An unexpected oasis has been discovered in a deep, rocky seamount environment south of Tasmania. The density and biomass of organisms in this deep sea community is 10 times greater than previously reported in this depth range, prompting a rethink on the ecological importance of such regions.

Scientists caught a glimpse of the exceptionally dense marine community at depths of 2000–2500 metres on a voyage to the South-east Commonwealth Marine Reserves by the Marine National Facility Research Vessel Southern Surveyor in December 2008. A detailed follow-up survey in January 2009 used the remotely operated vehicle Jason provided by the Woods Hole Oceanographic Institution.

“Before this find we thought that deep-ocean biomass densities were generally highest at the edge of the continental shelf at about 1000 m depth and then declined with depth, except near unusual circumstances such as hydrothermal vents,” said Ron Thresher of CSIRO. “Our observations on Australia's southern seamounts force us to rethink this generality. On average we saw three times the density of visible organisms at 2000–2500 m than in comparable images for cold water coral reefs at 1000–1300 m.”

The newly discovered community contains huge numbers of three benthic (bottom dwelling) organisms: large barnacles (Tetrachaelasma tasmanicum), an undescribed anemone and several large bamboo corals or gorgonians, also unknown to science. Dr Thresher said the community probably fed on carbon sinking from areas of high surface productivity in the Sub-Antarctic Zone of the Tasman Sea. The sources of this carbon are still unknown. Nor is it certain how widespread the community is, because few of these deep, rocky environments have been sampled worldwide. These conditions, and deep seamounts, are widely distributed around the southern mid-latitudes, however, suggesting this dense community may be widespread.

The ABE and Jason survey was the deepest conducted in Australian waters and spanned 820–4011 m. It obtained coral samples for paleoceanographic study of the Southern Ocean and determined biodiversity values for the extensive portion of South-east Commonwealth Marine Reserves found below 1600 m, (the depth limit of previous surveys in this area). Overall, seven depth-stratified assemblages were identified, ranging from a naturally sparse coral and gorgonian dominated community topping the seamounts to a low diversity sediment plain assemblage below about 3900 m.

The two most striking new finds were an intact, but entirely dead coral reef between roughly 1500 m and 1700 m, and the very high biomass zone centred around 2300 m. The dead reef was comprised of Solenosmilia variabilis, the same species that forms the current intact reef between 950 m and 1300 m. Its presence suggests large-scale changes in ocean circulation during the last millenia. Work on dating the fossil reef is under way.

Dr Thresher said the survey highlighted the lack of information on deep-sea biota worldwide and the potential for unanticipated impacts of deep-sea exploitation. "Globally, fishing, oil, gas and mineral exploration and extraction now take place at the depths occupied by the community,” he said. “We have previously assumed that biological communities at these depths were very sparse, and hence that the biological impacts of fishing, mining and other activities were likely to be slight. Our observations indicate that this assumption may not always be valid.”

Photo:  Isidid gorgonians (Keratoisis spp) and anemones at 2540 m. Jason dive #392, at 45° 17.70′′ S; 146° 06.72′′ E.  2009. Ron Thresher, CSIRO

Related information:
Thresher R. E., Adkins J.,  Fallon S.J., Gowlett-Holmes K., Althaus F., Williams A. (2011). Extraordinarily high biomass benthic community on Southern Ocean seamounts, Scientific Reports (Nature), 1:119.

Thresher R.E., Tilbrook, B., Fallon, S., Wilson, N.C. and Adkins, J. (2011). Effects of chronic low carbonate saturation levels on the distribution, growth and skeletal chemistry of deep-sea corals and other seamount megabenthos, Marine Ecology Progress Series, 442: 87-99.

Thresher R.E., Williams A., Adkins, J., Althaus F., Gowlett-Holmes K., lderslade, P., Dowdney, J., Fallon, S., Trull, T., Cho, W., Gagnon, A., Staples, D. and McEnnulty, F. (in review). Diversity, assemblage composition and biomass of the deep and very deep reef megabenthos on seamounts off southern Tasmania, Marine Ecology Progress Series.