6
David THISTLE
must be tempered for some variables and some
locations.
Pressure
Pressure increases by one atmosphere (10
5 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 fishes that use a gas-filled
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 modifications 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 efficient 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.
Bottom-water temperature
Bottom-water temperatures generally decrease with
increasing depth, reaching ~2ºC 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 profiles of mean temperature versus depth for the
open ocean. Modified from Pickard and Emery (1990). Reproduced
by permission of Butterworth Heinemann.
0
200
400
600
800
1000
1200
1400
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
Temperature ( C)
o
Depth (m)
Minimum
temperature
Maximum
temperature
Fig. 2.3. Annual temperature variation in the western North Atlantic
illustrating the diminishing amplitude of seasonal variation with
depth. Modified 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 floor 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-floor 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
David THISTLE
must be tempered for some variables and some
locations.
Pressure
Pressure increases by one atmosphere (10
5 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 fishes that use a gas-filled
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 modifications 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 efficient 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.
Bottom-water temperature
Bottom-water temperatures generally decrease with
increasing depth, reaching ~2ºC 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 profiles of mean temperature versus depth for the
open ocean. Modified from Pickard and Emery (1990). Reproduced
by permission of Butterworth Heinemann.
0
200
400
600
800
1000
1200
1400
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
Temperature ( C)
o
Depth (m)
Minimum
temperature
Maximum
temperature
Fig. 2.3. Annual temperature variation in the western North Atlantic
illustrating the diminishing amplitude of seasonal variation with
depth. Modified 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 floor 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-floor 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