Transcript 1 Sounding the Deep

2 The Oceanic Environment
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
©Jeffrey S. Levinton 2001
The Ocean
Geography and Bottom Features
©Jeffrey S. Levinton 2001
©Jeffrey S. Levinton 2001
The Ocean and Marginal Seas
• The worlds oceans: oceans and marginal
seas
• Oceans cover 71% of earth’s surface
• Southern hemisphere 80%, Northern
hemisphere 61%
• 84% deeper than 2000m
• Greatest depth ~ 11,000 m in Marianas
Trench
Marginal Seas
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1.
2.
3.
4.
Examples: Gulf of Mexico, Mediterranean Sea
Affected strongly by
regional climate,
precipitation-evaporation balance,
river input of fresh water and dissolved solids,
limited exchange with the open ocean (e.g., sill
partially cutting Mediterranean from
Atlantic)
Marginal Seas 2
• Often have recent history of major
change
• Mediterranean: completely dry a few
million years ago
• Baltic Sea: less than 11,000 years old
Marginal Seas 3
•
•
•
•
Marginal Seas (Gulf of Mexico, Baltic Sea):
influenced more by drainage and local climate:
1. River input (Baltic Sea)
2. Precipitation - evaporation balance
(Mediterranean)
• 3. Restricted circulation (Black Sea,
Mediterranean)
• 4. Geological history (Baltic)
Ocean as a Receptacle
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Particulate mineral matter
Dissolved salts
Particulate organic matter (POM)
Dissolved organic matter (DOM)
Atmospheric precipitation
Volcanic sources
Water
Water Relationships in the
Ocean
Topographic Features
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Continental shelf (1° slope)
Continental slope (2.9° slope)
Continental Rise
Abyssal Plain
Submarine Canyons
Oceanic Ridge Systems
Slope
Abyssal plain
Mid-ocean ridge
Abyssal plain
Marginal sea
Volcanic island
Depth (km)
Topographic Features 2
Eurasian
Eurasian
American
Caribbean
Philippine
Arabian
Pacific Cocos
Nazca
American African
Antarctic
Earth’s surface is divided into plates: borders are ridge systems, faults
The Oceanic Crust: Crust is formed at ridges,
moved laterally, and destroyed by subduction,
which forms trenches
Continental
crust
Inactive fault
Continent
Trench
Oceanic
crust
Intermediate, deepfocus earthquakes
Ridge
Fault
Continental
Crust
Mantle
The Ocean
Seawater Properties
©Jeffrey S. Levinton 2001
Water molecule
• Asymmetry of charge distribution on
water molecule - increases ability to
form bonds with ions - makes water
excellent solvent
Water properties
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•
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High heat capacity (0.9)
High heat of evaporation (590 cal/g)
High dissolving power
High transparency (absorbs
infrared, ultraviolet)
Latitudinal Gradient of Sea
Water Temperature
Vertical Temperature Gradient:
Open Tropical Ocean
Vertical Temperature Gradient:
Shallow Temperate Ocean
• In shallow shelf regions with strong seasonal
temperature change - get seasonal thermocline.
