THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
67
eddy as it forms along a front is largely determined
by their vertical migratory behaviour. The species that
are non-migratory are passively advected within the
eddy and so tend to persist within it. Those that
are advected into adverse conditions show signs of
malnourishment and physiological stress as the ambient
conditions deteriorate (Wiebe and Boyd, 1978). Species
with diel vertical migration initially entrapped in the
newly formed eddy tend to get spun out of it relatively
quickly, as they traverse differential shears within the
water column during their vertical excursions. Thus,
within rings and eddies, the assemblages of species
change more rapidly than might otherwise be expected.
This generates chaotic heterogeneity in the distributions
of pelagic species with fractal characteristics similar to
that of the eddy structure of the water. Behavioural,
feeding and reproductive strategies in pelagic species
can be expected to be adapted to this heterogeneity
in the biotic and abiotic environment. The effects
of mesoscale features are transmitted to deep-sea
environments. Many eddies extend all the way from
the surface to the bottom and are one mechanism
whereby “benthic storms” are generated in deep water
(Weatherly and Kelley, 1985; Kontar and Sokov, 1994).
They may also transport the planktonic species and
the pelagic larvae of benthic species far beyond the
bounds of their normal distributional ranges, as has
been observed for planktonic foraminifers (Fairbanks
et al., 1980).
Other features that generate heterogeneity within
pelagic environments include:
(1) “Streamers”, which are tongues of cold water that
can be seen in satellite images extending >100 km
offshore from upwelling regions (e.g., Mittelstaedt,
1991).
(2) Shelf-break fronts, which develop as a result
of internal waves breaking beneath the surface,
enabling vertical mixing to take place (e.g., New
and Pingree, 1990).
(3) The influence of seamounts often results in local
enhancement of primary production downstream,
so that secondary and tertiary producers also
accumulate in their vicinity (Haury et al., 2000).
Currents flowing over seamounts are accelerated
and destabilized by the shoaling topography, the socalled “Taylor column” effect, so that eddies and
internal waves are developed downstream (Nof,
1985). Taylor column effects can extend 1000 m
or so above the top of a seabed feature, so that
even quite deep features can induce perturbations
of the wind-mixed layer. There is enhancement
of both primary and secondary production around
seamounts, so that they are frequently utilized as
sites for the aggregation and spawning of oceanic
species, and also for colonization by suspensionfeeding benthos.
UPWELLING
At latitudes >40º the main mechanism re-supplying
nutrients to the euphotic zone is the seasonal mixing
that occurs when winter cooling breaks down the
stratification. But, in the subtropics and tropics where
the seasonal thermocline persists throughout the year,
the re-supply of nutrients via vertical mixing is limited
except where there is upwelling. As discussed above,
localized upwelling occurs along divergent fronts
around eddies and along some of the major oceanographic features. Much more significant upwelling
occurs in eastern boundary coastal regions, where trade
winds blow along the coast towards the equator. The
effects of Coriolis Force (a turning force generated
away from the equator by the rotation of the Earth)
result in the winds pushing the surface water offshore;
it is then replaced by cool, nutrient-rich water from
below the thermocline. There are five main areas where
upwelling occurs: off California, off Chile and Peru, off
Northwest and South-west Africa, and in the Arabian
and Somali Seas. Coastal upwelling tends to be an
episodic rather than a continuous process, and is linked
to periods of windy weather. Moreover, the winds that
favour upwelling tend to blow more strongly and more
frequently during specific seasons (Summerhayes et al.,
1995a). High productivity is stimulated by the nutrientrich subthermocline waters being drawn up to the
surface. These upwelling regions are important centres
for fisheries. Upwelling also occurs in the open ocean,
notably along the equator in the Central and Eastern
Pacific, and in the Eastern Atlantic. The winds on either
side of the equator blow divergently away from it, so
that upwelling occurs along a narrow band close to
the equator. In the Arabian Sea upwelling also occurs
well off-shore – again as a result of wind divergence
generated by the Findlater jet (a region where the wind
curl reverses).
