 40 50 60 70 80 90

0 1000 2000 3000 4000 5000 6000

Brest Glenan

Maerl thalli density per 0.1m² sample

Numberof speciesper 0.1m²sample

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Predators/Omnivores

5 10 15 20 25 30

Speciesrichness

Suspension feeders

500

Maerl thalli density

1000 1500 2000 2500 3000

Difference response between trophic groupsin the bay of Brest

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3 - Superimposed heterogeneity: seasonal macroepiphytic algae

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Example of the effect of super-imposed heterogeneity

7

Winter Spring Autumn Summer 0

1 2 3 4 5 6 7 8 9 10 11

0 1 2 3 4 5 6

Log (soft macroalgae biomass )

NumberofMelitidspecies

Epiphytic macroalgae 0

1 2 3 4 5 6 7

sept-96 oct-96 déc-96 mars-97 mai-97 juil-97 sept-97 oct-97 déc-97 mars-98 mai-98

Log (Biomass)

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3 - The effect of disturbance on maerl beds

Trawling / Dredging Fish Farming

- Homogenesisation of the sediment reduction of structural complexity

- Drop in live maerl cover on the long term - Reduction of biodiversity

- Reduction of the nursery role of maerl beds

- Clogging of maerl interstices - Increase of the TOC

- Anoxic episods

- Drop in live maerl cover - Drop in faunal diversity

- Dominance in opportunistic species

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Conclusions

- Maerl beds biodiversity is high compared to adjacent homogeneous sediment - Sediment heterogeneity created by the presence of maerl fragments over sediment is related to the presence of numerous epifaunal species

- Varying maerl fragments density is also correlated with species richness

- Super-imposed heterogeneity (such as seasonal soft epiphytic algae) may also be a factor increasing diversity. The presence of large epi or endo- megafauna, or of maerl binding species (sponges, macroalgae, big bivalves) may also have the same effect.

-We also need to clearly identify all trophic sources (seston, microphytobenthos, macroalgae …) as it could also be a factor increasing diversity.

- Human activities severely impair maerl beds architectural structure and diversity.

- Maerl beds are good candidates for pragmatic conservation as charismatic habitat, associated with a high number of species.

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Diversité des ressources trophiques et des microhabitats

S - A- B Paramètres synthétiques basiques

S - A- B

Paramètres synthétiques élevés

Les fonds de maerl et les autres habitats sédimentaires : hétérogénéité de l’habitat et biodiversité

Homogeneous sediment

Heterogeneous sediment

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Complexité

0 2 4 cm

S, A, B

SYNTHESIS

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Atelier de restitution du REBENT Bretagne -Rennes, 16 mai 2006

WHY ARE MAERL BEDS SO SPECIES RICH ?

1 - Sedimentary heterogeneity*

A complex tri-dimensional structure: results from

the BIOMAERL

Maerl beds on shallow-water muddy mixed sediments

Maerl on coarse shallow-water sediments

2 - Architectural complexity*

3 - Superimposed heterogeneity: seasonal macroepiphytic algae

3 - The effect of disturbance on maerl beds

Conclusions

The complex architecture of maerl beds