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Ocean Circulation and Marine Life relationships between the mean biological, nutrient and physical patterns


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Title: Ocean Circulation and Marine Life relationships between the mean biological, nutrient and physical patterns

Ocean Circulation and Marine Liferelationshi
ps between the mean biological, nutrient and
physical patterns
Our purpose is an examination of oceanic
observations for evidence that yields
understandings of how mean biological, chemical
and physical patterns are are related over long
oceanic spatial and temporal scales. To do so,
restrictions are imposed on the data and its
analytical treatment that naturally limit the
interpretation of the resulting patterns. Data
averaging procedures are chosen to smooth away
spatial variations less than 100 kilometers in
size because that is a value characterizing
current instabilities and the resulting eddies.
These features reduce property anomalies by time
dependent turbulent mixing and, because of this,
we eliminate them from the present consideration.
So the patterns that we use should not exhibit
changes over distances that are less than 100 km
and, as well, the patterns are considered
constant in time. Our flow models are based on
the concept of balanced forces which means
currents can change from place-to-place but at
any place they do not change in time. So, all
the other data that we will use, chemical and
biological, should be, as near as possible, of
the same character, steady-state. We will assume
that is so.
Do atmospheric and oceanic physical processes
influence an oceans biological distributions ?
Joe Reid and Scripps colleagues published a study
of the relationships between ocean circulation
and marine life in 1978. This work was the
initial joint exam-ination of ocean-wide
biological provinces and physical processes. It
led to definitive evidence of a significant
correlation between steady-state flow and
steady-state biological patterns. In the study,
the underlying horizontal and vertical motions
are explained by physical theories that account
for the affect of gravity, wind and the rotation
of the earth. A basin-wide overall circulation
pattern for a hemisphere expresses the
dom-inance of the wind through geostrophic
balances and convergence-divergence processes
encompassing the anticlockwise and clockwise
gyres. As well, the differences in the climate
and related vertical circulations contribute
significantly to the over-all maintenance of
different habitats for life forms. As a preview,
the subpolar clockwise gyres that have
equatorward extensions along eastern ocean
boundaries are cold, high in nutrients, and have
low numbers of species in which each has a
relatively large biomass. To the contrary, the
subtropical clockwise gyres are warm, low in
nutrients, and have relatively high numbers of
species each of which has a relatively low
biomass. As we work with the patterns, develop an
understanding of how these steady, mean
relationships prevail.
the Geostrophic model applied to a northern
hemisphere subtropical gyre
use the Ekman model results to understand the
convergence and divergence processes
subpolar gyre divergence
subtropical gyre convergence
equatorial region divergence
Oceanographers have observed and studied data
bearing on biological, chemical and physical
relationships for more than a century. For our
immediate studies alone, we have pointed out some
of those concerned and given credit for very
early success. Collections and classification led
to the studies of marine organisms in terms of
their relationships to the environment. In the
mid-1830s, Mullers net tows and
examin-ations with microscopes led to the
identification of the planktonic basis of the
food web and fundamental reasons for the
variations in oceanic nutrient patterns. The
American biologists Agassiz and Bigelow
contributed to changes that emphasized
correlation of organisms with one another and
with the environment. In chemistry, Buchanans
pioneer observations on the Challenger led to
Dittmars work that identified the major
dissolved ionic constituents, Forchhammer and
Marcets constancy principles and Knudsens
definition and determination of salinity. The
determination of the composition of plankton
organisms by Vinogradov in the 1930s and
observations at sea of the important nutrients
phosphate, silicate and compounds of nitrates all
but erased distinct lines separating chemistry
and biology. For physics, the French physicist
Coriolis solved the importance of the earths
rotation. Scandanavians Helland-Hansen and
Bjerknes theoretical work on currents in
variable density ocean waters (geostrophy) that
was made possible by Nansens advances in
temperature and salinity accuracies and Ekmans
theory of wind-driven currents. Their works
contain the concepts and results for OCNG 251s
steady-state, force-balanced explanation of
oceanic flow.
