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Water as an EnvironmentWater as an Environment
Oxygen ProfilesOxygen ProfilesLightLight
Part 2Part 2
Oxygen in Aquatic SystemsOxygen 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 algaeRemoval of oxygen: respiration by plants, animals and bacteria, decomposition
Vertical Oxygen ProfilesVertical Oxygen Profiles
Typical mid-summer temperature and oxygen profile
Typical spring temperature and oxygen profile
Vertical Oxygen ProfilesVertical Oxygen Profiles
Fall overturn in progressTypical mid-summertemperature and oxygen profile
Stratification and vertical distribution Stratification and vertical distribution of phytoplankton (algae)of phytoplankton (algae)
Compensation Depth (1% surface PAR)
Implications of oxygen profiles Implications of oxygen profiles in Aquatic Systemsin Aquatic Systems
Vertical distribution of organismsVertical distribution of organismsBenthic (bottom dwelling) animals Benthic (bottom dwelling) animals must be able to tolerate low DO or be must be able to tolerate low DO or be able to moveable to moveAquatic invertebrates can often Aquatic invertebrates can often tolerate lower DO than fish can.tolerate lower DO than fish can.
Nutrient and contaminant Nutrient and contaminant regeneration from sediments regeneration from sediments
Intermittent Stratification and Intermittent Stratification and Hypoxia in western Lake ErieHypoxia in western Lake Erie
Hypoxic episode in western Lake Erie
Mayfly
Importance of Light in Aquatic Importance of Light in Aquatic SystemsSystems
HeatingPhotosynthesisPredator-Prey Interactions
How light is measured
Light meter
Secchi disk
Light
potentially damaging
(PAR 400-700)
heat
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• absorbed
attenuation
Reflection
• angle (season, time of day, latitude)• meteorological conditions• wave action• ice and snow
Light attenuation of ice and snow
Direct solar radiation (QS)
Absorption
Indirect solar radiation (QH)Reflection (QR)
Upward scattering (QU)
(QW)
(QW) = long-wave radiation radiated back into the atmosphere
(QA)
(QA) = long-wave radiation returning from the atmosphere
Net Radiation Surplus = QS + QH + QA – QR – QU - QW
At night: Net Radiation Surplus = QA - QW
Energy Balance for a Lake
0
20
40
60
80
100
120
140
0 500 1,000 1,500 2,000
kd = 0.05 ocean, very clear
Light (umol/m2/s)d
epth
(m
)
kd = 10 very turbid lake
kd = 0.1
kd = 0.5most lakes
Light attenuation (or extinction)
decreases as a fixed proportion of light remaining at each depth
attenuation coefficient (k)
ln (light at surface) - ln (light at depth z)
depth z=
large k indicates that light is absorbed rapidly
I = Irradiance
IZ = I0 e-kz
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
Dep
th (
m)
Light Intensity (μE m-2 sec-1)
1
2
3
4
5
Allen Lake (MI) – Light Intensity vs. Depth
Secchi depth (3.7 m)
Compensation depth
Sep 2008
500 1500 2500
Above water surface
0
2
4
6
8
10
12
Light attenuation exercises
Depth (m) Light(uE m-2 s-1)0 6303 2505 1757 15610 11413 9315 7817 5420 2723 15.625 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
attenuation coefficient (k)
ln (light at surface) - ln (light at depth z)
depth z=
large k indicates that light is absorbed rapidly
I = Irradiance
IZ = I0 e-kz
Light AttenuationAbsorption water itself (red light) colored DOC “gelbstoff” (uv, blues) Particles (silt, clay, algae)
Light AttenuationScattering Particles (silt, clay, algae, rock flour) Size of particles is important
Fine particles will scatter more light than equivalent weight of larger particles
Effects of dissolved and suspended matter on absorption of light at various wavelengths
Increasing DOM[gelbstoff]
Water and dissolved substances tend to absorb light of specific colors.
Particles tend to absorb or scatter light more evenly across the spectrum
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