Transcript CHAOS Verhulst’ population model

Ocean Circulation and Marine Life
relationships between the mean
biological, nutrient and physical patterns
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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.
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Do atmospheric and oceanic physical processes influence
an ocean’s 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 examination 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 dominance 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,
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mean relationships prevail.
the Geostrophic model applied to a northern hemisphere subtropical gyre
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use the Ekman model results to understand the convergence and divergence processes
subpolar gyre
divergence
subtropical gyre
convergence
equatorial region
divergence
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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-1830’s, Muller’s
net tows and examinations 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, Buchanan’s pioneer observations on the Challenger led to Dittmar’s
work
that identified the major dissolved ionic constituents, Forchhammer and
Marcet’s constancy principles and Knudsen’s definition and determination of salinity.
The determination of the composition of plankton organisms by Vinogradov in the
1930’s 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 earth’s
rotation. Scandanavians Helland-Hansen and Bjerknes’ theoretical work on currents
in variable density ocean waters (geostrophy) that was made possible by Nansen’s
advances in temperature and salinity accuracies and Ekman’s theory of wind-driven
currents. Their works contain the concepts and results for OCNG 251’s steady-state,
force-balanced explanation of oceanic flow.
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Boyle
Buchanan
Forchhamer
Dittmar
Marcet’s wife, Jane
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Food webs are illustrations that
portray energy transfer pathways
between populations. Food pyramids do the same. In the pelagic
open ocean, waters can be
nutrient-limited with low productivity 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 phytoplankton, 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 food’s primary production.
Photosynthesis fixes most energy utilized in the ocean. The process is carried out by autotrophic 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
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oxygen during which water and carbon dioxide are produced.
CORIOLIS
EKMAN
NANSEN
BJERKNES
HELLAND-HANSEN
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MULLER
BIGELOW
AGASSIZ
VINOGRADOV
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Mean Ocean Surface Currents
This schematic of the world’s 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.
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The world ocean’s average surface currents are closely
correlated with mean atmospheric wind patterns.
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 gyre’s clockwise
motion and producing transport toward its center. The converse holds for the
Alaska and Oyashio Currents.
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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 understanding of a continuous circulation
path around its periphery. Remember
that the mound is skewed, adjacent to
the western boundary, because of the
land’s rotation eastward into the
ocean’s waters and the water’s inertia.
The resulting subtropical gyre can be
studied in terms of a geostrophic current theory that balances center-toedge pressure gradient forces [derived
from the temperature salinity observations] with the theoretical Coriolis
force calculated at the appropriate
latitude.
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winds, system by system,
for comparisons
distributions of the global
marine surface winds
and
patterns of mean ocean
surface currents
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General Light, Nutrient and Productivity Distributions
The availability of light that can
sustain the photosynthetic process
[white triangle] is affected by the
angle of incidence of solar radiation 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 descending 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 interval during high laitude summers.
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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
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an area at a given moment is referred to as the standing crop.
This schematic of a northern hemisphere ocean, implicitly bounded to the
north, east and west by land, represents
and simplifies the North Pacific Ocean.
The two gyres, Subtropical and Subpolar, 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 distributions.
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The oceanic region we will be working with is that above and to the right of the darker blue lines.
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WATERS SINKING ALONG ISOPYCNAL [CONSTANT DENSITY] SURFACES
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 Intermediate 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.
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Control of Primary Production
Primary production is quite low in much of the world’s 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 plant’s characteristic color.
Cholorophyll a is an important pigment whose absorption band peaks 670 to 695 nm.
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penetration of solar radiation into the ocean
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EQUATOR
SUBTROPICAL GYRE
SUBPOLAR GYRE
North Pacific Ocean
SCHEMATIC VERTICAL SECTION
SURFACE THROUGH MESOPELAGIC ZONES
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Water movement within the schematic diagram of the North Pacific.
Be sure to go over the physical attributes [ at the equators ? ]
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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 ?
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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.
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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 group’s horizontal distribution is dependent on a circulation 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.
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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.
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Can you give an explanation for the absence of this species in the Pacific equatorial region ?
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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 characteristics 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.
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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.
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The blue shading marks the temperature at 200 m and partially outlines the subpolar gyre.
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The southward extension is a current that you should know!
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 current’s name ?] . The
dark green shade is a faster, more organized portion of the ACC.
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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.
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Note the northward extension of this
equatorial species on the western side
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Krill : on whale dinner plates at most good Antarctic restaurants. Some of
the biggest [Mysticetes, baleen whales] can feed on some of the smallest 35
[zooplankton]. Much more efficient that way.
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Antarctic krill gathered from a Fin whale’s stomach
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Conceptual Model Summary
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FINIS
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the Geostrophic model applied to a northern hemisphere subtropical gyre
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SEASONAL JOINT PROGRESSIONS OF MAJOR OCEANIC ENVIRONMENTS
and
ASSOCIATED RELATIVE PHYTO- AND ZOOPLANKTON ABUNDANCES
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