PhD thesis of Auriane Jones (Université Bretagne-Loire) defended on 14/12/2017
The goal of this PhD was to understand (1) the effect of a structural ecosystem engineer on the diversity and functioning of benthic communitiesand (2) the impact of disturbances on the functioning of the habitat built by the engineer species. In this context, we used the reef habitat built bythe gregarious polychaete Sabellaria alveolataas a case study, and more precisely, the Sainte-Anne reef, located in the Mont-Saint-Michel Bay (Brittany, France). A zone characterized by soft sediments (fine to muddy sand), 1.5 km from the Sainte-Anne reef and on the same bathymetric level, represented the control site, non-influenced by the engineer species.
First, this physical ecosystem engineer induces a very strong structural change since it transforms a soft sediment into hard three-dimensional structures elevated above the seabed. The preliminary study revealed that the presence of a S. alveolata reef increases the stock of benthic chlorophyll biomass (e.g. microphytobenthos) in the local soft sediments and increases the species richness and macrofauna abundance, forming an original assemblage of species.The biogenic structures, via the modification of the local environmental conditions (grain-size distribution and organic matter content), also lead to the formation of an original community in the soft sediments under its direct influence. The reef habitat constitutes a biodiversity hotspot.
Furthermore, the establishment of S. alveolatahas important functional consequences in terms of food web structure and trophic interactions, estimated using carbon and nitrogen stable isotopes. Indeed, this ecosystem engineer increases the local pool of trophic resources (macro and microalgae,bacterial mats) through structural, abiotic and biotic changes. The locally produced trophic resources are consumed in the biogenic structures by a diverse community of trophic specialists, while in the adjacent soft sediments, they are consumed by a few trophic generalists. Overall, S. alveolataleads to an increase in the trophic niche of benthic communities(convex hull and standard ellipse) and of the benthic-pelagic coupling via the strong abundance of associated suspension-feeders(e.g.Magallana gigas, Mytiluscf.galloprovincialis). At the same time, the potential trophic competition between the engineer and the other primary consumers is very lowas revealed using mixing models.
In addition, the organic matter and nutrient cycling (biogeochemical fluxes) areenhanced by the establishment of S. alveolata, a positive effect mainlylinked to the engineer itself, which linearly structures the measured fluxes (e.g.oxygen demand).Notwithstanding, an intermediate functional diversity measured as the functional dispersion, maximizes the biogeochemical functioning; stressing the positive influence the associated species can have on the reef functioning via their biological traits.
Finally,along a disturbance gradient, Iobserved a replacement (turn-over) and an abundance increase of the associated macrofauna along with its homogenization. Regarding the reef functioning, the resource use (isotopic niche) and the global biogeochemical functioning appeared maximal where 10the reef was intermediately disturbed, estimated bythe density of adult S. alveolata. Overall, our results reveal the importance of(1)facilitation (i.e.positive species interactions)in the functioning of this engineered habitat, (2) limiting direct anthropogenic disturbances to the biogenic structures and (3) considering the actual engineered structures in association with the adjacent soft sediments, under an enlarged definition of a reef.In addition, this work indicates an overall positive effect of S. alveolataon all the measured fluxes and functions, while the use of a disturbance gradient hints towards a new conservation goal for these engineered habitats where their resilience capacity could be optimal.