Marcets wife, Jane
Food webs are illustrations that portray energy
transfer pathways between populations. Food
pyra-mids do the same. In the pelagic open ocean,
waters can be nutrient-limited with low
produc-tivity and populated by extremely small
organisms in large, complex food webs. In
upwelling regions, waters can be high in
nutrients and production. As a rule, the more
productive an environment, the fewer levels in
the web and the larger the organisms that
populate it. The term plankton includes all of
the marine organisms that do not stem currents.
There are phyto-
plankton, the plant plankton, animal plankton
known as zooplankton, and bacterioplankton that
provide a decomposition stage for returning
organic to nutrient material. Phytoplankton
require an external source of energy for the
synthetic fixation of carbon. In this regard,
solar radiation light values and variability play
a strong role in foods primary production.
Photosynthesis fixes most energy utilized in the
ocean. The process is carried out by
auto-trophic organisms that use water as a
hydrogen donor and liberate oxygen. At a higher
food pyramid level heterotrophs consume the
plants and, in turn, can serve as food
themselves. The reverse of photosynthesis is
known as respiration, the combination of fixed
carbon and oxygen during which water and carbon
dioxide are produced.
Mean Ocean Surface Currents
This schematic of the worlds wind-driven
currents illustrates the ocean-to-ocean
similarities. Flow pattern differences are
primarily a function of boundary differences.
The Atlantic and Pacific Oceans have wide range
due to their near pole to pole extents. To
generalize, in a given northern hemisphere, there
are equatorial counter-currents, equatorial
currents, a central subtropical gyre and a
subpolar gyre, south to north. We will use a
Pacific example because the basin is large enough
to more easily sort out the various currents.
The world oceans average surface currents are
closely correlated with mean atmospheric wind
learn the names, locations and flow
directions of the above currents
This is particularly evident in the case of the
North Pacific Gyre when it is juxtaposed with the
overlying NE Trade and Westerly winds. This
system applies a clock-wise torque at the ocean
surface forcing the gyres clockwise motion and
producing transport toward its center. The
converse holds for the Alaska and Oyashio
The convergence of waters toward the center of
the subtropical gyres is shown by the blue Ekman
transport vectors. Recall that the depth over
which the transport occurs depends on the
magnitude of the wind speed. The result of this
convergence is a mound of water most often simply
shown as disk with a raised central portion in
order to facilitate an under-standing of a
continuous circulation path around its periphery.
Remember that the mound is skewed, adjacent to
the western boundary, because of the lands
rotation eastward into the oceans waters and the
waters inertia. The resulting subtropical gyre
can be studied in terms of a geostrophic cur-rent
theory that balances center-to-edge pressure
gradient forces derived from the temperature
salinity observa-tions with the theoretical
Coriolis force calculated at the appropriate


winds, system by
system, for comparisons
distributions of the global marine surface winds
and patterns of mean ocean surface
General Light, Nutrient and Productivity
The availability of light that can sustain the
photosynthetic process white triangle is
affected by the angle of incidence of solar
radia- tion with respect to the ocean surface.
Latitude represents this relationship. Nutrient
concentration blue area is related to the
upwelling process and the amount of detrital
material de-scending into mesopelagic zones of
adjacent biological provinces. Productivity
black lines shows a continuous process in the
tropics two peaks, related to spring and fall
winds, associated with temperate climates and a
single growth inter-val during high laitude
A relative view of productivity values for
tropical, temperate and polar regions . The
generation of biomass is taken as a measure of
productivity. One more time Why is there a
single peak in the North polar curve? What is
meant by the word biomass? It is the weight of
one animal or plant species species biomass or
can be used for all the species in a community
community biomass expressed per unit area or
per unit volume. The biomass in an area at a
given moment is referred to as the standing crop.
This schematic of a northern hemi-sphere ocean,
implicitly bounded to the north, east and west by
land, represents and simplifies the North Pacific
Ocean. The two gyres, Subtropical and Sub-polar,
are wind-driven surface currents in response to
the spin torque applied by the wind systems.
