THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
51
available in the wind-mixed layer (Dugdale et al.,
1995), and is closely linked to the growth and death
of diatoms. Diatoms are a group of phytoplankton that
have cell walls made out of two cylinders of silica that
fit together like a pill-box. Their production dominates
the spring bloom. But as the nutrients become depleted
in the euphotic zone, they have the tendency to clump
together into aggregations. These aggregations contribute to the formation of “marine snow” – aggregates
that can range in size from 1 mm to 1 cm and which
sink rapidly at speeds of several hundreds of metres
per day (Lampitt and Antia, 1997). The term “marine
snow” was coined to describe the large quantities of
flocculent material first seen in the lights of the early
submersibles. Seasonal pulses of sedimenting particles
are a regular feature in many oceanic areas, and have
been quantified using sediment traps. Sediment traps
function like recording rain gauges, and are moored for
up to six months at a range of depths in the ocean to
quantify the fluxes of sinking particles (e.g., Honjo and
Manganini, 1993). Time-lapse cameras deployed on the
sea-bed have photographed the deposition of copious
quantities of detrital material on to the sea-floor at
abyssal depths in the North-east Atlantic within a few
weeks of the collapse of the spring bloom at the surface
(Lampitt, 1985; Thiel et al., 1988/89; Rice et al., 1994;
Smith et al., 1996). Below the deepest range of vertical
migration all organisms are dependent on the fluxes of
organic matter that sink from the surface layers. The
larger the particle or aggregate is, the faster it will
sink, and so the less likely it is to be intercepted by
detritivores on its journey down to the sea-bed. Also
there will be less time for it to be degraded by microbial
activity. Thus, the size range of the primary producers
strongly influences the ultimate fate of the carbon fixed,
and the quantities of organic material that reach the
deep-living communities both within the water and on
the sea-bed.
At low latitudes the re-supply of nutrients from
subthermocline depths occurs either as a result of largescale upwelling events (Summerhayes et al., 1995b)
(see pp. 67–68), or smaller-scale vertical mixing that
occurs around the meandering edges of eddies and
along fronts (see pp. 65–67) as a result of the
conservation of vorticity. (Vorticity is the mechanism
used by skaters when they spin, either increasing their
rotation by drawing their arms in and over their heads,
or slowing it by throwing their arms out wide). There
is also some resupply by vertical diffusion, but this
is a very slow process. The biological response to
vertical mixing and upwelling depends very much on
the characteristics of the deep water being brought
up to the surface. In the North Atlantic where the
formation of deep water is actively going on, relatively
little time has passed since the deep waters were at the
surface, and they are described as being “young”. The
young waters that have been at the surface relatively
recently contain plenty of dissolved oxygen (Mantyla
and Reid, 1983), but relatively low concentrations of
regenerated nutrients. This is clearly shown in Fig. 3.3,
which illustrates the differences in the concentrations of
nitrate between the deep waters of the various oceans
(Levitus et al., 1993). Thus, in winter in the Bay of
Biscay, although the water is mixed to very considerable depths (>500 m: Parsons, 1988), it does not
boost the nutrient concentrations as much as it might
in other oceans, because the deep water contains only
moderate concentrations of nutrients. Consequently,
phytoplankton production in the spring is not nearly as
high in the North Atlantic as it is in the North Pacific,
even though the vertical stirring is restricted to the uppermost 150 m. The subthermocline waters in the North
Pacific are “older” and so contain richer quantities of
regenerated nutrients (Fig. 3.3), but they also contain
only about half the amount of dissolved oxygen.
Chemical profiles
The concentration profiles of dissolved substances vary
according to the extent to which the substances are
involved in biological processes. Thus, some metals
and ions behave like the nutrients, and are controlled
by biological activity, particularly in the euphotic zone
(e.g., barium and cadmium). Typically these occur in
low concentrations in the upper water column, because
they are removed as part of the downward flux of
organic material. In deep water their concentrations
increase as chemical and microbial breakdown of the
sinking organic matter releases them back into solution.
Other substances are considered to be intermediate
in their characteristics, because, although they are
involved in biological processes, other dynamic processes are dominant in regulating their concentrations
(e.g., copper). Oxygen concentrations are particularly
interesting. As discussed, concentrations of dissolved
oxygen generally reflect the age of the water –
the length of time since the water was last at the
surface – because the oxidation of the organic matter in
midwater progressively uses up the available oxygen.
