THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
49
(e.g., Martin et al., 1990; de Baar and Boyd, 2000).
Other areas where this same phenomenon of high
nutrient, low chlorophyll (HNLC) occurs include the
North Pacific and Eastern Tropical Pacific. Another
notable feature of the Southern Ocean is that there
are exceptionally high sedimentation rates of silicate
beneath the Antarctic Convergence (Shimmield et al.,
1994).
There are a number of smaller seas separated from
the main ocean basins, which have very different
hydrological and ecological characteristics (see Chapter 9). The Mediterranean and Red Seas provide
exceptional deep-sea environments, both pelagic and
benthic. Shallow sills at the Strait of Gibraltar and
Bab-el-Mandab prevent deep interchange with the
neighbouring oceans. Both are situated at latitudes
which are arid, so that freshwater inputs by precipitation and riverine run-off are much lower than the
evaporative losses from the surface. This imbalance
is compensated for by surface inflows through the
Straits, but inside the Seas the surface salinities of
these inflows increase, and there is localized formation
of bottom water which creates uniquely warm, highsalinity deep water. In the Western Mediterranean
deep-water temperatures and salinities are 12.6ºC and
>36, respectively, in the Eastern Mediterranean 13.4ºC
and >37, and in the Red Sea 21.6ºC and >38. At
depths >300 m, both seas are almost isothermal, and
the warm temperatures in the deep water enhance the
rates of bacterial degradation of sedimenting particles,
so that pelagic standing crops decline very much
faster with depth than in the open oceans. Recent
sampling from RRS Discovery in the Alboran Sea
between Spain and Algeria confirmed that below 500–
600 m there were almost no living copepods in the
water column (c.f. Weikert and Koppelmann, 1996).
Also, there is almost no bathypelagic community of
species; instead, a few components of the mesopelagic
fauna extend their bathymetric ranges to unusually
great depths. The micronekton samples collected from
depths >2000 m contained a small biomass consisting
of a single species of caridean shrimp, Acanthephyra
eximia. Little benthopelagic sampling has been carried
out in either sea, but the deep communities appear
to be supported either by fast-track large-package
inputs or by downslope turbidity flows. Maybe these
seas present conditions analogous to those that prevailed globally during the Mesozoic, when deep-water
temperatures were generally very much warmer than
today.
OCEAN GRADIENTS AND PRIMARY
PRODUCTION
Many of the fundamental ecological processes are
strongly influenced by the vertical structure of the water
column, the most important of these processes being
primary productivity. Rates of primary production are
influenced by the availability of light and nutrients.
In the open ocean the nutrient supply is strongly
influenced by the degree to which vertical mixing
occurs, and the resulting density structure of the upper
water column. At low latitudes the density structure is
predominantly determined by temperature, particularly
by the depth of the thermocline, the zone of steepest
temperature gradient. However, at high latitudes in
the Arctic and Norwegian Sea, the range of water
temperatures is small. Hence the density structure of
the water is generated principally by the surface layer of
low-salinity sea water resulting from the huge outflows
of fresh water discharged into the Arctic by the big
Siberian rivers and the Mackenzie River in North
America.
The uppermost few tens of metres of the water
column are kept well mixed by the turbulence generated
by the wind. This uniform layer is called the “windmixed layer”. It is isothermal and isohaline, with
similar nutrient concentrations throughout. In the North
Atlantic at high latitudes during winter, the windmixed layer extends down to depths of several hundreds
of metres, but in the North Pacific a strong salinity
gradient maintains the stratification and restricts the
wind-mixed layer to the uppermost 150 m or so.
Phytoplankton cells have a very limited ability to
regulate their depth, so that the turbulent mixing results
in the cells spending most of the time at depths where
the light is far too dim for photosynthesis. During
springtime in the North Atlantic, the surface waters
tend to become warmer as the strength and duration of
the solar radiation increase. However, it is usually not
until there have been a few calm days that the surface
water eventually warms sufficiently for it to stratify
thermally. The depth to which any turbulent mixing
occurs is then limited to the top of the thermocline. The
phytoplankton cells then are no longer being churned
down into deep water but stay suspended at depths
where there is ample sunlight for photosynthesis. The
near-surface zone illuminated by enough sunlight to
support photosynthesis is termed the “euphotic zone”.
The phytoplankton population starts to grow rapidly.
