Submarine structures made by leaking gases

compiled by Dieter Boedeker

 

1. European Unions Natural Marine Habitat types definitions: 
    1180 Submarine structures made by leaking gases

PAL.CLASS.: 11.24

 

1.1. Definition of the habitat

Submarine structures consist of sandstone slabs, pavements, and pillars up to 4 m high, formed by aggregation of carbonate cement resulting from microbial oxidation of gas emissions, mainly methane. The formations are interspersed with gas vents that intermittently release gas. The methane most likely originates from the microbial decomposition of fossil plant materials. The first type of submarine structures is known as “bubbling reefs”. These formations support a zonation of diverse benthic communities consisting of algae and/or invertebrate specialists of hard marine substrates different to that of the surrounding habitat. Animals seeking shelter in the numerous caves further enhance the biodiversity. A variety of sublittoral topographic features are included in this habitat such as: overhangs, vertical pillars and stratified leaf-like structures with numerous caves.

The second type is called carbonate structures within “pockmarks”. “Pockmarks” are depressions in soft sediment seabed areas, even 45 m deep and a few hundred meters wide. Not all pockmarks are formed by leaking gases and, of those formed by leaking gases, many do not contain substantial carbonate structures and are therefore not included in this habitat type. Benthic communities consist of invertebrate specialists of hard marine substrata and are different from the surrounding (usually) muddy habitat. The diversity of the infauna community in the muddy slope surrounding the “pockmark” may also be high.

1.2. Characteristic species:

“Bubbling reefs”

Plants: If the structure is within the photic zone, marine macroalgae may be present such as Laminariales, other foliose and filamentous brown and red algae.

Animals: A large diversity of invertebrates such as Porifera, Anthozoa, Polychaeta, Gastropoda, Decapoda, Echinodermata as well as numerous fish species are present. Especially the polychaete Polycirrus norwegicus and the bivalve Kellia suborbicularis are associated species of the “bubbling reefs”.

“Pockmarks”

Plants: Usually none.

Animals: Invertebrate specialists of hard substrate including Hydrozoa, Anthozoa, Ophiuroidea and Gastropoda. In the soft sediment surrounding the pockmark Nematodae, Polychaeta and Crustacea are present.

1.3. Associated habitats:

“Bubbling reefs” can be found in association with the habitat type ”sandbanks, which are covered by sea water all the time (1110)” and “reefs (1170)”.

1.4. Geographical distribution and regional varieties:

Shallow water examples of “bubbling reefs” colonised by macroalgae and/or animals are observed in Danish waters in the littoral and sublittoral zone from 0 to 30 m water depth. They are present in the northern Kattegat and in the Skagerrak and follow a NW-SE direction parallel to the Fennoscandian fault line.

“Pockmarks” are found in many areas of the European shelf seas. Deep water examples of pockmarks with benthic fauna communities exists at approximately 100 m water depth in the UK part of the North Sea as depressions in areas of predominantly muddy seabed. Examples of extensive areas with pockmarks are found on the Galician coast (Spain) at the bottom of Rias at a more shallow water depth compared to the pockmarks in the North Sea. Present emission of gas has been reported, as well as other inactive pockmarks filled by more modern sediments. Another difference with the “bubbling reefs” of the Danish coast is that gas stocks are closer to the present bottom surface.

1.5. Corresponding categories:

HELCOM classification: All subtypes under “Bubbling reefs (2.10)”.

EUNIS: Relevant types under A3.C.

1.6. References:

Hansen, J.M. (1988). Koraller i Kattegat, kortlægning. Miljøministeriets, Skov- og Naturstyrelsen.

Hovland, M. & Judd, A.G. (1988). Seabed Pockmarks and seepages: Impact on Geology, Biology and the Marine Environment. Graham & Trotman, London. 245pp.

Jensen, P. et al. (1992). “Bubbling reefs” in the Kattegat: submarine landscapes of carbonate-cemented rocks support a diverse ecosystem at methane seeps. Mar. Ecol. Prog. Ser., vol. 83:103-112.

