Small scale variations in the microbenthic loop of Posidonia oceanica meadows: an experimental interpolation design.

Small scale variations in the microbenthic loop of Posidonia

oceanica meadows: an experimental interpolation design

Dorothée PETE

_{, Fabian LENARTZ}

_{, Branko VELIMIROV}

_{, Jean-Marie}

BOUQUEGNEAU

_{and Sylvie GOBERT}

1_{MARE Centre, University of Liège (ULg), BELGIUM.}

2_{General Microbiology, Medecine University of Vienna, AUSTRIA.}

e-mail : Dorothee.Pete@ulg.ac.be

Fig. 1: Sampling site and sampling grid.

Acknowledgements:

This study is financially supported by a grant from the “Fond National de la Recherche Scientifique”, Belgium, and the research project ARC.RACE 0510/333.

Introduction:

In the sediment compartment of Posidonia oceanica meadows, there is a severe lack of information on small scale variations. In an attempt to understand spatial variations of the microbenthic loop (bacteria, organic matter, microphytobenthos and meiofauna), an experiment based on interpolation methods was led in the research station STARESO (Calvi Bay, Corsica, France), in March 2008, at 10 m depth (Fig.1).

ST AR ESO 1,25 m 25 cm

Methods:

A sampling grid (Fig.1) was put in the P. oceanica meadow. Twelve points were selected ramdomly to assess and describe the spatial distribution and variations of seagrass density, bacteria, organic matter (OM), microphytobenthos and nutrients (NO₂

-+ NO3-, NH4+, HPO42-) by taking sediment cores.

Spatial distributions were calculated by DIVA (Data Interpolating Variational Analysis), a method adapted by the GHER (MARE Centre, ULg, Belgium).

Results:

Small scale variations are found for all the measured parameters. They are higher in the first centimeter of the sediment cores and for bacterial biomass and abundance (Fig.2). Significant variations are also observed with sediment depth, except for OM.

Correlations between bacterial and OM biomass (r=0,80), and between microphytobenthos biomass and HPO42-(r= 0,67) are also shown (Fig.3).

Fig. 2: Distribution maps forP. oceanica density, HPO42-, biomass of

organic matter (OM), microphytobenthos, biomass and abundance of bacteria in the first centimeter of the sediment cores.

40 45 50 32 10 12 200 700_{P. oce} an ica de n sit y (s h oot .m -2 ) 1,6 0,6 HP O 4 2-(µ M ) OM bi om as s (m g .g -1 se di m en t dry we ig ht) 500 Bi om ass of m ic ro p h yto b ent ho s (µ g C. g -1 se di m en t dry we ig ht) Abu n d an ce of bact er ia (10 8 ce lls .g -1 se di m en t dry w eig h t) 120 Bi om ass of b act er ia (µ g C . g -1 se di m en t dry w eig h t) 40 45 50 32 10 12 200 700_{P. oce} an ica de n sit y (s h oot .m -2 ) 1,6 0,6 HP O 4 2-(µ M ) OM bi om as s (m g .g -1 se di m en t dry we ig ht) 500 Bi om ass of m ic ro p h yto b ent ho s (µ g C. g -1 se di m en t dry we ig ht) Abu n d an ce of bact er ia (10 8 ce lls .g -1 se di m en t dry w eig h t) 120 Bi om ass of b act er ia (µ g C . g -1 se di m en t dry w eig h t) 45 50 32 10 12 200 700_{P. oce} an ica de n sit y (s h oot .m -2 ) 1,6 0,6 HP O 4 2-(µ M ) OM bi om as s (m g .g -1 se di m en t dry we ig ht) 500 Bi om ass of m ic ro p h yto b ent ho s (µ g C. g -1 se di m en t dry we ig ht) Abu n d an ce of bact er ia (10 8 ce lls .g -1 se di m en t dry w eig h t) 120 Bi om ass of b act er ia (µ g C . g -1 se di m en t dry w eig h t) 10 12 200 700_{P. oce} an ica de n sit y (s h oot .m -2 ) 1,6 0,6 HP O 4 2-(µ M ) OM bi om as s (m g .g -1 se di m en t dry we ig ht) 500 Bi om ass of m ic ro p h yto b ent ho s (µ g C. g -1 se di m en t dry we ig ht) Abu n d an ce of bact er ia (10 8 ce lls .g -1 se di m en t dry w eig h t) 120 Bi om ass of b act er ia (µ g C . g -1 se di m en t dry w eig h t) 200 700_{P. oce} an ica de n sit y (s h oot .m -2 ) 1,6 0,6 HP O 4 2-(µ M ) OM bi om as s (m g .g -1 se di m en t dry we ig ht) 500 Bi om ass of m ic ro p h yto b ent ho s (µ g C. g -1 se di m en t dry we ig ht) Abu n d an ce of bact er ia (10 8 ce lls .g -1 se di m en t dry w eig h t) 120 Bi om ass of b act er ia (µ g C . g -1 se di m en t dry w eig h t) 200 700_{P. oce} an ica de n sit y (s h oot .m -2 ) 1,6 0,6 HP O 4 2-(µ M ) OM bi om as s (m g .g -1 se di m en t dry we ig ht) 500 Bi om ass of m ic ro p h yto b ent ho s (µ g C. g -1 se di m en t dry we ig ht) Abu n d an ce of bact er ia (10 8 ce lls .g -1 se di m en t dry w eig h t) 120 Bi om ass of b act er ia (µ g C . g -1 se di m en t dry w eig h t) Bi om ass of mi cr op h yt ob en t hos ( µ g C .g -1 se di m en t dr y we ig h t) HPO4 2-OM b iom as s (m g .g -1 se di m en t dr y w ei gh t) Total biomass of bacteria (µg C.g-1

sediment dry weight)

Bi om ass of mi cr op h yt ob en t hos ( µ g C .g -1 se di m en t dr y we ig h t) HPO4 2-Bi om ass of mi cr op h yt ob en t hos ( µ g C .g -1 se di m en t dr y we ig h t) HPO4 2-OM b iom as s (m g .g -1 se di m en t dr y w ei gh t) Total biomass of bacteria (µg C.g-1

sediment dry weight)

Fig. 3:Correlations between microphytobenthos and

HPO42-and biomasses of OM and bacteria.

Conclusions:

Small scale variations in the sediment are

important in P.oceanica meadows and must be

taken into account in sampling strategies.