Example: coastal NE United States thermocline found in summer continental shelf,
with surface warmer wind-mixed layer, deeper
cooler water
• Winter: vertical temperature distribution much
more uniform and cold, from surface to
bottom. No strong thermocline
Salinity
• Definition: g of dissolved salts per 100g of
seawater; units are o/oo or ppt
• Controlled by:
+ evaporation, sea-ice formation
- precipitation, river runoff
Salinity in open ocean is 32-38 o/oo
Important elements in seawater
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Chlorine (19,000 mg/l)
Sodium (10,500
Magnesium (1,300)
Sulfur (900)
Calcium (400)
Potassium (380)
Bromine (65)
Carbon (28 - variable)
Principle of Constant Element Ratios
• Ratios between many major
elements are constant all over the
ocean, even though salinity varies
Principle of Constant Element Ratios 2
• Why? Because residence time of
elements is much greater than time
to mix them evenly throughout ocean
by water currents (ca. 1000 y)
Principle of Constant Element Ratios 3
• Residence time of Na, Cl, Sr is on the
order of millions of years
Principle of Constant Element Ratios 4
• Principle does not apply to elements
that cycle rapidly, especially under
influence of biological processes (e.g.,
sulfur, phosphorous)
Measurement of Salinity
• Chlorinity: g of chlorine per 1000 ml
of seawater
• Salinity = 1.81 x chlorinity
• Measured by chemical titration
Measurement of Salinity 2
• Conductivity - increases with
increasing salinity, has to be
corrected for temperature
Measurement of Salinity 3
• Optical refraction - more salt, more
refraction of light, uses
refractometer
Temperature
• Oceanic range (1.9 - 40 °C) less than
terrestrial range (-68.5-58 °C)
• Deep ocean is cold (2 - 4) degrees
Heat Changes in the Ocean
Additions
Losses
Latitudinal gradient Back radiation of
of solar heating
surface
Geothermal heating Convection of heat
to atmosphere
Internal Friction
Evaporation
Water Vapor
Condensation
Seawater density (mass/volume)
• Influenced by salt, no maximum density at 4 °C
(unlike freshwater)
• Density measure of seawater at temperature t
t= (density - 1) x 1000
t increases with increasing salinity
t increases with decreasing temperature
Special significance: vertical density
gradients
Latitudinal salinity gradient
Excess of evaporation over ppt in mid-latitudes
Excess of ppt over evaporation at equator
The Ocean
Circulation in the Ocean
©Jeffrey S. Levinton 2001
Coriolis Effect - Earth’s Rotation
Latitude
Equator
Eastward Velocity
(km/h)
1670
30° N. latitude
1440
60° N. latitude
830
Coriolis Effect - Movement of fluids, in relation to
earth beneath, results in deflections
Coriolis Effect and Deflection
• Surface winds move over water
• Coriolis effect causes movement of water
at an angle to the wind (to right in
northern hemisphere)
• Water movement drags water beneath,
and to right of water above
• Result: Shifting of water movement Ekman Spiral (actually friction binds
water together and all water moves at a
right angle to wind (right of wind in n.
hemisphere)
Coriolis Effect and Deflection 2
• Surface winds move over water
• Coriolis effect causes movement of water
at an angle to the wind (to right in
northern hemisphere)
• Water movement drags water beneath,
and to right of water above
• Result: Shifting of water movement Ekman Spiral (actually friction binds
water together and all water moves at a
right angle to wind (right of wind in n.
hemisphere)
Coriolis Effect and Deflection 3
• Surface winds move over water
• Coriolis effect causes movement of water
at an angle to the wind (to right in
northern hemisphere)
• Water movement drags water beneath,
and to right of water above
• Result: Shifting of water movement Ekman Spiral (actually friction binds
water together and all water moves at a
right angle to wind (right of wind in n.
hemisphere)
Coriolis Effect and Deflection 4
• Surface winds move over water
• Coriolis effect causes movement of water
at an angle to the wind (to right in
northern hemisphere)
• Water movement drags water beneath,
and to right of water above
• Result: Shifting of water movement Ekman Spiral (actually friction binds
water together and all water moves at a
right angle to wind (right of wind in n.
hemisphere)
Coastal Winds + Coriolis Effect =
Upwelling
Southern hemisphere: water moves to the left of wind
Oceanic Circulation
• Wind-driven surface circulation
• Density-driven thermohaline
circulation
Wind-driven Circulation
• Driven by heating of air near equator,
which rises, moves to higher latitude,
falls, creating circulation cells that are
affected by Earth’s rotation. Air moves
surface water
• Prevailing westerlies (40°N & S latitude)
• Trade winds (toward the west)
Wind-driven Circulation 2
• Combination of wind systems and shapes
of ocean basins create cyclonic flow
known as gyres
• Rotation of earth tends to concentrate
boundary currents on west sides of ocean
- creates concentrated currents such as
Gulf Stream
Wind-driven Circulation 3
Wind systems
Westerlies
NE Tradewinds
Doldrums
SE Tradewinds
Westerlies
Surface currents
Subpolar
gyre
Subtropical
gyre
Subtropical
gyre
West wind drift
Thermohaline Circulation
• Water in the ocean can be divided into
water masses, identified by distinct
temperature, salinity, and other physicochemical characteristics
Thermohaline Circulation 2
• Thermohaline circulation is movement of
ocean water controlled mainly by density
characteristics
• Controlled by (1) Location of formation
of water, (2) density, (3) Coriolis effect to
a degree
Thermohaline Circulation 3
• Water masses formed in high latitude
surface waters - water is cold, often high
salinity because of ice formation
• Waters sink, move at depth towards
lower latitude
• Water masses each have a characteristic
depth, because of their density, which is
largely a function of their high latitude
surface origin
Thermohaline Circulation 4
AABW=Antarctic Bottom Water; AAIW = Antarctic Intermediate
Water; NADW = North Atlantic Deep Water
Circulation Recap
• Coriolis effect - rotation of Earth, prop.
