Bubbling reefs in the Kattegat: submarine landscapes of carbonate-cemented rocks support a diverse ecosystem at methane seeps

Abstract

Methane seeps in shallow waters In the northern Kattegat off the Danish coast form spectacular submarine landscapes -the 'bubbling reefs' -due to carbonate-cemented sandstone structures which are colonized by brightly coloured animals and plants. These structures may be 100 m2 in area and consist of pavements, complex formations of overlying slab-type layers, and pillars up to 4 m high. The carbonate cement (high-magnesium calcite, dolomite or aragonite) is 13C-depleted, indicating that it originated as a result of microbial methane oxidation. It is believed that the cementation occurred in the subsurface and that the rocks were exposed by subsequent erosion of the surrounding unconsolidated sediment. The formations are interspersed with gas vents that intermittently release gas, primarily methane, at up to 25 1 h-' The methane most likely originated from the microbial decomposition of plant material deposited during the Eemian and early Weichselian periods, i.e. l00 000 to 125 000 years B.P. Aerobic methane oxidation in the sediment was restricted Lo the upper 4 cm in muddy sand and to the upper 13 cm In coarse sand. Maximum aerobic methane oxidation rates ranged from 4.8 to 45.6 pm01 dm-3 d". The rock surfaces and epifauna around the seeps were also sites of methane-oxidizing activity. Integrated sulphate reduction rates for the upper 10 cm of muddy sand gave 4.2 to 26.6 mm01 m-2 d-' These rates are higher than those previously reported from similar water depths in the Kattegat but did not relate to the sediment methane content. Since gas venting occurs over several km2 of the sea floor in the Kattegat it is likely to make a significant local contribution to the cycling of elements in the sediment and the water column. The rocks support a diverse ecosystem ranging from bacteria to macroalgae and anthozoans. Many animals live within the rocks in holes bored by sponges, polychaetes and bivalves. Stable carbon isotope composition (6'") of tissues of invertebrates from the rocks were in the range -17 to -24 'A, indicating that methane-derived carbon makes little direct contribution to their nutrition. Within the sediments surrounding the seeps there is a poor metazoan fauna, in terms of abundance, diversity and biomass. This may be a result of toxicity due to hydrogen sulphide input from the gas.

Ecological implications of surficial marine gas hydrates for the associated small-sized benthic biota at the Hydrate Ridge (Cascadia Convergent Margin, NE Pacific)

“…Olu et al (1997) found that abundances of non symbiotic detritivorous nematodes at mud volcanoes associated with gas hydrates were 1 to 2 orders of magnitude higher than those recorded from other deep-sea environments at similar water depths. Similarly, enhancement of meiofaunal production stimulated by bacterial chemosynthesis has also been found in cold seep habitats not associated with surficial gas hydrates, both in shallow water environments and in the deep sea (Jensen 1986, Shirayama & Ohta 1990, 1996, Dando et al 1991, Jensen et al 1992, Dando et al 1994, Zimmermann 1999. At methane seep sites in the Black Sea Luth et al (1999) detected no elevated biomasses of the SSBB.…”

“…This syntrophic consortium releases sulfide into the porewater, which serves as an electron donor for chemosynthetic sulfide oxidising bacteria producing extra C org (autochthonous C org carbon source) in addition to C org settled from the euphotic zone (allochthonous C org source). Aerobic methane oxidation using oxygen as a terminal electron acceptor at the sediment surface significantly contributes to the sedimentary carbon turnover at cold seep environments or organically enriched sediments (Dando et al 1991, Jensen et al 1992, Schmaljohann 1996, but aerobic methane oxidation in bacterial mat sediments associated with SGH is of minor importance (Boetius et al 2000). Its significance in clam field sediments is not known.…”

Leptonemella species (Desmodoridae, Stilbonematinae), benthic marine nematodes with ectosymbiotic bacteria, from littoral sand of the North Sea island of Sylt: taxonomy and ecological aspects

“…Jensen 1987b) and to stages of lysis in the intestine (Ott and Novak 1989;Ott et al 1991). Jensen et al (1992) consider that digestion occurs in the posterior part of the gut. Accordingly, in the present study bacteria were observed to be concentrated in the anterior part only of the intestine.…”

Hidden secrets of the Northern Adriatic: “Tegnúe”, peculiar reefs

“…According to the best-supported theories, their rocky cores formed over the centuries -the result of the cementing of muddy sandy sediments -by the precipitation of carbonates on beach sediments or after the ascent of methane from the sea bottom through the sediments themselves [22]. Methane most likely came from the microbial decomposition of fossil plant material, and also helped to form these complex structures in the Baltic Sea [23]. In particular, the term adopted by Habitats Directive of EU (Natura 2000, Interpretation Manual of European Union Habitats, Eur 25, October 2003, No 1180), for similar formations is "Submarine structure made by leaking gases".…”