Jørgensen, N.O. et al. (1989). Holocene methane-derived dolomite-cemented sandstone pillars from Kattegat, Denmark. Mar. Geol., vol. 88: 71-81.

Jørgensen, N.O. et al. (1990). Shallow hydrocarbon gas in the nothern Jutland-Kattegat region, Denmark. Bull. Geol. Soc., vol. 38: 69-76.

Laier, T. et al. (1991). Kalksøjler og gasudslip i Kattegat, seismisk kortlægning af området nordvest for Hirsholmene. Miljøministeriet, Danmarks Geologiske Undersøgelse.

 

2. Additional HELCOM information

HELCOM has so far only assessed bubbling reefs, whereas pockmarks, although most likely present in the Baltic Sea area, have not been assessed so far. This needs, however, to be done by all Baltic Sea EU member states, because they are obliged to take all appropriate steps to avoid further deterioration of all submarine structures made by leaking gases. This includes the obligation to protect this natural habitat type within the Natura 2000 network, and thus to designate as many SACs as necessary to guarantee its favourable conservation status.

All known bubbling reefs in the Danish waters are designated under the Habitats Directive. Further, the specific localities of individual reefs are protected to conservation purposes.

2.1 Description of the habitat

Bubbling reefs are submarine landscapes of carbonate-cemented (magnesium calcite, dolomite or aragonite) sandstones supporting a diverse ecosystem at methane seeps. They are a result of microbial methane oxidation in the sediment. Due to erosion of the surrounded unconsolidated sediment they are exposed as structures up to 100 m², consisting of pavements, complex formations of overlying slab-type layers and pillars up to heights of 4 m. The formations are interspersed with gas vents that intermittently release gas, primarily methane, at up to 25 litres per hour (HELCOM 1998).

2.2 Distribution

Bubbling reefs are only known from a relatively small shallow area in the Kattegat as well as very few localities in the North Sea. They are primarily found in the Danish territorial waters, with one or two localities in the Swedish part of the Kattegat. The most southerly occurring bubbling reef is at the Great Middelground Bank, about 50 km north of Zealand.

2.3 Importance (sub-regional, Baltic-wide, global)

Bubbling reefs are, as such, unique in Europe, probably world-wide. They are hot spots for the biodiversity. This means that they are of global importance. Less than 50 localities covering areas from a few square meters to 1.5 km² are known. 

2.4 Status of threat/decline

Bubbling reefs are under threat or decline in the Kattegat, the only HELCOM sub-region where they occur. According to the HELCOM Red List of Biotopes and Biotope Complexes (HELCOM, 1998) bubbling reefs are "Potentially endangered" because of their rarity.

2.5 Threat/decline factors

According to the HELCOM Red List of Biotopes and Biotope Complexes (HELCOM, 1998) threats are caused by: Eutrophication, pollution, fishing, tourism and recreational activities (e.g. diving).

2.6 Options for improvement

Programs and measures for a drastic reduction of eutrophication and pollution are needed. Trawling above bubbling reefs must be forbidden. Tourism and recreational activities may only take place if they do not affect the habitat and species. EU member states have to follow Article 6 (3) of the Habitats Directive: Plans and projects which are not directly connected with or necessary to the management of a Natura 2000 site but likely to have a significant effect thereon, either individually or in combination with other plans or projects, shall be subject to appropriate assessment of its implications. As for all natural habitat types an inventory and a monitoring and assessment programme (also for human activities) is obligatory for EU Member States.

2.7 References

European Commission (2007). Guidelines for the establishment of the Natura 2000 network in the marine environment. Application of the Habitats and Birds Directives. Appendix I: Marine Habitat types definitions. Update of “Interpretation Manual of European Union Habitats” (EU interpretation manual).

HELCOM (1998). Red List of Marine and Coastal Biotopes and Biotope Complexes of the Baltic Sea, Belt Sea and Kattegat - Including a comprehensive description and classification system for all Baltic Marine and Coastal Biotopes. HELCOM-Baltic Sea Environment Proceedings 75, Helsinki Commission. 115 pp.