ESONET: Porcupine/Celtic

The Porcupine Seabight and Abyssal Plain area has been an important area for bio-geochemical flux studies in the past but is also a very productive fisheries and oil-gas exploration area. It is a stable margin with little evidence of seismicity, but does have important deep water habitats.

The 'Atlantic Frontier', to the west of Ireland, is endowed with a diverse and rich assemblage of marine environments and associated habitats and fauna. At the European continental margin, water depth increases over a relatively short distance from about 150m to 4500m. Ireland's extensive offshore territory is considered one of the most promising for petroleum hydrocarbons (i.e. crude oil and natural gas). The Porcupine area forms a focal point of European deep sea fisheries and is closely located at one of the main arteries of global shipping. The continental margin features a high geomorphological variety with abundant canyons, broad- and narrow banded slopes, a vast intersection into the margin (Porcupine Seabight, PSB), in combination with a variety of smaller mesoscale geomorphological structures such as carbonate mounds. This large geomorphological variability provides the basis for a multifaceted habitat- and species diversity. Consequently, the area represents a major genetic and biochemical reserve of the European continental margin. One of the most spectacular ecosystems of the Irish EEZ are aphotic coral ecosystems, widely distributed along the NW-European continental margin. In the North Atlantic the major reef constructing coral is the colonial azooxanthellate Lophelia pertusa that has the potential to build substantial reefs in the aphotic zone. The reefs themselves provide a series of habitats for thousands of species that live permanently or temporarily in the coral ecosystem. Compared to off-reef environments, the richness of species and biomass can be ten times higher in the reef environment. Like their tropical cousins, deep-water coral reefs play an important role in the life cycle of demersal fishes. There is convincing evidence that many fishes deposit their egg cases between the corals (sharks, rayfishes). Others form huge schools of fish in the summit regions of the reefs for a certain time period (redfish, cod). For this reason, deep-water reefs are substantial for fishes acting as nursery, breeding and spawning sites.

Deep water coral
Underwater Images of deepwater coral and associated fauna. Images from ACES project (Atlantic Coral Ecosystem Study)

Adjacent to the Irish continental margin lies the Porcupine Abyssal Plain (PAP). Surface water layers during winter form a mixed layer as deep as 800m driven by thermally convective overturning and wind forcing. With warming and reduced storm frequencies in spring, the water column becomes more stable and an upper mixed layer of about 50m thickness is established, leading to a major phytoplankton bloom. PAP lies south of the main stream of the North Atlantic Current and is subject to return flows from this coming from the West and Northwest. Processes at the seabed are dynamically coupled to upper mixed layer processes geared by atmospheric forcing. The downward flux of particulate matter from the upper part of the water column has a profound effect on ocean biogeochemistry and hence on the global climate; export below the winter mixed layer may isolate it from the upper ocean for decades or centuries. Over the last decade, a dramatic change has occurred in the abundance of megafauna living over a vast expanse of the PAP. Many taxa, particularly sea cucumbers, on the abyssal seafloor at a depth of about 4850m have all increased significantly in abundance.

Sea cucumbers feeding
Photograph of the sea floor at the Porcupine Abyssal Plain (4800m depth) from 'Bathysnap'. Sea cucumbers Amperima are visible feeding in green detritus recently deposited from the surface.

In the last 8 years a dramatic change has occurred in the abundance of megafauna living over a vast expanse of the PAP. Many taxa, particularly, sea cucumbers on the abyssal seafloor at a depth of about 4850m have all increased significantly in abundance. The sea cucumber species, Amperima rosea has increased in abundance from just 2-3 individuals per hectare to more than 6000. This increase occurred suddenly in 1995/1996. The increase in number of the large invertebrates by at least two orders of magnitude led to a significant increase in the rate at which the seabed was reworked. Before 1996 it took 2.5 years for the animals to reprocess the sediment surface. After 1996 it took less than 6 weeks with fundamental consequences to the functioning of the ecosystem. The species that have increased most in abundance are specialist feeders on phytodetritus, the seasonal peak deposition of detrital organic matter on the seabed derived from primary production in surface waters. Recent work on using chlorophyll and carotenoid pigments as tracers of the organic matter holothurians feed on has shown that each species feeds on a slightly different fraction of the phytodetritus.

Amperima per hectare
Numbers of Amperima per hectare on the Porcupine Abyssal Plain during the years 1988-2000.

There is already a substantial data base from previous EU and various national programmes from the Celtic Margin and Porcupine Abyssal Plain on which to build. Ships of opportunity contribute significantly with frequent transects by the Continuous Plankton Recorder since 1949 and pCO2 transects under the EU programme CAVASSO. Sites of significance encompass a main cable route from the shelf through the Porcupine Seabight into the Porcupine Abyssal Plain focussing on a carbonate mound/ coral reef ecosystem (Belgica Mounds), a vast sponge ecosystem (Phaeronema Belt), a mid Bight station, the mouth of the PSB and the BENGAL Station on the PAP. Branches are proposed to the Hovland Carbonate Mound province, to the Goban Spur and to a canyon system, the later acting as a rapid conduit between shelf and deep sea. The high levels of biological productivity in the area support rich and diverse marine communities, including rich fishable stocks and probably many species yet unknown to science.

Map of Porcupine network
The proposed Porcupine regional network in the North East Atlantic Ocean.

3D map of Procupine network
3D map of the proposed Porcupine regional network.