Food web structures of subtidal benthic muddy habitats: evidence of microphytobenthos contribution supported by an engineer species

Stable carbon and nitrogen isotopic composition (delta C-13 and delta N-15) of benthic organisms was investigated to understand the effects of a gregarious tubicolous amphipod species (Haploops nirae) on benthic food web structures and test whether drastic changes in species composition cause changes in food web structure. The habitat engineered by this species was sampled and compared with the adjacent uncolonized Amphiura filiformis muddy habitat in winter and summer. The isotopic signatures of macrofaunal and megafaunal species associated with both habitats were analysed along with potential food sources at each sampling period. Similar food web structures for each habitat (and each season), with high delta N-15 ranges spanning over 3 trophic levels, were reported although omnivory was less frequent in the H. nirae habitat. The amphipod H. nirae appears to play a baseline role, with the lowest delta N-15 values and delta C-13 values indicating that it primarily feeds on phytoplankton. Bayesian mixing models were used to estimate the contributions of potential food sources to the diet of the species accounting for most of the biomass in each habitat and showed that the epibionts covering tube mats are a key contribution to the food webs of both habitats. Identified as benthic diatoms, they may minimize interspecific food competition between dominant suspension feeders such as Polititapes virgineus or H. nirae within a habitat. This finding highlights the contribution of microphytobenthic producers to subtidal (similar to 30 m deep) muddy habitat food webs, suggesting that the mucopolysaccharides composing the H. nirae tubes support the growth of a heterotrophic diatom population. As an engineer species, H. nirae acts as a facilitator for diatoms which ultimately sustain the secondary production of the H. nirae habitat as well as the immediate surrounding habitats

Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on the specific diversity and functional structure of benthic communities

Haploops nirae is a gregarious tubiculous amphipod which extended its habitat over thousands of hectares in shallow waters of South Brittany bays (Bay of Biscay, Atlantic) over the last decades and created uniquely large and dense tube mats. In the bay of Concameau, we investigated the specific diversity (i.e. species richness and species composition) and the functional structure (using biological traits) of the macrofauna associated with this Haploops community as a comparison with several surrounding soft-sediment communities to determine the effect of this engineer species on ecosystem functions. We showed that the occurrence of Haploops tubes and individuals significantly modifies sediment features (e.g. change in sediment grain size, increase in C and N organic content) but also largely affect species diversity and benthic composition. The species richness was significantly higher in Haploops community but the species assemblage associated with Haploops habitat was very homogeneous compared to the neighboring habitats and unique with 33% of all species exclusively found in this community. Multivariate analysis (dbRDA) revealed that Haploops density was by far the factor explaining the variation in species composition of benthic communities. No differences in species diversity and assemblage were detected in relationship to Haploops density. A biological trait analysis performed on the whole ecosystem (Haploops included) revealed that Haploops largely dominates the functional structure of the Haploops community by its own functional traits. When performed on selected traits of the associated fauna only (Haploops excluded) the functional structure of the Haploops community was characterized by a greatly reduced proportion of small to medium long lived, sensitive to disturbance, free living or burrowing/tube-building filter-feeding species. H. nirae appears to be a bioengineer and a foundation species that largely modifies its hydro-sedimentary features, controlling diversity and abundances of associated species, and creating a complex set of positive and negative interactions so that a unique benthic assemblage is found in sediments they colonized

Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on the specific diversity and functional structure of benthic communities

