TREE 1194 No of Pages 8 Review Hadal trenches the ecology of the deepest places on Earth Alan J Jamieson Toyonobu Fujii Daniel J Mayor Martin Solan and Imants G Priede Oceanlab University of Aberdeen Newburgh AB41 6AA UK Hadal trenches account for the deepest 45 of the oceanic depth range and host active and diverse biological communities Advances in our understanding of hadal community structure and function have until recently relied on technologies that were unable to document ecological information Renewed international interest in exploring the deepest marine environment on Earth provides impetus to re evaluate hadal community ecology We review the abiotic and biotic characteristics of trenches and offer a contemporary perspective of trench ecology The application of existing rather than the generation of novel ecological theory offers the best prospect of understanding deep ocean ecology The hadal environment The deepest areas of the ocean commonly referred to as the hadal zone 6000 11 000 m 1 represent 1 2 of the global benthic area see Glossary but they constitute the deepest 45 of the vertical depth gradient They are almost exclusively comprised of trenches representing spatially disjunct environments separated by shallower areas Figure 1 Box 1 Hadal trenches remain one of the least understood habitats on Earth Marine biozones that are based on observed faunal transitions with depth 2 have been used as a convenient means to divide the ocean into a series of realms Indeed species composition density biomass and diversity of hadal zones often contrast to that of the surrounding abyssal area This nomenclature ignores that depth is continuous and that hadal trenches are intrinsically linked to shallower ecosystems Topography geographical isolation and spatio temporal variation in food supply as well as elevated hydrostatic pressure and low temperature are all factors that might have encouraged speciation and thus shaped present faunal assemblage structure The first major trench sampling campaigns were conducted during the early 1950s on the Danish Galathea and Russian Vitjaz global expeditions Using trawl and grab methods the diversity abundance and biomass of benthic epifaunal and infaunal invertebrates were described Of the 300 metazoan species documented in this relatively sparse data set 1 58 were thought to be endemic a level comparable to neighbouring abyssal environments Since all subsequent hadal reviews 1 3 4 have primarily been based on these two data sets the current perception of Corresponding author Jamieson A J a jamieson abdn ac uk hadal trench ecosystems lacks an up to date ecological interpretation Research efforts have continued over the last 30 years although they have been sporadic and uncoordinated Recent advances in technological capacity e g 5 6 provide impetus for a renewed wave of hadal exploration Here as a first step towards synthesising and integrating available knowledge we provide a contemporary perspective on hadal trench environments and argue that the separation of environments by depth zonation alone is likely to hamper our understanding of deep ocean ecology Hydrographic and physical characteristics of hadal trenches It is now known that the bottom water of the hadal trenches is not stagnant and that deep currents flow through and ventilate the trenches 7 For example the deep water flowing through the West Pacific Trenches originates from the Southern Ocean There are two major water masses present in the deep Pacific 1000 m the Lower Circum Polar Water LCPW and the North Pacific Deep Water NPDW 8 The LCPW enters the Pacific from Glossary Adiabatic a process in which when a fluid is compressed its pressure increases and its temperature rises without the gain or loss of any heat Allochthonous an external source of food Autochthonous an internal source of food Benthic organisms living on or in the seabed Biogeographical province a biological subdivision of the surface of the Earth incorporating both faunal and floral characteristics Biozone biological depth zones littoral 0 1000 m bathyal 1000 3000 m abyssal 3000 6000 m hadal 6000 11 000 m Carrion food falls the deposit of dead or decaying flesh on the seafloor e g fish jellyfish or cetacean carcasses Deposit feeding a feeding strategy whereby organisms acquire food by ingesting large volumes of sediment and extract nutrients from the small organic fraction of the ingested sediment Eurybathy the ability to occupy a wide range of depths Eurythermic the tolerating of a wide temperature range Heterotrophic requiring complex organic compounds of nitrogen and carbon for metabolic synthesis Meiofauna organisms passing through a 250 500 mm sieve and retained on a 41 63 mm sieve Necrophagy feeding on carcasses Ocean acidification the ongoing decrease in the pH of the Earth s oceans caused by uptake of carbon dioxide from the atmosphere Ossified hardened Particulate Organic Matter POM particles of organic solids 0 4 mm suspended within the water column Phytopigments a pigment that undergoes a physical or chemical change upon exposure to light Primary production the production of organic compounds from atmospheric or aquatic carbon dioxide principally through the process of photosynthesis Stenobathy confined or restricted to a small depth range 0169 5347 see front matter 2009 Elsevier Ltd All rights reserved doi 10 1016 j tree 2009 09 009 Available online xxxxxx 1 TREE 1194 No of Pages 8 Review Figure 1 Major hadal trenches of the World a Sunda Java Trench 7450 m b Philippine Trench 10 540 m c Marianas Trench 10 989 m d Izu Bonin IzuOgasawara Trench 9810 m e Japan Trench 8412 m f Kurile Kamchatka Trench 10 542 m g Aleutian Trench 7820 m h Tonga Trench 10 800 m i Kermadec Trench 10 047 m j Middle America Trench 6662 m k Cayman Trench 7093 m l Puerto Rico Trench 8385 m m Peru Chile Trench 8055 m and n South Sandwich Trench 8428 m the south and flows northward 9 10 in a clockwise direction passing through the trenches on the west of the Pacific i e the Kermadec Trench and the Tonga Trench 7 Through the Samoan Passage it flows northwest across the equator to the east Mariana Basin and into a north and westward flow The westward branch flows through the west Mariana Basin 8 and through the Izu Ogasawara Japan and Kuril Kamchatka Trenches before heading southwards around the Emperor Seamounts towards the Aleutian Trench As for the NPDW trench currents then flow westwards back around the Aleutian and Box 1