to sine of latitude, Right deflection in N.
hemisphere, Left deflection in S.
hemisphere - upwelling, deflection of
currents
Circulation Recap 2
• Surface circulation - driven by planetary
winds, which are controlled by heating,
convection, Coriolis effect - gyres, eastern
boundary currents
Circulation Recap 3
• Thermohaline Circulation - driven by
density, sinking, surface water brought to
deep sea - water masses determined by
density
El Niño - Global Phenomenon
• Periodic - every few years
• Warm water moves easterward across
Pacific Ocean
• Eastern tropical and subtropical Pacific
becomes warm, thermocline deepens
• Causes mortality of clams, fishes, from
heat shock
• Strongly affects weather in eastern
Pacific, storms increase; droughts in
western Pacific
The Ocean
Coastal Processes
©Jeffrey S. Levinton 2001
Waves
• Dimensions
Wave Length L
Amplitude H
Velocity V
Waves
• When depth < L/2: waves “feel bottom”
• When H/L > 1/7: wave is unstable and
collapses (breaks)
Beaches
• Many beaches exposed to direct wave and
erosive action
• Some sandy beaches are more protected,
especially some that are very broad and
dissipate wave energy at the low tide
mark and only for a few meters from
waterline
Beaches 2
• Profile more gentle in summer; fall and
winter storms cause erosion and a steeper
profile
Beaches 3
• Longshore currents, riptides are common
features, causing erosion and transport of
sand
Wave Refraction
Tides 1
• Gravitational attraction between ocean
and moon + sun (moon has 6x the effect
of the sun)
Tides 2
• Water on earth facing moon has net
gravitational attraction, water facing
away from earth has less attraction centrifugal force causes high tides here
Tides 3
• Water on earth surface at right angles to
moon is pulled either away or toward
moon - results in low tides here
Tides 4
• Spring tides - greatest tidal range, highest
high and lowest low - happens when
earth, moon and sun are in line
• Neap tides - least tidal range - happens
when earth, moon, and sun form
approximate right angle
Tides 5
Spring
Tide
m
E
m
Sun
m
Neap
Tide
Sun
E
m
E = Earth
m = Moon
Tides 6
• Tides differ in different areas; function of
basin shape, basin size, latitude
• Results in strong regional differences in
the amplitude of tides, the evenness of the
expected two high and two low tides per
lunar day (e.g., in Pacific NW, there is
only one strong low tide per day, while
the other “low” tide is nearly as high as
the high tides
Tides 7
Connecticut - even tides
Washington State - uneven
Estuaries 1
• Body of water where freshwater source
from land mixes with seawater
• Often results in strong salinity gradient
from river to ocean
• Salinity may be higher at bottom and
lower at top, owing to source of river
water that comes to lay on top of sea
water below, or mixes with the sea water
to some degree
Estuaries 2
Chesapeake Bay
with summer surface
salinity. Dark blue
areas: tributaries
have salinity
< 10 o/oo
Estuaries
types
3
Fresh water layer
Sea water
Highly stratified estuary
10
20
Moderately stratified estuary (wind, tide mixing)
10
20
Vertically homogeneous estuary
30
Circulation in a coastal fjord
• Reduced exchange of fjord’s bottom waters,
combined with density stratification and
respiration results in low oxygen in bottom
waters
Ocean
Restricted circulation
Sill
The End