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