These winds also force transports with depth,
movements we describe with Ekman theory that are
part of vertical convection cells. The
subtropical gyre and its clockwise flow imply
that the winds also produce the inward
transport we call convergence.
Downwelling occurs within the subtropical gyre
due to increased density brought about by
evaporation at the surface. In the subpolar
gyre, the counter-clockwise motion of the surface
waters and the winds are a tell-tale for
divergence, that is, upwelling in the central
portion of this gyre and outward flow. These
aspects of flow and their connections can be
thought of as part of convection cells whose flow
patterns impact nutrient and biological
The oceanic region we will be working with is
that above and to the right of the darker blue
When extreme climatic conditions create a
localized pool of surface waters, that is, a new
water mass denser than adjacent water masses,
downwelling will occur because of gravity. The
OCNG 251 concept that represents the sinking
process is called sinking along isopycnal
constant density surfaces during which the new
water mass moves with a vertical component of
flow along constant density surfaces until its
density matches the density of deeper, adjacent
waters. Then this new water mass begins to move
horizontally, partially driven by sinking of
additional new waters back in the source region
and partially by its own momentum. The case
shown above is a schematic of the generation of
Antarctic Inter-mediate Waters along a continuous
front around Antarctica that is the juncture at
the surface of the northern edge of the cold,
fresh Antarctic Circumpolar Current and the warm,
salty Central Waters of subtropical gyres. The
T,S vertical distribution at A reflects the
horizontal surface distribution in the source
region A to B. Now apply such a sinking
concept to waters created by evaporation
occurring at the surface within the North Pacific
subtropical gyre centered at about 30 degrees
north latitude. Spreading out in all directions
from the center of the gyre, we can consider that
there is horizontal motion towards the gyre
periphery within the mesopelagic zone. As this
occurs, the waters pick up nutrients developed by
bacterial decomposition of detrital material
raining down from above. The waters are
recharged with nutrients at depth in this
manner and exported, at depth, from the
subtropical gyre.
Control of Primary Production
Primary production is quite low in much of the
worlds ocean and extremely high in a few regions
where a positive climatic influence becomes
paramount. In general, there are four
controlling factors light, horizontal and
vertical mixing, nutrient availability and
herbivore grazing. The balances that are struck
between these influences determine the patterns
evident in planktonic geographical
distributions. Photosynthetic chemistry requires
energy from solar radiation. The light intensity
arriving at the sea surface varies with latitude,
season, and time of day. Throughout the
epipelagic water column, both the quantity and
quality of the light depends on the concentration
of suspended material and dissolved organic
matter. With penetration, the light is
selec--tively absorbed as a function of
wavelength by water molecules, suspended
particles and dissolved organic material. Light
penetration is deepest in the clear water near
the center of subtropical gyres nearly 90 is
absorbed by a depth of 70 meters with the
greatest absorption being in the first few meters
at the infrared and red end of the solar
radiation spectrum. The photosynthetic process is
effectively out of business by a depth of 150
meters. Blue-green and blue portions are the
shortest wavelengths of the solar spectrum and
penetrate deepest. However, by 1000 meters, the
deep of the mesopelagic zone, there is virtually
no light left to be absorbed. A variety of
light-absorbing pigments allows different
phytoplankters to carry out carbon production at
different wavelengths. Overall, light of
wavelengths from 400 to 720 nanometers (nm)
provide the energy for plant production. Each
pigment is tuned to a narrow wavelength band and
this is responsible for a plants characteristic
color. Cholorophyll a is an important pigment
whose absorption band peaks 670 to 695 nm.
penetration of solar radiation into the ocean

Water movement within the schematic diagram of
the North Pacific. Be sure to go over the
physical attributes at the equators ?
For comparison an actual N-S section of the
nutrient inorganic phosphate - phosporus.
Why is this nutrient lowest in concentration
under the center of the subtropical gyre ?