International audienceHaploops nirae is a gregarious tubiculous amphipod which extended its habitat over thousands of hectares in shallow waters of South Brittany bays (Bay of Biscay, Atlantic) over the last decades and created uniquely large and dense tube mats. In the bay of Concarneau, we investigated the specific diversity (i.e. species richness and species composition) and the functional structure (using biological traits) of the macrofauna associated with this Haploops community as a comparison with several surrounding soft-sediment communities to determine the effect of this engineer species on ecosystem functions. We showed that the occurrence of Haploops tubes and individuals significantly modifies sediment features (e.g. change in sediment grain size, increase in C and N organic content) but also largely affect species diversity and benthic composition. The species richness was significantly higher in Haploops community but the species assemblage associated with Haploops habitat was very homogeneous compared to the neighboring habitats and unique with 33% of all species exclusively found in this community. Multivariate analysis (dbRDA) revealed that Haploops density was by far the factor explaining the variation in species composition of benthic communities. No differences in species diversity and assemblage were detected in relationship to Haploops density. A biological trait analysis performed on the whole ecosystem (Haploops included) revealed that Haploops largely dominates the functional structure of the Haploops community by its own functional traits. When performed on selected traits of the associated fauna only (Haploops excluded) the functional structure of the Haploops community was characterized by a greatly reduced proportion of small to medium long lived, sensitive to disturbance, free living or burrowing/tube-building filter-feeding species. H. nirae appears to be a bioengineer and a foundation species that largely modifies its hydro-sedimentary features, controlling diversity and abundances of associated species, and creating a complex set of positive and negative interactions so that a unique benthic assemblage is found in sediments they colonized

Investigating isotopic functional indices to reveal changes in the structure and functioning of benthic communities

1. With the use of stable isotopes, new concepts have emerged based on the idea that the ecological niche can be approximated by the isotopic niche defined as a δ-space area with isotopic δ values as coordinates. This study aims to (i) redefine functional indices originally based on quantitative biological traits of species and demonstrate the ecological significance of newly defined isotopic functional indices (IFI) in a δ-isotopic space, (ii) compare IFI using biomass data with existing unweighted isotopic indices using only isotopic compositions. 2. Using a community-wide approach, we tested IFI using isotopic compositions of a large set of associated species from two marine benthic communities widely reported in coastal shallow waters: the common Amphiura filiformis muddy-sand community and the engineered Haploops nirae sandy-mud community. Biomass and isotopic composition (13C and 15N) of all species were measured during four seasons. 3. IFI were calculated in the isotopic space defined by the two communities, and variations were analysed: (i) isotopic functional richness indices measure the overall extent of the community trophic niche. They are higher in the Haploops community due to a higher diversity in food sources but also to longer food chains. (ii) isotopic functional evenness indices quantify the regularity in species distribution and the density in species packing. They showed that the biomass is concentrated at the edges of the food web in the Haploops community, outside the isotopic range of the main food source. (iii) isotopic functional divergence indices quantify the degree to which species distribution maximizes the divergence. They showed a larger utilization of secondary food sources in the Haploops community. 4. The IFI variations responded according to expectations overall, based on the extensive knowledge of those communities. Results highlighted that IFI weighted with species biomass provide new insights into how the structure of energy accumulation as biomass between species is likely to underpin community structure and the interplay between structural components of richness, diversity and evenness of biomass distribution

Investigating isotopic functional indices to reveal changes in the structure and functioning of benthic communities

1. With the use of stable isotopes, new concepts have emerged based on the idea that the ecological niche can be approximated by the isotopic niche defined as a δ-space area with isotopic δ values as coordinates. This study aims to (i) redefine functional indices originally based on quantitative biological traits of species and demonstrate the ecological significance of newly defined isotopic functional indices (IFI) in a δ-isotopic space, (ii) compare IFI using biomass data with existing unweighted isotopic indices using only isotopic compositions. 2. Using a community-wide approach, we tested IFI using isotopic compositions of a large set of associated species from two marine benthic communities widely reported in coastal shallow waters: the common Amphiura filiformis muddy-sand community and the engineered Haploops nirae sandy-mud community. Biomass and isotopic composition (13C and 15N) of all species were measured during four seasons. 3. IFI were calculated in the isotopic space defined by the two communities, and variations were analysed: (i) isotopic functional richness indices measure the overall extent of the community trophic niche. They are higher in the Haploops community due to a higher diversity in food sources but also to longer food chains. (ii) isotopic functional evenness indices quantify the regularity in species distribution and the density in species packing. They showed that the biomass is concentrated at the edges of the food web in the Haploops community, outside the isotopic range of the main food source. (iii) isotopic functional divergence indices quantify the degree to which species distribution maximizes the divergence. They showed a larger utilization of secondary food sources in the Haploops community. 4. The IFI variations responded according to expectations overall, based on the extensive knowledge of those communities. Results highlighted that IFI weighted with species biomass provide new insights into how the structure of energy accumulation as biomass between species is likely to underpin community structure and the interplay between structural components of richness, diversity and evenness of biomass distribution

