BIOL 4120: Principles of Ecology
Lecture 4: Aquatic
Environment
Dafeng Hui
Room: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Topics for this class:
4.1 Global water cycling between Earth and
atmosphere
4.2 Physical properties of water (H2O)
4.3 Light in aquatic environments
4.4 Temperature in aquatic environments
4.5 Oxygen (O2) in aquatic environments
4.6 Acidity in aquatic environments
4.7 Water movement in fresh water and marine
environments
4.8 Tides in marines coastal environment
4.9 Transition zone (estuaries)
4.1 Global Hydrologic (water) cycle
between Earth and atmosphere Cycle
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Water is essential for life
(75-95% weight of living
cell)
Over 75% of the Earth’s
surface is covered by
water
• Oceans contain 97%.
• Polar ice caps and
glaciers contain 2%.
• Freshwater in lakes,
streams, and ground
water make up less
than 1%.
(Saltwater and fresh water)
Water Cycles between Earth and
the Atmosphere
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The water (or hydrologic) cycle is
the process by which water travels in
a sequence from the air to Earth and
returns to the atmosphere
Solar radiation is the driving force
behind the water cycle because it
provides energy for the evaporation
of water
The Hydrologic Cycle
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Distribution of water is not static (processes)
Precipitation
(PPT)
Interception
Infiltration
Groundwater
recharge
Runoff
Evaporation
(E)
Transpiration
(T)
Global water budget
Land
Pools (10^3 km3):
Glaciers: 29,000
Groundwater:4,000
Lake: 229
Soil: 67
Fluxes (km3/yr):
PPT: 111,000
ET: 71,000
River flow:40,000
Ocean
Pools:
Ocean:1.37*10^6
Fluxes:
PPT:385,000
ET: 425,000
Water Reservoirs and fluxes
Renewal Time
300-11,000 yrs
12,000 yrs
60-300 yrs
330 days
7-11 days
7 days
(Horne & Goldman, 1994)
4.2 Water Has Important Physical
Properties


In water (H2O), the atoms are
asymmetrically bound to one another
The hydrogen atoms share an
electron with the oxygen atom
through a covalent bond
• Because electrons are unequally shared
and spend more time around oxygen,
water is considered a polar molecule

Because of their polarity, water
molecules bond with one another due
to hydrogen bonding
4.2 Physical properties of water (H2O)
Basic Structure
1. Covalent bonding of 2H + O atoms
2. Polar-covalent bond
3. Inter-molecule attraction
4. H-bonds among water moleculars
Physical and chemical properties

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High specific heat capacity:
Specific Heat: 1.0 (also called Heat Capacity)
•
calories required to raise 1 g H2O 1oC high
•
(e.g. from 10 to 11oC)
Latent heat: energy released or absorbed in the
transformation of water from one state to another.
1 calorie to raise 1oC; 536 calories to change 100oC
water to vapor; 86 calories ice to 1oC water
Peculiar density-temperature relationship
density increases as T decreases (when T> 4oC), then
decrease to 0oC, freezing (ice)
Cohesion
Due to the hydrogen bonding, water molecules tend to
stick firmly to each other, resisting external forces that
would break the bonds (drop of water, transpiration).
Properties (cont.)

Surface tension-strong
attraction within the
water body and
weaker attraction in
the surface caused
that molecules at the
surface are drawn
downward.
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High viscosity:
Viscosity: property of a material that measures
the force necessary to separate the molecules
and allow passage of an object through liquid.
Frictional resistance is 100 times greater than air.
Water is 860 times denser than air.
• Organisms in water have similar density to
water, the neutral buoyancy helps against the
force of gravity, thus require less investment
in structure material such as skeletons
• Organisms in deep water need to adapt to the
high pressure (20 to 1000 atm).
4.3 Light in aquatic environments
Reflection
Interception by
particles: absorb
or scatter by
Absorption by
water:
shortwave+
Red, yellow,
green and violet
Only blue
wavelength to
penetrate deeper
water
Change in light quantity and quality have important implications to life in
aquatic environments.
4.4 Temperature in aquatic
environments
Temperature change is
the result of the
exponential decline in
solar radiation with water
depth.
T profile in the ocean
The vertical profile of T
varies seasonally.
Fall turnover: cool dense
surface water sinks,
displace the warmer
water below, create
uniform T.

