Dauby, P.

Dupont, A.

Gobert, S.

Lepoint, G.

Michel, L.

Nyssen, F.

Schnitzler, J.G

Open Marine Archive

 - 53 -
ORGANISMS AS ECOSYSTEMS ENGINEERS: THE CASE OF AMPHIPOD 
GRAZERS FROM POSIDONIA OCEANICA MEADOWS 
Michel Loïc1, Joseph G. Schnitzler2, Alessandra Dupont1, Sylvie Gobert2, Fabienne Nyssen1, Patrick 
Dauby1 and Gilles Lepoint2 
1 Laboratory of Systematics and Animal Diversity, University of Liège, 13 Allée du VI Août, 4000 
Liège, Belgium 
E-mail: Loic.Michel@ulg.ac.be  
2 Laboratory of Oceanology, University of Liège, 13 Allée du VI Août, 4000 Liège, Belgium 
 
Posidonia oceanica is a seagrass endemic to the Mediterranean Sea, and is able to form large 
monospecific areas, called meadows. These meadows shelter high biomasses and biodiversities of 
amphipod crustaceans. Moreover, It is now established that several species of these amphipods 
feed on the macro-epiphytes present on the leaves of the seagrass.  
Here, we performed in situ experiments to assess whether this grazing activity could impact the 
dynamics of the leaves’ epiphytic cover, and thus influence the functioning of the meadow as an 
ecosystem. We used microcosms containing monospecific populations of 3 amphipods taxa 
(Apherusa chiereghinii, Dexamine spiniventris and Gammarus spp.), and placed them directly in the 
meadow, at a depth of 10m. 
 
Biomasses of erected macroalgae and erected animals (hydrozoans, bryozoans) were lower in all 
grazed treatments. However, none of the studied taxa seemed to consume encrusting epiphytes, 
either vegetal or animal. This selective grazing pressure by amphipods may release encrusting 
epiphytes from competition for space, light and/or nutriments with the fast-growing erected algae, 
and could thus play an important role in the structuring of the epiphytic cover from P. oceanica 
leaves. Moreover, this top-down control might keep erected algae biomass to a normal, sustainable 
level, therefore also benefiting the seagrass itself. 
 
Our results also indicate that amphipod trophic activity caused nitrogen enrichment in both grazed 
(erected algae) and non-grazed (encrusting algae & seagrass leaves) vegetal tissues. A plausible 
interpretation could be that sloppy feeding and excretion by the grazers enhanced availability of 
this nutrient, which is typically limiting for photosynthesis in shallow P. oceanica meadows. This 
emphasizes the fact that grazing is not a simple negative interaction, but that it can also benefit the 
primary producers.  
 
Our results thus indicate that amphipods from P. oceanica meadows seem to be bound to the 
epiphytic cover of the leaves by complex and multilateral trophic interactions, and have an indirect 
influence on the seagrass itself. Amphipods may therefore play an important part in the functioning 
of the epiphyte/seagrass/grazer system of these meadows, and thus act as ecosystems engineers. 
 
This abstract is dedicated to the freshly born Adèle and Côme. 


Organisms as ecosystems engineers: The case of amphipod grazers from <i>Posidonia oceanica</i> meadows

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