Investigating isotopic functional indices to reveal changes in the structure and functioning of benthic communities

International audienceWith the use of stable isotopes, new concepts have emerged based on the idea that the ecological niche can be approximated by the isotopic niche defined as a δ-space area with isotopic δ values as coordinates. This study aims to (i) redefine functional indices originally based on quantitative biological traits of species and demonstrate the ecological significance of newly defined isotopic functional indices (IFI) in a δ-isotopic space, (ii) compare IFI using biomass data with existing unweighted isotopic indices using only isotopic compositions.Using a community-wide approach, we tested IFI using isotopic compositions of a large set of associated species from two marine benthic communities widely reported in coastal shallow waters: the common Amphiura filiformis muddy-sand community and the engineered Haploops nirae sandy-mud community. Biomass and isotopic composition (13C and 15N) of all species were measured during four seasons. IFI were calculated in the isotopic space defined by the two communities, and variations were analysed: (i) isotopic functional richness indices measure the overall extent of the community trophic niche. They are higher in the Haploops community due to a higher diversity in food sources but also to longer food chains. (ii) isotopic functional evenness indices quantify the regularity in species distribution and the density in species packing. They showed that the biomass is concentrated at the edges of the food web in the Haploops community, outside the isotopic range of the main food source. (iii) isotopic functional divergence indices quantify the degree to which species distribution maximizes the divergence. They showed a larger utilization of secondary food sources in the Haploops community. The IFI variations responded according to expectations overall, based on the extensive knowledge of those communities. Results highlighted that IFI weighted with species biomass provide new insights into how the structure of energy accumulation as biomass between species is likely to underpin community structure and the interplay between structural components of richness, diversity and evenness of biomass distribution

Role of pockmarks in diversity and species assemblages of coastal macrobenthic communities

We used existing bathymetric data to study the macrofauna of a geophysical pockmark field restricted to a benthic habitat engineered by the tubiculous amphipod Haploops nirae in South Brittany (France). Stations inside and outside pockmarks of different morphometric characteristics (location, size, depression depth) were sampled for macrofauna and environmental parameters (sediment characteristics, organic matter, chl a, hydrogen sulfide and methane concentrations). Diversity indices showed higher species richness inside pockmarks, especially for species with medium to high abundances. Most sediment cores showed low methane but high hydrogen sulfide concentrations. We hypothesised that after eruption, the remaining residual methane from pockmark sediments is oxidised by seawater sulfate and accounts for the high sulfide concentrations found at increasing depth in our samples and the low methane concentrations. We found no relationship between sediment profiles and morphometric features of the pockmarks. Macrofauna assemblages inside vs. outside pockmarks appeared to be different. Pockmarks appear to increase connectivity among habitats and heterogeneity within habitats, thereby creating local hotspots that allow the settlement of species that cannot otherwise develop in Haploops tube mats. Multivariate analyses distinguished 4 groups of pockmarks and control stations. We assumed that deeper pockmarks were created more recently than shallow pockmarks and that each pockmark is at a different stage of evolution, hence explaining the large variability in the characteristics of pockmark groups. This explains why previous investigations have found contradictory results when comparing macrofauna species diversity and composition between areas inside and outside pockmarks. Finally, we propose and discuss a successional stage model for pockmarks

Geophysical exploration of an active pockmarks field in the Bay of Concarneau, southern Brittany and implication for resident suspension feeders. Geo-marine Letters

International audienc