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 mid-summer temperature and oxygen profile Typical spring temperature and oxygen profile

Vertical Oxygen Profiles Fall overturn in progress Typical mid-summer temperature and oxygen profile

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 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. 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 (PAR ) 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 (Q S ) Absorption Indirect solar radiation (Q H ) Reflection (Q R ) Upward scattering (Q U ) (Q W ) (Q W ) = long-wave radiation radiated back into the atmosphere (Q A ) (Q A ) = long-wave radiation returning from the atmosphere Net Radiation Surplus = Q S + Q H + Q A – Q R – Q U - Q W At night: Net Radiation Surplus = Q A - Q W Energy Balance for a Lake

,0001,5002,000 k d = 0.05 ocean, very clear Light (umol/m 2 /s) depth (m) k d = 10 very turbid lake k d = 0.1 k d = 0.5 most 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 I Z = I 0 e -kz

 Each wavelength of light has its own attenuation coefficient (k)  Since we are concerned with photosynthesis, we generally talk about K PAR Absorption of light of various wavebands in a typical lake

Depth (m) Light Intensity (μE m -2 sec -1 ) Allen Lake (MI) – Light Intensity vs. Depth Secchi depth (3.7 m)Compensation depth Sep Above water surface

Light attenuation exercises Depth (m) Light(uE m -2 s -1 ) Given the light profile at left, what is k PAR ? 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 I Z = I 0 e -kz

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

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 k PAR and Secchi depth Rule of thumb: k = 1.7/Z SD In non-humic lakes