Flowing body of water (stream or river) has a quite different T
profile. Must take into account all the factors
• Water and its depth
• Temperature
Altitude
• Geography
Biotic environment
4.5 Oxygen concentration in
aquatic environment
High O2 in the
surface due to
diffusion
O2 is
dissolved in
water
O2
concentration
in water is
determined
by solubility
and diffusion.
Vertical profile of oxygen in the
Atlantic Ocean
O2
declines
to
minimum
O2 zone
(5001000 m).
The
increase
caused by
influx of
O2 rich
cold
water
sank in
the polar
water.
4.6 Water functions as a solvent
Solution: a homogeneous liquid
with 2 or more substances mixed.
Solvent: dissolving agent
Solute: substance that is dissolved
Aqueous solution: water as
solvent
Ions: Compounds of electrically
charged atoms
Cations: positive
Anions: negative
Practical salinity units (PSU,
o/oo): grams of chlorine per
kilogram of water.
Ocean: 35 unit, Fresh water:
0.065-0.30 unit)
4.7 Acidity has a widespread influence
on aquatic environments
Acidity: the abundance of hydrogen ions (H+) in
solution.
Alkalinity: abundance of hydroxyl ions (OH-) in
solution
Acidity in water is related to carbon dioxide (CO2).
Forms of Carbon
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Carbon-bicarbonate equilibrium
• Carbon dioxide:
CO2
• Carbonic acid:
H2CO3
• Bicarbonate:
HCO3• Carbonate:
CO32-
CO2 + H2O H2CO3 HCO3- + H+ CO32- + 2H+

Measurement: pH =-log([H+])
(value between 1-14)
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Pure water: 7 Acidic: <7 Alkaline: >7
Ocean water tends to be slightly
alkaline with a pH range of 7.5-8.4
Proportions of the forms
of CO2 in Relation to pH
Free
Bicarbonate
Carbonate
pH
CO2
HCO3–
4
0.996
0.004
1.26 x 10-9
5
0.962
0.038
1.20 x 10-7
6
0.725
0.275
0.91 x 10-5
7
0.208
0.792
2.60 x 10-4
8
0.025
0.972
3.20 x 10-3
9
10
0.003
0.000
0.966
0.757
0.031
0.243
CO3=
4.8 Water movements shape fresh
water and marine environments
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Water movement
in the fresh water
• Stream flow rate
(velocity of a
stream)
• Determined by
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Shape and steepness
of stream
Intensity of rainfall
If Flow Rate>50
cm/s, remove all >5
mm in diameter
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Water movement in the ocean (lake)
• Wave
• Generated by wind
Upwelling:
4.9 Tides dominate the marine
coastal environment
3.27
3.49
Centrifugal force:
3.38
Tides: due to
the
gravitational
pulls of the
Sun and the
Moon.
Tides are not entirely regular, nor are they the
same all over the Earth.
Atlantic: semi-daily; Gulf of Mexico: one daily
tide; Pacific: mixed tides.
Intertidal Zonation
Dramatic shifts in env.
Conditions:
Inundation and exposure
Intertidal zone: area lying between the water line of high and low tide.
4.10 Transition zone (Estuary)
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Estuary: place where fresh water joins
and mixes with the saltwater.
Features:
• Considerable fluctuation in temperature, both
daily and seasonally.
• Large variation in salinity (vertical and
horizontal)
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Tidal overmixing: unstable salinity, saltwater on
surface tends to sink as lighter fresh water in bottom
rises, and mixing takes place from the surface to the
bottom.
Limited number of species and high production.
Vertical and horizontal stratification
of salinity from river mouth to the
estuary
The END