Multi approach Study of the Larval Dispersal of the Honeycomb Worm Sabellaria alveolata: Sustainability of Threatened Biogenic Reefs

Transcription

1 Ocean Sciences Meeting Orlando, 5th March 2008 Multi approach Study of the Larval Dispersal of the Honeycomb Worm Sabellaria alveolata: Sustainability of Threatened Biogenic Reefs Sakina Dorothée Ayata 1, Céline Ellien 2, Stanislas Dubois 3, Frank Dumas 3, Solenne Farcy 1, Frédérique Viard 1 & Éric Thiébaut 1 Adaptation et Diversité en Milieu Marin, Station Biologique de Roscoff, UPMC CNRS, France 2 Biologie des Organismes Marins et Écosystèmes, MNHN UPMC CNRS, Paris, France 3 DYNECO, Ifremer, Brest, France 1

2 Introduction Sabellaria alveolata, a reef builder polychaete European intertidal coasts Bentho pelagic life cycle: Connectivity through larval dispersal Engineer species: Biodiversity hotspots Threatened Natural heritage : Anthropogenic disturbances => Sustainability of the reefs? S. alveolata reefs in Europe

3 The biggest Sabellaria alveolata reefs are located in France, in the Bay of Mont Saint Michel

4 Problematic Assessing the sustainability of the reefs The reefs of the Bay of Mont Saint Michel (BMSM) Why here? Local hydrodynamical features that favor retention? Functionning: Exchanges? Source Sink hypothesis? The biggest reef in Europe (LR): 3.5 km long, surface of 2.23 km² A smaller reef 15 km away (SR): 1 km long, surface of 0.29 km² Small Reef Hydrodynamical barrier? Small reef (SR) of Champeaux Large reef (LR) of Sainte Anne Large Reef 5 km Mont Saint Michel

5 Methods Using combined approaches Bio physical modelling In situ surveys larval distribution life history parameters dispersal patterns variability Dispersal & Connectivity Genetic analysis larval exchanges?

6 in situ larval surveys Larval presence and distribution: When & Where? In situ sampling: (Dubois et al., 2007) 1) Seasonal survey: Larval presence and life history traits Extended spawning period, with 2 main peaks: May & September Planktonic larval duration (PLD): 6 to 10 weeks Larval mortality: 0.09 d 1 2) Spatial survey: Larval distributions Residual circulation 0.1m.s 1 Abundances July 2002 larvae.m 2: Gyre SR LR SR LR Larval retention

7 Using combined approaches Demographic study: Fecundity per spawning: SR: larvae LR: larvae Life history traits In situ surveys Bio physical modelling Dispersal & Connectivity: Retention in the Bay? Larval exchanges??????? Hydrodynamical model (MARS 3D) Realistic forcing (tides, winds..) 3D: 10 σ layers, 800m x 800m Biological model Genetic analysis Reproductive outputs Eulerian larval transport Larval mortality Gregarious settlement

8 Bio physical modelling (1/3) Larval retention at the bay scale Settlement rate and variability of the hydrodynamical conditions: Spawnings simulated from March to October, under realistic meteorological conditions of 2002 Hydrodynamical barrier? Settlement rate (%) at the Bay scale 6E-3 Small Reef LargeReef SW W 5E-3 SW S 4E-3 3E-3 2E-3 1E-3 0E+0 Mar Mar Apr Apr May May Jun Jun 22 Jul 10 Jul 23 Spawning dates Aug Aug Sep Sep Oct Oct SW and W winds favor the larval retention at the bay scale favored retention during the first main spawning period in 2002

9 Bio physical modelling (2/3) Where do the larvae go? Meteorological forcing induces strong variability in dispersal patterns: Following of larval cohorts released from the Large Reef (LR) SR LR SR LR Larval release from the LR the 23 May 2002 Larval release from the LR the 21 August 2002 The larvae are constrained within the bay (strong W winds) > retention at the bay scale The larvae are transported out of the bay (weak NE winds) > larval export and loss

10 Quantifying larval supply and exchanges Bio physical modelling (3/3) Variability of the number of settled larvae Spawnings from March to October, under realistic hydrodynamical conditions of the years 2000 to 2004 Larvae settled in the SR Retention Retention in SR From LR 5,E+09 Exchange Retention Number of settled larvae 4,E+09 Exchange LR 3,E+09 Larvae settled in the LR Retention in LR From SR 2,E+09 5,E+09 Some years, strong larval supply from LR Exchanges in both directions 1,E+09 4,E+09 0,E+00 3,E+09 A M ,E+09 SR J J A S O A M 2002 Spawning dates J J A S 2004 O A M J J A S O Lig ne 51

11 Using combined approaches Demographic study Life history traits In situ surveys Bio physical modelling Dispersal & Connectivity: Retention at the bay Variability and favorable windows for strong suply from LR Bi directionnal exchanges Genetic Genetic analysis analysis (of adults)??? Output of larval settlement and effective exchanges? 4 microsatellite loci 15 sites

12 Genetic analysis Gene flow between the two reefs? 1) Same genetic diversity Source sink Hypothesis No decrease of the genetic diversity in the SR 2) Important exchanges Light but significant genetic difference: Fst=0.006 (P<10 5) But: no genetic isolation by the distance Inter reef Intra reef Free exchanges between the two reefs Genetic distance F/(1 F) Geographic distance (ln)

13 Conclusions & Prospects Life history traits Bio Physical modelling In situ surveys Dispersal & Connectivity: Retention at the bay scale Variability and favorable windows for strong suply from LR Bi directionnal exchanges Variability of the hydrodynamical barrier SR 5 km Genetic analysis LR But...

14 Conclusions & Prospects Life history traits Bio Physical modelling In situ surveys Dispersal & Connectivity: Retention at the bay scale Variability and favorable windows for strong suply from LR Bi directionnal exchanges Variability of the hydrodynamical barrier SR 5 km Genetic analysis LR Damaged reef in LR Sustainability of the reefs? Strong degradation of LR since 2002 => Viability analysis population dynamic model, including larval supplies => Local conservation policies

15 Frank Dumas Sakina Ayata sb roscoff.fr Céline Ellien Stanislas Dubois Solenne Farcy Thanks for your attention! Frédérique Viard Eric Thiébaut Bibliography: Dubois S., Comtet T., Retière C., Thiébaut E. (2007) Distribution of Sabellaria alveolata larvae (Polychaeta: Sabellariidae) in the Bay of Mont Saint Michel, France. Marine Ecology Progress Series, 346: