A Study of Electra Posidoniae Gautier, a Bryozoan Strictly Found As (Rio De Janeiro) November 2012 Epiphyte of the Seagrass Posidonia Oceanica (L.) Delile
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ISBW10 Buzos A study of Electra posidoniae Gautier, a bryozoan strictly found as (Rio de Janeiro) November 2012 epiphyte of the seagrass Posidonia oceanica (L.) Delile. Gilles Lepoint1, Olivier Mouchette1, Corine Prélaprat2, Sylvie Gobert1 1Laboratoire d’Océanologie, Centre MARE, Université de Liège A B C 2Stareso S.A., Station de Recherche Sous marine et océanographique, Calvi, Corse Introduction: Aa B C d *Seagrass epiphytes have often important role in ecosystem functioning, such as food web suppliers, *Some species (such as the bryozoa Electra posidoniae Gautier) (Fig.1) are specialised in the colonization of plant substrate and b often dominate epiphytic community of long life-span seagrasses such as Posidonia oceanica (L.) Delile, 0.5 mm *Because epiphytic compartment dysfunction is often implied in 5 mm human-induced seagrass decline, it is important to understand Figure 1: A. The seagrass Posidonia oceanica (L.) Delile and its epiphytic community; B. a colony of dynamics and life traits of its specific component in pristine area. the bryozoans Electra posidoniae Gautier on P. oceanica leaf, C: E. posidoniae colony (zoom). Aim: In our study, colonization patterns, biomass, carbon stable isotope composition of E. posidoniae (Fig. 1), a species strictly found on the leaves of the seagrass P. oceanica, were assessed monthly at 10 metre depth in the Revellata Bay (Calvi, Corsica, Mediterranean sea). E. posidoniae E. Results I: 2500 600 ) P. oceanica leaves, their algae epiphytes and E. posidoniae shown seasonal growth and decay (Fig. 2), -1 / 500 epiphytes other 2000 E. posidoniae is almost absent in winter, E. posidoniae is a early spring species, with a biomass maximum in May, 400 1500 Epiphytic algae continued to develop in summer, till leaf abscission, 300 dry mass (mg DM .shoot DM (mg mass dry Contribution of E. posidoniae to epiphytic biomass and to total above ground biomass varied be- 1000 dry mass (mg DM.shoot (mg mass dry tween 0.5 and 48% and, between 0.1 and 10%, respectively. 200 500 100 P. oceanica P. 0 0 250 N D J F M A M J J A S O N D J -1 ) 2003 2004 200 Figure 2: Dry mass per shoot of P. oceanica leaves, total epiphytes and of E. posi- doniae at 10 m depth from November 2002 to December 2003 in the Revellata Bay (Calvi, Corsica, Mediterranean Sea) 150 Results II: 100 Larvae and Colonies were mostly settled on the internal face of P. oceanica leaves (not shown) → WHY (Functional hypothesis vs Competition hypothesis) and HOW (larval chemotaxy?) ? 50 Recruitment occurs between February and June, leaving a 6 month gap without significant Number of colony shoot per colonization 0 N D J F M A M J J A S O N D J → WHICH REPRODUCTIVE STRAGTEGY?, 2 2003 2004 Maximum colony number (229 per shoot) corresponded to more than 100,000 colonies per m at 10 m depth in the P. oceanica meadow of the Revellata Bay Figure 3: Number of E. posidoniae colony by shoot of P. oceanica at 10 m depth → WHICH IMPORTANCE FOR BENTHIC-PELAGIC COUPLING? from November 2002 to December 2003 in the Revellata Bay (Calvi, Corsica, Mediterranean Sea) . 13 P. -12 Delta C values of 10 Results III: oceanica -14 In March and April, composition of carbon stable isotopes of E. posido- 8 niae, expressed by delta 13C, was closed to the delta 13C of water column phytoplankton -16 (w/w) Ratio C/N 6 → MAJOR CONTRIBUTION OF WATER COLUMN TO E. POSIDONIAE DIET Delta 13C values of epilithic -18 biofilms C (per mille) (per C 13 4 Nevertheless, at the end of phytoplankton bloom, these values tended Delta to increase to less negative values -20 Delta 13C values of phytoplankton → CONTRIBUTION OF OTHER SOURCES? (epiphytic microflora and/or P. 2 oceanica detritic material) -22 -24 0 C/N ratios (w:w) tends to increase in July (increase of lipid reserves or de- N D J F M A M J J A S O N D J crease of food quality ?) 2003 Phytoplankton bloom period 2004 → RELATION TO REPRODUCTION PERIOD LARVAL SUPPLY? Figure 4: Elemental and stable isotope compositions of E. posidoniae according to sampling time, measured using EA-IRMS (VarioMicro-Isoprime 100, Elementar). Colonies of one shoot were pooled to make measure- ments. Delta 13C values were obtained on decalcified samples. Range for phytoplankton and for P. oceanica from Lepoint et al., 2000, Mar. Biol. and Lepoint el al., 2003, Bot.Mar. Range for biofilm grown on artificial substrate: Vermeulen 2012, PhD thesis. Take home message: The bryozoa Electra posidoniae is a early spring colonizer of P. oceanica, both in relation to seasonal seagrass growth and to phytoplankton occurrence, This animal species contributes significantly to epiphytic biomass and, probably, to pelagic-benthic coupling in this system, Preliminary results using stable isotopes of carbon could indicate alternative food sources during post-bloom. Acknowledgments : Thank you to STARESO crew for their welcome and for access to scientific facilities. G.L is Research-Associate at FRS-FNRS. This study was supported by a post doctoral fellowship and a FRFC project from FRS-FNRS Corresponding authors E-mail : [email protected]. Authors bibliography is available at institutional repository site: http://orbi.ulg.ac.be/ .