The rate at which it is used up depends on how
51
available in the wind-mixed layer (Dugdale et al.,
1995), and is closely linked to the growth and death
of diatoms. Diatoms are a group of phytoplankton that
have cell walls made out of two cylinders of silica that
fit together like a pill-box. Their production dominates
the spring bloom. But as the nutrients become depleted
in the euphotic zone, they have the tendency to clump
together into aggregations. These aggregations contribute to the formation of “marine snow” – aggregates
that can range in size from 1 mm to 1 cm and which
sink rapidly at speeds of several hundreds of metres
per day (Lampitt and Antia, 1997). The term “marine
snow” was coined to describe the large quantities of
flocculent material first seen in the lights of the early
submersibles. Seasonal pulses of sedimenting particles
are a regular feature in many oceanic areas, and have
been quantified using sediment traps. Sediment traps
function like recording rain gauges, and are moored for
up to six months at a range of depths in the ocean to
quantify the fluxes of sinking particles (e.g., Honjo and
Manganini, 1993). Time-lapse cameras deployed on the
sea-bed have photographed the deposition of copious
quantities of detrital material on to the sea-floor at
abyssal depths in the North-east Atlantic within a few
weeks of the collapse of the spring bloom at the surface
(Lampitt, 1985; Thiel et al., 1988/89; Rice et al., 1994;
Smith et al., 1996). Below the deepest range of vertical
migration all organisms are dependent on the fluxes of
organic matter that sink from the surface layers. The
larger the particle or aggregate is, the faster it will
sink, and so the less likely it is to be intercepted by
detritivores on its journey down to the sea-bed. Also
there will be less time for it to be degraded by microbial
activity. Thus, the size range of the primary producers
strongly influences the ultimate fate of the carbon fixed,
and the quantities of organic material that reach the
deep-living communities both within the water and on
the sea-bed.
At low latitudes the re-supply of nutrients from
subthermocline depths occurs either as a result of largescale upwelling events (Summerhayes et al., 1995b)
(see pp. 67–68), or smaller-scale vertical mixing that
occurs around the meandering edges of eddies and
along fronts (see pp. 65–67) as a result of the
conservation of vorticity. (Vorticity is the mechanism
used by skaters when they spin, either increasing their
rotation by drawing their arms in and over their heads,
or slowing it by throwing their arms out wide). There
is also some resupply by vertical diffusion, but this
is a very slow process. The biological response to
vertical mixing and upwelling depends very much on
the characteristics of the deep water being brought
up to the surface. In the North Atlantic where the
formation of deep water is actively going on, relatively
little time has passed since the deep waters were at the
surface, and they are described as being “young”. The
young waters that have been at the surface relatively
recently contain plenty of dissolved oxygen (Mantyla
and Reid, 1983), but relatively low concentrations of
regenerated nutrients. This is clearly shown in Fig. 3.3,
which illustrates the differences in the concentrations of
nitrate between the deep waters of the various oceans
(Levitus et al., 1993). Thus, in winter in the Bay of
Biscay, although the water is mixed to very considerable depths (>500 m: Parsons, 1988), it does not
boost the nutrient concentrations as much as it might
in other oceans, because the deep water contains only
moderate concentrations of nutrients. Consequently,
phytoplankton production in the spring is not nearly as
high in the North Atlantic as it is in the North Pacific,
even though the vertical stirring is restricted to the uppermost 150 m. The subthermocline waters in the North
Pacific are “older” and so contain richer quantities of
regenerated nutrients (Fig. 3.3), but they also contain
only about half the amount of dissolved oxygen.
Chemical profiles
The concentration profiles of dissolved substances vary
according to the extent to which the substances are
involved in biological processes. Thus, some metals
and ions behave like the nutrients, and are controlled
by biological activity, particularly in the euphotic zone
(e.g., barium and cadmium). Typically these occur in
low concentrations in the upper water column, because
they are removed as part of the downward flux of
organic material. In deep water their concentrations
increase as chemical and microbial breakdown of the
sinking organic matter releases them back into solution.
Other substances are considered to be intermediate
in their characteristics, because, although they are
involved in biological processes, other dynamic processes are dominant in regulating their concentrations
(e.g., copper). Oxygen concentrations are particularly
interesting. As discussed, concentrations of dissolved
oxygen generally reflect the age of the water –
the length of time since the water was last at the
surface – because the oxidation of the organic matter in
midwater progressively uses up the available oxygen.
The rate at which it is used up depends on how