HOOPLA

The SEM-REV experimental station (off the coast of Le Croisic) is located in an area where fields of Haploops (tubicolous amphipods) have been identified. The sedimentary structures built by Haploops constitute a unique ecosystem that is home to specific fauna and therefore represent a potential source of biodiversity for the maritime part of the Loire estuary. However, very little is known about the interactions between Haploops and their environment. The role of these Haploops fields on flows (sedimentary and geochemical) and on the concentration of certain contaminants will be examined, with a focus on the analysis of foraminifera.

The main objective is to assess the potential impact of wind turbine anchors on this specific habitat and to determine the appropriate indicators for monitoring the evolution of disturbances caused to the seabed by the installation of such structures. Is it possible to calculate a resilience time?

•  Physicochemical characterization of microhabitats formed by colonized sediments
• Description of the ecology of foraminifera in these habitats
• Understanding the dynamics of Haploops fields and their impact on nutrient recycling
• Impact of the SEM-REV station’s semi-buried cables on this ecosystem
• Development of ecological status indices based on these indicators

Demonstrator

Impact of the SEM-REV station’s laid or buried structures on the geographical distribution of Haploops fields
Comparison of foraminifera ecology and geochemistry in “bare” sediments, sediments colonized by Haploops, and sediments impacted by structures.

Impact of SEMREV’s export power cable installation on the seabed

The burial of the SEM-REV offshore test site’s power cable in 2012 left different marks in each sedimentary facies crossed, which are fading at uneven rates. These marks can be seen on bathymetric maps and sonar imagery.

In the fluid sandy mud east of the Haploops settlement, images obtained in 2017 and 2018 show no signs of the cable route (blue square). Here, the ecosystem has returned to its original state in less than four years.

In the bench at Haploops, a deep irregular trench is clearly visible in 2017 on the bathymetric map (orange square). This trench does not appear on the imagery. In 2018, some parts of this trench appear to have been filled in (red ovals), indicating a resilience time of at least six years.

In the muddy sand with gravel, the cable route is visible on both the imagery and the bathymetric maps. In this facies, the scar is wide, straight, and relatively shallow (green square). Since the cable was laid (2018 images), the sediments in this trench appear to be finer and/or more fluid. No signs of a return to the initial state are detectable after 7 years.

Impact diagnosis protocol: biotic indices

Biotic indices are tools that summarize information on ecosystem quality in a simple way, such as a numerical value that can be associated with a characteristic environmental condition. These indices are based on different taxonomic groups that serve as bioindicators. Some indices are based on species diversity, while others are based on ecological groups of species.

In the latter case, species are assigned to ecological groups based on observations of their reactions to various environmental stresses (e.g., organic enrichment, see representation above). The method used to assign species to each ecological group varies from one index to another, as does the way in which ecological groups are integrated into the index calculation.

Impact of the installation of the export power cable on foraminifera communities

In the case of a relatively small impacted area (a few meters wide), foraminifera appear to be a suitable study tool. A large number of individuals representing the biodiversity of the environment can be obtained by collecting a small amount of sediment. This characteristic makes it possible to study small ecosystems and replicate samples to reinforce subsequent statistical analyses.

In the case of the cable trench study, a transect was carried out from the bottom of the trench (H1) to the unimpacted middle of the Haploops spp. bench (H4), with an intermediate point on the trench slope (H2). Three sediment cores dedicated to the analysis of foraminiferal communities were collected by divers at each station. The deployment of shipboard corers or grabs from the deck of the vessel has a location accuracy of a few square meters, which makes their use impossible for such a spatial approach.

Five years after the installation of the SEM-REV cable (in 2017), few differences have been observed in the foraminifera communities between the reference station and the two impacted stations. The differences are concentrated in the vertical distribution of the communities and probably reflect the absence of the Haploops bench in the cable trench (see associated core photos).

Ecological status of the different stations and limitations of biotic indices

Two biotic indices (the Atlantic Foram-AMBI and the Mediterranean TSI) were tested using foraminifera fauna data along two transects, within the Haploops bench and in the adjacent sandy mud.

Although the two indices do not place the stations in the same ecological status (due to differences in calculation and corrections applied), the trends are identical:

• A clear distinction can be made between the two sites, with the uncolonized sandy mud consistently showing poorer results, a sign of a stressed environment.
• A decreasing gradient in ecological status can be observed between the Haploops bench and the center of the trench, highlighting that the trench remains an impacted environment, even after five years.

In order to apply these indices, it was necessary to assign certain major species that were not previously assigned, or to modify the ecological group proposed in the scientific literature, taking into account our ecological observations of local assemblages.

The corrections applied to the TSI-Med do not take into account the trophic specificities of the Bay of Biscay, which biases the absolute values of the calculated ecological states. The ecology of certain species needs to be better defined in order for the assignment to an ecological group to be completely reliable.

GEOCHEMISTRY

Deep sedimentary geochemistry

Pockmarks indicate eruptive methane emissions on the seabed. However, methane can also rise from its reservoir through quieter but continuous processes through the sediments by diffusion. Five-meter cores taken from the two sedimentary facies monitored in HOOPLA show a methane production zone closer to the surface in the Haploops and pockmarks facies than in the bare mud facies. As methane is a reduced carbon chemical species, it oxidizes and disappears fairly quickly when it comes into contact with sulfate in seawater, which prevents it from diffusing into the water above the sediment. It can be noted that near the surface in the Haploops facies, the vertical distribution of sulfate is disrupted and made more homogeneous than in the bare mud facies. This is due to the numerous abandoned Haploops tubes, which form a tangle of conduits that allow seawater to circulate, thus erasing the chemical gradients in the sediment.

Surface sedimentary geochemistry

Thanks to innovative techniques (DET-2D), we are able to map the chemical compounds dissolved in the sediment at a submillimeter scale. We can therefore study in detail the effect of the entanglement of Haploops tubes on the distribution of nitrate, phosphate, and ammonium for the two facies studied. For Haploops sediments, we can observe that phosphate and ammonium have a vertical organization that corresponds to the effect of water circulation. For ammonium, this results in less marked variations in concentration. The consequence of this circulation is the effective oxidation of ammonium, generating an excess of nitrate in the water column near the bottom compared to the bare mud facies. This could contribute to local primary production, which would be a less sporadic source of food than the suspension of organic matter during methane eruptions.