The Brown Bank
DISCLOSE has visited the Brown Bank area three times. Scroll down and discover what we found!
The Brown Bank area
This is the Brown Bank area (in red). It's at the border of the Dutch Continental Shelf, in an area that is characterized by large subtidal sand banks. The largest is called the Brown Bank Ridge and is about 20 meters high.
These sand banks are formed by the (low: green, high: red) coming from the English Channel twice a day and result in a very at the seafloor.
Here, you see a more detailed bathymetry map of the Brown Bank. You clearly can see the crest of the sand bank (red) and the much deeper troughs on both sides (blue).
However, if you look closer, for instance in the upper left corner of the area, you can also spot These structures are several meters high, and have a wave length of 100 - 300 m. Due to the (tidal) current, these sand waves migrate several meters per year.
Expeditions
DISCLOSE has joined 5 expeditions. Following the joined expedition with OCEANA that visited the Brown Bank in summer 2017, DISCLOSE performed another two dedicated expeditions to the Brown Bank area in autumn 2017 and May 2019.
Here you see the research vessel Pelagia, which was our home during many of the expeditions. Photo by Nadia Hijner.
In August 2017, DISCLOSE joined the OCEANA expedition that observed life at the seafloor with a Remotely Operated Vehicle (ROV) at four locations, which resulted in the discovery of ross worm reefs (Sabellaria spinulosa).
Ross worm reefs at the Brown Bank. Picture by OCEANA.
In Autumn 2017, we focused on the effects of the large-scale tidal sand dune on life at the seafloor. For this, we obtained detailed bathymetry and backscatter data with the multibeam-echosounder for an area of around 15 km2, performed video transects (black) to collect footage and collected bottom samples (red).
The towed video system used in the video transects. Photo by Danielle Prins de Jonge.
The discovered ross worm reefs were the main subject of this expedition. Our aim was to investigate the spatial extent and ecological effects of these reefs at three locations, by collecting data with a multi-beam echosounder, video transects (black), and bottom samples (red).
The box core device employed to obtain sediment and faunal samples of the seafloor. Photo by Karin van der Reijden.
Extrapolating bottom samples
The expedition in Autumn 2017 yielded many bottom samples, which were all sieved over a 0.5-mm sieve.
The sieved bottom sample of station '17C', containing among others some shell fragments, a worm (right) and a small brittle star (right). Photo by Danielle Prins de Jonge.
Some samples contained only a few species like this one; others had much more organisms. Shell fragments and larger sediment grains were also caught in the sieve, which lead to many hours of processing in the lab!
Species composition
Analysis revealed that the large-scale sand bank affected the distribution of species living in the seafloor. The troughs housed different species than the crest, and the slopes. This can be observed in the ordination plot underneath. This figure shows the similarity of the different sampling locations, based on the species encountered in the samples. The symbols and colors represent the position of the sampling location with respect to the large-scale sand bank and smaller-scaled sand waves respectively. Samples with the same symbols are generally grouped closer than symbols with other symbols.
Ordination plot for species abundances, showing the sampling position with regard to sand bank and sand wave topology. (From: Mestdagh et al, 2020)
For the samples in the "Beyond" locations with respect to the sand bank (shown with a circle-symbol), two specific groups can be observed: a yellow and a blue group. These groups show that species distributions are also affected by the smaller sand waves!
Extrapolation
We know what species are found in the samples. But we aimed for a method to estimate how these species are distributed in between the sampling locations.
The data collected with the multibeam echosounder (MBES) could help us here! The MBES is an acoustic measuring device, often attached underneath the vessel, that emits sound waves and records their echos (backscatter). The time needed for an echo to return to the MBES is used to determine the exact water depth (see figure underneath). That is how we obtained the detailed bathymetry information of the Brown Bank area.
A schematic drawing of an multibeam echosounder system that provides for detailed bathymetry data.
In addition, the intensity of the echo (backscatter data) provides more information about the seafloor. Mud results in a different echo than sand or rock. A data hence provides detailed information on the distribution of sediment types.
Within DISCLOSE, the method to analyse backscatter data has been improved, and we now can derive estimates for the and as well from the backscatter data! Volume scattering represents the 'packedness' of the sediments, while seafloor roughness is an estimate of how flat the seabed surface is.
The species distributions in the bottom samples were highly dependent on the water depth and the volume scattering of their location. The much more available MBES-data could therefore be a very good method to better estimate the distribution of species in between sampling locations.
Figure showing the relation between the species observed at the sampling locations and their explanatory variables (Bathymetry, TOC (Total Organic Carbon), and σ2 (volume scattering)). (From: Mestdagh et al., 2020)
You can find the associated scientific papers underneath for the backscatter classification (Koop et al., 2019), the volume scattering and seafloor roughness (Amiri-Simkooei et al., 2019), and the use of these parameters to extrapolate species observations between samples (Mestdagh et al., 2020).
Ross worm reef discovery
In summer 2017, OCEANA and DISCLOSE discovered ross worm reefs (Sabellaria spinulosa) at the Brown Bank, using an ROV. These vulnerable reefs provide a habitat for many species.
Find the associated scientific paper underneath.
In May 2019, we revisited the reefs to estimate their spatial extent and ecological effects. We gathered acoustic surveys with a multibeam echonsounder, and gathered information on the species present by video transects and bottom samples.
We show that the Sabellaria reefs locally increase the overall density of organisms and the species richness, for organisms living on top of the seafloor.
Overall density (C) and species richness (D) for Sabellaria, Rubble and Sand habitats, based on video transects collected in 2019 and 2017. (From: van der Reijden et al., 2021)
Find the associated scientific paper underneath.
Megaripples
The detailed bathymetry collected in 2017 revealed that the Brown Bank area accommodates another type of submerged sand dune: megaripples. These small-scale megaripples have a wavelength of around 10-30 m and a height of ~0.5 m. They are dominantly found in the troughs of the large-scale sand dune and have a east-west orientation.
Based on the backscatter classification of the multibeam echosounder data, we revealed that the echo intensity varies over megaripples. Higher intensities are observed in the throughs, lower ones at the crests, and a mixture of hard and low intensities at the lee (steep) slope.
The average intensity classes over a megaripple cycle, showing higher intensity (red) in the troughs, and lower intensity (green) at the crests. (From Koop et al., 2019)
The combination of video footage and detailed bathymetry maps demonstrated that the Sabellaria reefs are dominantly located within the megaripples troughs.
This observation is not yet explained, but two main hypotheses have been formulated:
- The migration of megaripples potentially limit the spatial extent of Sabellaria reefs as they periodically suffocate the worms when 'walking' over a reef.
- The megaripples provide some protection to bottom-trawling fisheries that are very active in the area, which restrict the Sabellaria reefs to the safe troughs of the megaripples.
