TREE 962 No of Pages 11 Review Abyssal food limitation ecosystem structure and climate change Craig R Smith1 Fabio C De Leo1 Angelo F Bernardino1 2 Andrew K Sweetman1 3 and Pedro Martinez Arbizu4 1 Department of Oceanography University of Hawaii 1000 Pope Road Honolulu HI 96822 USA Instituto Oceanogra fico Universidade de Sa o Paulo Prac a do Oceanogra fico 191 CEP 05508 120 Sa o Paulo SP Brazil 3 Norwegian Institute for Water Research Gaustadalle en 21 NO 0349 Oslo Norway 4 DZMB Forschungsinstitut Senckenberg Suedstrand 44 D 26382 Wilhelmshaven Germany 2 The abyssal seafloor covers more than 50 of the Earth and is postulated to be both a reservoir of biodiversity and a source of important ecosystem services We show that ecosystem structure and function in the abyss are strongly modulated by the quantity and quality of detrital food material sinking from the surface ocean Climate change and human activities e g successful ocean fertilization will alter patterns of sinking food flux to the deep ocean substantially impacting the structure function and biodiversity of abyssal ecosystems Abyssal ecosystem response thus must be considered in assessments of the environmental impacts of global warming and ocean fertilization The nature of abyssal habitats Based on faunal distributions and environmental characteristics the abyssal seafloor occurs between ocean depths of 3000 and 6000 m 1 2 Abyssal ecosystems are truly vast covering 54 of the Earth s surface 3 they are essentially a network of plains and rolling hills punctured by seamounts and subdivided by mid ocean ridges island arcs and ocean trenches Figure 1 Several ecological generalizations can be made about abyssal habitats 4 5 The abyssal seafloor is mostly covered by fine sediments medium sands to clays Figure 2 It is also characterized by an absence of in situ primary production except at spatially rare hydrothermal vents and cold seeps welloxygenated waters and by low temperatures of 0 5 3 08C 3 4 Most of the abyssal seafloor experiences low current velocities and little sediment erosion however in some regions e g beneath western boundary currents 3 6 sediment erosion can be frequent Much of the habitat structure of abyssal sediments is biogenic consisting of the tests of giant protozoans and the burrows mounds and tracks of megabenthos 3 4 Figure 2 Hard substrates associated with manganese nodules and fault scarps occur in many parts of the abyss and these substrates support faunal assemblages distinct from those in sediments 4 5 7 Figure 2 Probably the most important ecological characteristic of abyssal ecosystems is energy limitation abyssal seafloor communities are considered to be food limited because benthic production depends on the input of detrital organic material produced in the euphotic zone thousands of Corresponding author Smith C R craigsmi hawaii edu meters above 4 Most of the organic flux arrives as an attenuated rain of small particles typically only 0 5 2 of net primary production in the euphotic zone which decreases inversely with water depth 8 and varies regionally with levels of primary production in the upper ocean 9 The small particle flux can be augmented by the fall of larger carcasses and downslope transport of organic material near continental margins 4 10 Because of the size and remoteness of the abyss ecosystem structure and function at the seafloor have historically been very poorly studied For example more than 80 of the hundreds of species of seafloor invertebrates collected at any abyssal station are new to science 11 13 Nonetheless it is recognized that local diversity i e on Glossary Abyssal ocean floor depths from 3000 to 6000 m Bathyal ocean floor depths from 200 to 3000 m Benthos organisms living on or in the seafloor Bioturbation intensity Db a parameter measuring the diffusive mixing intensity of bioturbation or reworking of sediment grains by sedimentdwelling animals Bottom up processes controls on biotic community structure operating from lower to higher trophic levels for example control of features of detritusbased food webs e g species structure biomass size distributions etc by detritus availability Deposit feeder an animal that ingests deposited particulate material which primarily consists of inert sediments of low food value mineral grains refractory organic matter etc Export efficiency the percentage of net primary production in the euphotic zones that sinks into the deep ocean i e below depths of about 500 m Export production the amount of organic matter produced in the ocean by primary production that sinks into the aphotic zone i e below about 500 m Macrofauna benthic animals passing through a 2 cm mesh or not recognizable in bottom photographs but retained on a 250 500 mm sieve Manganese nodules rock concretions typically 5 10 cm in diameter on the seafloor formed of concentric layers of iron and manganese hydroxides Megafauna animals recognizable in bottom photographs and video images or collected in bottom trawls with mesh 2 cm Meiofauna benthic animals passing through a 250 500 mm sieve and retained on a 41 63 mm sieve Particulate organic carbon POC organic carbon in particulate form i e that can be filtered out of a sample much of which sinks through the water column providing a primary food source for abyssal ecosystems Pulse chase experiments experiments used to study ecosystem function by adding isotopically labeled phytoplankton detritus to intact sediment communities The uptake and respiration of the phytodetritus is then tracked over time e g 33 Sediment mixed layer depth the thickness of surface sediments actively reworked by benthic animals The main agents of reworking are depositfeeding and burrowing animals Working species collections collections of animals that have been sorted to the species level but where the bulk of species have provisional identifications species A B etc and remain undescribed 0169 5347 see front matter 2008 Elsevier Ltd All rights reserved doi 10 1016 j tree 2008 05 002 Available online xxxxxx 1 TREE 962 No of Pages 11 Review Trends in Ecology and Evolution Vol xxx No x Figure 1 Map of the abyssal seafloor namely bottom depths of 3000 6000 m light blue Depths greater than 3000 m are dark blue depths of 0 3000 m are gray and landmasses are black or off white Map created with GeoMapApp spatial scales 0 1 1 m2 in abyssal sediments can be moderate to high e g with 50 species for every 150 individuals of polychaetes and more than 100