Secchi Disk Observations
These observations assess water clarity in terms
of the depth at which the observer decides the
disc can no longer be seen. Just east of the
International Date Line, at about ten degrees
north latitude, the water is so clear that the
depth is greater than sixty meters. There is
very little in the way of suspended material in
the water column at this location.
This is the world-wide distribution of the
euphausiid group Stylocheiron Maximum described
by Brinton in 1975. The vertical zonation
discovered for these vertically migrating
zooplankters ranges from the surface to 700
meters over the epipelagic and mesopelagic. In
general, any groups horizontal distribution is
dependent on a circula-tion system, either an
oceanic gyre or current-countercurrents. These
physical features maintain both water masses and
their characteristics. For most euphausiid
species, there is a conspicuous correlation
between their presence and surface temperature.
However, the case above shows that this
particular group is exceptional, spanning much of
tropical and temperate climates while defining
their absence in the polar, very cold, and in
regions of extreme oceanic warmth, as in the
Arabian Sea.
Using pelagic animals as indicators of water
masses and motion is a concept as old (or older)
as the Challenger Expedition era. We can use the
converse to seek reasons for the distribution of
certain plankton relative to others and isolation
of species from one another. It has been
confirmed that there is good agreement between
many pelagic species limits and water mass
boundaries. Bob Bieri 1959 found this to be
most pronounced in the Eastern Pacific where
currents are relatively sluggish, allow more
distinct and permanent temperature and salinity
discontinuities, and do not displace the
populations very far before they are eliminated
by adverse environmental factors. As well,
observations indicate that the abundance of
species within their distribution is dependent on
the availability of food as well as a water mass
temperature and salinity.
Can you give an explanation for the absence of
this species in the Pacific equatorial region ?
Subtropical gyres are the most widespread of
biological provinces. They have more common
species than other provinces. Euphausia brevis
is a globally distributed species with five
subpopulations and, in the Atlantic and Pacific
Oceans, they are clearly separated by the current
and water mass char-acteristics of the equatorial
zone. In the western North Pacific, this species
exhibits distribution extensions to the north and
south brought about by the development of western
boundary currents.
One may not expect exactly the same species
across the equator. Euphausia hemigibba is
found in four of the subtropical gyres, however a
fifth gyre, the South Pacific, has a sibling
species, E. gibba. There are other subtropical
species that are not as tightly bound to separate
gyres and transgress the equatorial barrier.
The blue shading marks the temperature at 200 m
and partially outlines the subpolar gyre. The
southward extension is a current that you should
What is the name of the current associated with
the red shading ?
This is a representation of the upper layer
horizontal circulation of the Pacific Ocean drawn
from ocean observations all made within a decade.
Flow between adjacent lines is relatively
stronger when the lines are closer together the
shaded regions color between two adjacent lines.
The blue shade shows the counterclockwise flow
around the subpolar gyre the yellow presents an
incomplete clockwise flow path around the
subtropical gyre, left incomplete because the
western portion of the observations were too many
years later to demonstrate continuity in the
pattern. The light green shade is a northern
portion of the clockwise Antarctic Circumpolar
Current that shows a northward extension along
the western side of South America Do you
remember this currents name ? . The dark green
shade is a faster, more organized portion of the
Extensions of colder waters can bring certain
species equatorward. For example, E. mucronata
in the South Pacific along the west coast of
South America. As well, E. pacifica, which is
slightly more tolerant of warm waters than T,
longipes, makes a southerly penetration along the
the California and Mexico coast. At the furthest
extension, individuals of these species are not
likely to be robust and capable of reproduction.
Note the northward extension of this
equatorial species on the western side
Krill on whale dinner plates at most good
Antarctic restaurants. Some of the biggest
Mysticetes, baleen whales can feed on some of
the smallest zooplankton. Much more efficient
that way.
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Antarctic krill gathered from a Fin whales
Conceptual Model Summary
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the Geostrophic model applied to a northern
hemisphere subtropical gyre
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