Hydrocarbon-based communities in the North Sea?

Variations in deep-sea methane seepage linked to millennial-scale changes in bottom water temperatures ~ 50-6 ka, NW Svalbard margin

During the last glaciation, the northern hemisphere experienced profound millennial-scale changes (termed Dansgaard-Oeschger (DO) events) in atmospheric and oceanic temperatures. In the North Atlantic, the fluctuations resulted in extremely unstable bottom water conditions with bottom water temperatures (BWT) varying up to > 5 °C. We have studied these changes in a core from 1,300 m water depth at Vestnesa Ridge, northwestern Svalbard margin to investigate a possible connection between BWT and seepage of methane from the seafloor covering the period ~ 50-6 ka. Beneath Vestnesa Ridge, gas hydrates containing vast amounts of methane are kept stable due to the high pressure and low temperatures. Release of gas is shown by numerous pockmarks on the seafloor. The pockmarks at 1,300 m water depth are presently inactive, but they bear witness of earlier activity. Our study shows that from ~ 50-6 ka, the core site experienced repeated increases in BWT and in the emissions of gas, both following the pattern of the DO events. This correspondence in time scale indicates that BWT was the primary forcing factor for the variability in methane release. However, the releases were delayed by up to > 1,000 years compared to the initial increase in BWT.

Effects of shallow-water hydrothermal venting on biological communities of coastal marine ecosystems of the western Pacific

This review is based on integrated studies of the composition, structure and function of shallow-water ecosystems in the western Pacific that are influenced by underwater gas-hydrothermal activity. Most of the data were collected from 1985 to 1997 by the Institute of Marine Biology of the Far East Branch of the Russian Academy of Science during expeditions to zones of modern volcanism. Gas-hydrothermal activity of volcanoes has a great influence on the physicochemical characteristics of the water column and plankton, and of bottom sediment and benthic communities. The abundance of nutrients (SiO(3)(2-), PO(4)(3-), NO(3)(-)), gases (CO(2), CH(4), H(2), H(2)S) and other reduced compounds (C(n)H(n), S(0), S(2)O(3)(2-), NH(4)(+)) in zones of shallow-water hydrothermal vents provides conditions for the use of two energy sources for primary production: sunlight (photosynthesis) and the oxidation of reduced compounds (bacterial chemosynthesis). In areas of shallow-water volcanic activity, chemosynthesis occurs not only in the immediate vicinity of venting fluid release but also in the surface layer of the water column, where it occurs together with intense photosynthesis. This surface photosynthesis is found below the layer of chemosynthesis, which is related to the distribution of hydrothermal fluids at the water surface. The contribution of each of these processes to total primary production depends on the physical and chemical conditions created by the vents and on the range and adaptation potential of the organisms. On the seabed in zones of shallow-water venting, microorganisms form mats that consist of bacteria of various physiological groups, microalgae, the products of their metabolism and sedimentary particles. Oxygenic photosynthesis of benthic diatoms, bacterial photosynthesis (anoxygenic photosynthesis) and autotrophic chemosynthesis in algobacterial and bacterial mats generate organic matter additional to that produced in the water column. The high rates of primary production, abundance of organic matter in the water column and intense development of benthic microflora ensure the formation of an abundant benthic fauna. In Kraternaya Bight, Matupi Harbour and Bay of Plenty, the macrozoobenthos has low species diversity. The taxonomic composition of the populations is determined by geographical region (temperate or tropical), by the character of the seabed (hard or soft bottoms, rigid lava flows or hydrothermal structures), by the temperature of bottom sediments and of volcanic fluids and lastly by the chemical composition of the vent fluid (sulfide or nonsulfide). In most of the surveyed areas the fauna is derived from locally common species or from opportunistic species that can form high-density populations in eutrophic waters. The benthic communities of shallow-water venting areas have many characteristics in common with communities subject to anthropogenic impact (thermal, residential or industrial) or to changes resulting from a sharp deterioration of the marine environment. In contrast to the fauna of deeper water hydrothermal communities (i.e., those that exist below 200 m), shallow-water venting communities lack obligate hydrothermal species. The structure and function of the pelagic and benthic communities in areas of shallow-water venting can be regarded as transitional between those of deep-water vent communities and the normal communities of the coast.