Water as an Environment
Oxygen Profiles
Light
Part 2
Oxygen in Aquatic Systems
Oxygen

is needed by aquatic organisms
< 3 mg/L is lethal to fish)
Oxygen
solubility in water decreases with
increasing water temperature (fig 2.4 )

At room temperature, water contains about 8.5 mg/L DO
Sources
of oxygen: atmosphere, plants and algae
Removal of oxygen: respiration by plants, animals
and bacteria, decomposition
Vertical Oxygen Profiles
Typical spring
temperature and oxygen profile
Typical mid-summer
temperature and oxygen profile
Vertical Oxygen Profiles
Typical mid-summer
temperature and oxygen profile
Fall overturn in progress
Stratification and vertical distribution
of phytoplankton (algae)
Compensation Depth
(1% surface PAR)
Implications of oxygen profiles
in Aquatic Systems
Vertical
distribution of organisms
Benthic (bottom dwelling) animals must
be able to tolerate low DO or be able to
move
Aquatic invertebrates can often tolerate
lower DO than fish can.

Nutrient
and contaminant
regeneration from sediments
Intermittent Stratification and
Hypoxia in western Lake Erie
Hypoxic episode in western Lake Erie
Mayfly
Importance of Light in Aquatic
Systems
Heating
Photosynthesis
Predator-Prey
Interactions
How light is measured
Light meter
Secchi disk
Light
potentially
damaging
heat
(PAR 400-700)
PAR = Photosynthetically available radiation
PAR: Photosynthetically-available radiation
[radiation usable in photosynthesis]
 Amount of light hitting water’s surface
depends on angle of sun & conditions:
• latitude
• season
• time of day
• cloud cover
 Light that hits surface is:
• reflected
• scattered
attenuation
• absorbed
Reflection
• angle (season, time of day, latitude)
• meteorological conditions
• wave action
• ice and snow
Light attenuation of ice and snow
Energy Balance for a Lake
Absorption
Direct solar radiation (QS)
Indirect solar radiation (QH)
Reflection (QR)
(QW)
Upward scattering (QU)
(QA)
(QW ) = long-wave radiation
radiated back into the
atmosphere
(QA) = long-wave radiation
returning from the
atmosphere
Net Radiation Surplus = QS + QH + QA – QR – QU - QW
At night: Net Radiation Surplus = QA - QW
Light (umol/m2/s)
0
500
1,000
1,500
0
depth (m)
20
40
60
80
100
kd = 0.05 ocean, very clear
kd = 0.1
kd = 0.5
120
140
most lakes
kd = 10 very turbid lake
2,000
Light attenuation (or extinction)
decreases as a fixed proportion of light remaining at each
depth
I = Irradiance
IZ = I0 e-kz
attenuation
coefficient
(k)
=
ln (light at surface) - ln (light at depth z)
depth z
large k indicates that light is absorbed rapidly
Each
wavelength of
light has its own
attenuation
coefficient (k)
Since we are
concerned with
photosynthesis,
we generally
talk about KPAR
Absorption of light of
various wavebands in a
typical lake
Allen Lake (MI) – Light Intensity vs. Depth
Light Intensity (μE m-2 sec-1)
Above water surface
0
500
1500
2500
Depth (m)
21
42
Secchi depth (3.7 m)
63
84
105
Compensation depth
Sep 2008
12
Light attenuation exercises
Depth (m)
0
3
5
7
10
13
15
17
20
23
25
Light(uE m-2 s-1)
630
250
175
156
114
93
78
54
27
15.6
10.5
Given the light profile at
left, what is kPAR ?
What is the depth of
10% light?
What is the
compensation depth
(1% light)?
Light attenuation (or extinction)
decreases as a fixed proportion of light remaining at each
depth
I = Irradiance
IZ = I0 e-kz
attenuation
coefficient
(k)
=
ln (light at surface) - ln (light at depth z)
depth z
large k indicates that light is absorbed rapidly
Light Attenuation
Absorption

water itself (red light)
 colored DOC “gelbstoff” (uv, blues)
 Particles (silt, clay, algae)
Light Attenuation
Scattering

Particles (silt, clay, algae, rock flour)
 Size of particles is important
Fine particles will scatter more light than equivalent weight of
larger particles

Water and
dissolved
substances tend
to absorb light of
specific colors.
Particles tend to
absorb or scatter
light more evenly
across the
spectrum
Effects of
dissolved and
suspended
matter on
absorption of
light at various
wavelengths
Increasing DOM
[gelbstoff]
What if you don’t have a light meter handy?
• For a given lake, there is usually a good
relationship between kPAR and Secchi depth
Rule of thumb: k = 1.7/ZSD
In non-humic lakes