In upwelling regions, standing crops of zooplankton
are very high, but are dominated by a very few species
which have life histories specially adapted to take
67
eddy as it forms along a front is largely determined
by their vertical migratory behaviour. The species that
are non-migratory are passively advected within the
eddy and so tend to persist within it. Those that
are advected into adverse conditions show signs of
malnourishment and physiological stress as the ambient
conditions deteriorate (Wiebe and Boyd, 1978). Species
with diel vertical migration initially entrapped in the
newly formed eddy tend to get spun out of it relatively
quickly, as they traverse differential shears within the
water column during their vertical excursions. Thus,
within rings and eddies, the assemblages of species
change more rapidly than might otherwise be expected.
This generates chaotic heterogeneity in the distributions
of pelagic species with fractal characteristics similar to
that of the eddy structure of the water. Behavioural,
feeding and reproductive strategies in pelagic species
can be expected to be adapted to this heterogeneity
in the biotic and abiotic environment. The effects
of mesoscale features are transmitted to deep-sea
environments. Many eddies extend all the way from
the surface to the bottom and are one mechanism
whereby “benthic storms” are generated in deep water
(Weatherly and Kelley, 1985; Kontar and Sokov, 1994).
They may also transport the planktonic species and
the pelagic larvae of benthic species far beyond the
bounds of their normal distributional ranges, as has
been observed for planktonic foraminifers (Fairbanks
et al., 1980).
Other features that generate heterogeneity within
pelagic environments include:
(1) “Streamers”, which are tongues of cold water that
can be seen in satellite images extending >100 km
offshore from upwelling regions (e.g., Mittelstaedt,
1991).
(2) Shelf-break fronts, which develop as a result
of internal waves breaking beneath the surface,
enabling vertical mixing to take place (e.g., New
and Pingree, 1990).
(3) The influence of seamounts often results in local
enhancement of primary production downstream,
so that secondary and tertiary producers also
accumulate in their vicinity (Haury et al., 2000).
Currents flowing over seamounts are accelerated
and destabilized by the shoaling topography, the socalled “Taylor column” effect, so that eddies and
internal waves are developed downstream (Nof,
1985). Taylor column effects can extend 1000 m
or so above the top of a seabed feature, so that
even quite deep features can induce perturbations
of the wind-mixed layer. There is enhancement
of both primary and secondary production around
seamounts, so that they are frequently utilized as
sites for the aggregation and spawning of oceanic
species, and also for colonization by suspensionfeeding benthos.
UPWELLING
At latitudes >40º the main mechanism re-supplying
nutrients to the euphotic zone is the seasonal mixing
that occurs when winter cooling breaks down the
stratification. But, in the subtropics and tropics where
the seasonal thermocline persists throughout the year,
the re-supply of nutrients via vertical mixing is limited
except where there is upwelling. As discussed above,
localized upwelling occurs along divergent fronts
around eddies and along some of the major oceanographic features. Much more significant upwelling
occurs in eastern boundary coastal regions, where trade
winds blow along the coast towards the equator. The
effects of Coriolis Force (a turning force generated
away from the equator by the rotation of the Earth)
result in the winds pushing the surface water offshore;
it is then replaced by cool, nutrient-rich water from
below the thermocline. There are five main areas where
upwelling occurs: off California, off Chile and Peru, off
Northwest and South-west Africa, and in the Arabian
and Somali Seas. Coastal upwelling tends to be an
episodic rather than a continuous process, and is linked
to periods of windy weather. Moreover, the winds that
favour upwelling tend to blow more strongly and more
frequently during specific seasons (Summerhayes et al.,
1995a). High productivity is stimulated by the nutrientrich subthermocline waters being drawn up to the
surface. These upwelling regions are important centres
for fisheries. Upwelling also occurs in the open ocean,
notably along the equator in the Central and Eastern
Pacific, and in the Eastern Atlantic. The winds on either
side of the equator blow divergently away from it, so
that upwelling occurs along a narrow band close to
the equator. In the Arabian Sea upwelling also occurs
well off-shore – again as a result of wind divergence
generated by the Findlater jet (a region where the wind
curl reverses).
In upwelling regions, standing crops of zooplankton
are very high, but are dominated by a very few species
which have life histories specially adapted to take