Artificial reefs

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Nanophytoplankton. Pelagic primary production : main OM source on artificial reefs. Retention of the predominant part of suspended OM (picophytoplankton in.
Are artificial reefs effective biomass producers or only concentrating devices ? A case study from the largest Mediterranean artificial reef system Pierre CRESSON*, Sandrine RUITTON, Marie-France FONTAINE, Mélanie OURGAUD, Mireille HARMELIN-VIVIEN Aix-Marseille University , Mediterranean Institute of Oceanography Marseille – France * Present address: IFREMER, Lab Environment and Resources, La Seyne sur Mer [email protected]

« Attraction vs. production » : A crucial question Artificial reefs : a globally used tool to manage coastal zones and to support small scales fisheries Higher fish biomasses in the vicinity of artificial reefs, with benefits for fisheries

Where did these fishes come from ? Were they attracted or produced ?

© S. Ruitton

The predominant question about artificial reefs in the last decades

« Attraction vs. production » : A crucial question Attraction hypothesis

Adapted from Osenberg et al. (2002), Powers et al. (2002) and Brickhill et al. (2005)

« Attraction vs. production » : A crucial question Attraction hypothesis

Production hypothesis

Adapted from Osenberg et al. (2002), Powers et al. (2002) and Brickhill et al. (2005)

« Attraction vs. production » : A crucial question Attraction hypothesis

Production hypothesis

Sustainable support to fisheries only from production Adapted from Osenberg et al. (2002), Powers et al. (2002) and Brickhill et al. (2005)

« Attraction vs. production » : A crucial question Underwater surveys of community change

Mathematical modeling Osenberg et al. 2002

Biomass (kg m-3)

© GIS Posidonie

Occurrence (%)

Area of the AR Organic matter production and transfers through trophic network  a crucial information but also a knowledge gap (Brickhill et al. 2005) C and N stable isotope ratios

C and N isotopic ratios d15N

Specific isotopic ratio of each OM production C : low enrichment at each trophic level

d13C : OM origin N : high enrichment at each trophic level

d15N : Trophic level +3 à 4 ‰

+1 ‰

d13C

Isotopic ratio integrated in the muscle : mean diet of the last months

Relative positions of all organisms in a trophic network Integrated view of the OM fluxes of a system 7

Largest artificial reef deployment in the Mediterranean 400 AR, of 6 architectural types on a 220 ha area, more than 27 000 m3

8

Main questions 1) Isotopic features of OM sources at the bases of the trophic networks 2) How are these sources used ?  Isotopic ratios of benthic invertebrates 3) Fish attraction / biomass production ?  Position of fishes in the AR trophic network

Main questions 1) Isotopic features of OM sources at the bases of the trophic networks 2) How are these sources used ?  Isotopic ratios of benthic invertebrates 3) Fish attraction / biomass production ?  Position of fishes in the AR trophic network

OM sources supporting invertebrates Whole dataset : 26 species – 7 taxonomic groups – 5 trophic modes Only main species/ groups presented here

Predominance of filter feeders Main OM source: Pelagic primary production

OM sources supporting filter-feeders Proportion of the source in the diet of bivalves Nanophytoplankton

Terrestrial inputs

Benthic primary production Phytoplankton

Pelagic primary production : main OM source on artificial reefs Retention of the predominant part of suspended OM (picophytoplankton in oligotrophic waters)

OM sources supporting invertebrates Whole dataset : 26 species – 7 taxonomic groups – 5 trophic modes Only main species/ groups presented here

Predominance of filter feeders Main OM source: Pelagic primary production

Benthic primary production Consumption by grazing or through a detrital pool

OM sources supporting invertebrates Whole dataset : 26 species – 7 taxonomic groups – 5 trophic modes Only main species/ groups presented here

Predators : integration of pelagic and benthic primary production AR production can support an invertebrate trophic network

Predominance of filter feeders Main OM source: Pelagic primary production

Benthic primary production Consumption by grazing or through a detrital pool

OM sources supporting invertebrates

Invertebrates: potential preys for fishes Where do fishes stand in the AR trophic network ?

Fish isotopic ratios

D15N ≈ 7‰ 3 trophic levels

D13C ≈ 2.5 ‰  At least 2 OM sources 16

Conclusion Trophic functionning of artificial reefs Fishes : biomass production based on local OM production or retention

AR = ↗ shelter + ↗ resources for invertebrates

Main OM sources: nanophytoplankton and local benthic PP

Mechanisms involved in carbon use

Mechanisms involved in carbon use

AR deployment enhances : Pelagic- benthic OM fluxes  Benthic production OM bacterial recycling

To enhance secondary production

Ecosystem modelling

Conclusion • Effective fish biomass production based on AR primary and secondary productions

• AR : Efficient tools to support sustanabily small scale fisheries if well managed

• Working base for future ecosystem modelling studies and to design efficient management policies

Photo Pierre Cresson 21

Thanks for listening !

Mediteranean raimbow wrasse (Coris 22 julis) on Marseilles’ artificial reefs

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