This rapid growth is maintained for as long as an
49
(e.g., Martin et al., 1990; de Baar and Boyd, 2000).
Other areas where this same phenomenon of high
nutrient, low chlorophyll (HNLC) occurs include the
North Pacific and Eastern Tropical Pacific. Another
notable feature of the Southern Ocean is that there
are exceptionally high sedimentation rates of silicate
beneath the Antarctic Convergence (Shimmield et al.,
1994).
There are a number of smaller seas separated from
the main ocean basins, which have very different
hydrological and ecological characteristics (see Chapter 9). The Mediterranean and Red Seas provide
exceptional deep-sea environments, both pelagic and
benthic. Shallow sills at the Strait of Gibraltar and
Bab-el-Mandab prevent deep interchange with the
neighbouring oceans. Both are situated at latitudes
which are arid, so that freshwater inputs by precipitation and riverine run-off are much lower than the
evaporative losses from the surface. This imbalance
is compensated for by surface inflows through the
Straits, but inside the Seas the surface salinities of
these inflows increase, and there is localized formation
of bottom water which creates uniquely warm, highsalinity deep water. In the Western Mediterranean
deep-water temperatures and salinities are 12.6ºC and
>36, respectively, in the Eastern Mediterranean 13.4ºC
and >37, and in the Red Sea 21.6ºC and >38. At
depths >300 m, both seas are almost isothermal, and
the warm temperatures in the deep water enhance the
rates of bacterial degradation of sedimenting particles,
so that pelagic standing crops decline very much
faster with depth than in the open oceans. Recent
sampling from RRS Discovery in the Alboran Sea
between Spain and Algeria confirmed that below 500–
600 m there were almost no living copepods in the
water column (c.f. Weikert and Koppelmann, 1996).
Also, there is almost no bathypelagic community of
species; instead, a few components of the mesopelagic
fauna extend their bathymetric ranges to unusually
great depths. The micronekton samples collected from
depths >2000 m contained a small biomass consisting
of a single species of caridean shrimp, Acanthephyra
eximia. Little benthopelagic sampling has been carried
out in either sea, but the deep communities appear
to be supported either by fast-track large-package
inputs or by downslope turbidity flows. Maybe these
seas present conditions analogous to those that prevailed globally during the Mesozoic, when deep-water
temperatures were generally very much warmer than
today.
OCEAN GRADIENTS AND PRIMARY
PRODUCTION
Many of the fundamental ecological processes are
strongly influenced by the vertical structure of the water
column, the most important of these processes being
primary productivity. Rates of primary production are
influenced by the availability of light and nutrients.
In the open ocean the nutrient supply is strongly
influenced by the degree to which vertical mixing
occurs, and the resulting density structure of the upper
water column. At low latitudes the density structure is
predominantly determined by temperature, particularly
by the depth of the thermocline, the zone of steepest
temperature gradient. However, at high latitudes in
the Arctic and Norwegian Sea, the range of water
temperatures is small. Hence the density structure of
the water is generated principally by the surface layer of
low-salinity sea water resulting from the huge outflows
of fresh water discharged into the Arctic by the big
Siberian rivers and the Mackenzie River in North
America.
The uppermost few tens of metres of the water
column are kept well mixed by the turbulence generated
by the wind. This uniform layer is called the “windmixed layer”. It is isothermal and isohaline, with
similar nutrient concentrations throughout. In the North
Atlantic at high latitudes during winter, the windmixed layer extends down to depths of several hundreds
of metres, but in the North Pacific a strong salinity
gradient maintains the stratification and restricts the
wind-mixed layer to the uppermost 150 m or so.
Phytoplankton cells have a very limited ability to
regulate their depth, so that the turbulent mixing results
in the cells spending most of the time at depths where
the light is far too dim for photosynthesis. During
springtime in the North Atlantic, the surface waters
tend to become warmer as the strength and duration of
the solar radiation increase. However, it is usually not
until there have been a few calm days that the surface
water eventually warms sufficiently for it to stratify
thermally. The depth to which any turbulent mixing
occurs is then limited to the top of the thermocline. The
phytoplankton cells then are no longer being churned
down into deep water but stay suspended at depths
where there is ample sunlight for photosynthesis. The
near-surface zone illuminated by enough sunlight to
support photosynthesis is termed the “euphotic zone”.
The phytoplankton population starts to grow rapidly.
This rapid growth is maintained for as long as an