Chapter 2 THE DEEP SEA FLOOR AN OVERVIEW David THISTLE The shelf break is at about 200 m depth in many parts of the ocean so the deep sea is said to begin at 200 m The deep sea oor is therefore a vast habitat covering more than 65 of the Earth s surface Sverdrup et al 1942 Much of it is covered by sediment but in some regions e g mid ocean ridges seamounts bare rock is exposed In the overview of environmental conditions that follows the information applies to both hard and soft bottoms unless differences are noted The ecosystems of hydrothermal vents and cold seeps are special cases and are described in Chapter 4 INTRODUCTION This chapter provides a general introduction to the ecosystem of the deep sea oor beginning with a description of the physical environment of the deep sea A section on how information is obtained about the deep sea oor ecosystem follows because knowledge of this ecosystem is greatly in uenced by the effectiveness of the available technology Introductions to the fauna of the deep sea where the substratum is sediment soft bottoms and where it is not hard bottoms follow The chapter concludes with a section on the pace of life in the deep sea Environmental setting The deep sea oor is an extreme environment pressure is high temperature is low and food input is small It has been characterized as a physically stable environment Sanders 1968 Below I review the major environmental variables and indicate circumstances under which these environmental variables constitute a biological challenge I also show that the image of the deep sea oor as monotonous and stable The geographic extent of the deep sea oor ecosystem The deep sea is usually de ned as beginning at the shelf break Fig 2 1 because this physiographic feature coincides with the transition from the basically shallow water fauna of the shelf to the deep sea fauna Sanders et al 1965 Hessler 1974 Merrett 1989 0 Bathyal zone Continental shelf 3 4 5 Shelf break Continental slope 2 Abyssal zone Depth km 1 Continental rise Abyssal plain 6 Distance from shore Fig 2 1 Diagrammatic cross section of the ocean showing the major physiographic features and major depth zones The sublittoral zone 0 200 m is not labeled and the hadal zone 6000 10 000 m is not shown Modi ed from Gage and Tyler 1991 Copyright Cambridge University Press 1991 Reprinted with the permission of Cambridge University Press 5 6 David THISTLE must be tempered for some variables and some locations Bottom water temperature Bottom water temperatures generally decrease with increasing depth reaching 2 C on the abyssal plain but the pattern varies with latitude and region Mantyla and Reid 1983 Fig 2 2 Above about 500 m in midlatitude temperature varies seasonally but with diminishing amplitude with increasing depth Figs 2 2 2 3 It should be noted that at high latitudes the vertical gradient in bottom water temperature is small Sverdrup et al 1942 A small vertical temperature gradient also occurs in regions where the bottom water is warm e g the Mediterranean Sea and the Red Sea Fig 2 2 Typical pro les of mean temperature versus depth for the open ocean Modi ed from Pickard and Emery 1990 Reproduced by permission of Butterworth Heinemann Temperature oC 2 0 2 4 6 8 10 12 14 16 18 20 22 24 0 200 400 Depth m Pressure Pressure increases by one atmosphere 105 Pascals for every 10 m increase in water depth so pressure varies from 20 atm at the shelf slope break to 1000 atm in the deepest parts of the trenches Pressure can affect organisms physiologically For example high deep sea pressures oppose the secretion of gas Many bottom associated deep sea shes that use a gas lled swim bladder to regulate their buoyancy Merrett 1989 overcome this problem in part by increasing the length of the retia mirabilia Marshall 1979 a component of the system that secretes gas into the swim bladder Pressure also affects an organism biochemically because the performance of proteins e g enzymes and lipid structures e g membranes changes with pressure For example any biochemical reaction that involves an increase in volume at any step in the transition from reactants to products will proceed more slowly as pressure increases Hochachka and Somero 1984 A species that lives in the deep sea must have adaptations that reduce or eliminate the pressure effects on reaction rates Such adaptations include modi cations of the enzymatic machinery e g changes to the amino acid sequence of an enzyme to reduce or eliminate volume changes during catalysis Siebenaller and Somero 1978 These adaptations come with a cost pressure insensitive enzymes are not as ef cient at shallow water pressures as are those of shallow water species Hochachka and Somero 1984 This requirement for molecular level adaptations has been postulated to constitute an evolutionary barrier that must have been overcome by those species that successfully entered the deep sea Minimum temperature Maximum temperature 600 800 1000 1200 1400 Fig 2 3 Annual temperature variation in the western North Atlantic illustrating the diminishing amplitude of seasonal variation with depth Modi ed from Sanders 1968 Reproduced by permission of the University of Chicago Press Copyright 1968 by the University of Chicago In summary most of the water overlying the deepsea oor is cold compared to that over most shallowwater habitats At depths below 800 m temperature is remarkably constant Fig 2 3 In the abyss temporal variation is measured in the second decimal place and occurs for example because internal tides and waves cause the oscillation of isothermal surfaces Hydrothermal vents are exceptions they occur in the cold deep sea but temperatures near them are elevated and variable see Chapter 4 The low temperatures have consequences for deepsea oor organisms because the cold reduces chemical reaction rates and shifts reaction equilibria toward reactants and away from products Hochachka and Somero 1984 To metabolize at reasonable rates deep sea species must have biochemical machinery that compensates For example low temperatures decrease THE DEEP SEA FLOOR AN OVERVIEW Oxygen ml l 1 0 1 2 3 4 5 6 7 1000 Depth m enzyme exibility and therefore catalytic rates This effect can be offset over evolutionary time by changes in the amino acid sequence of an enzyme to reduce the number of weak interactions e g hydrogen bonds that stabilize its three dimensional structure Hochachka and Somero 1984 The necessity for such adaptation to low temperatures like that to