Light and the Aquatic Ecosystem
Why is light important? Ultimate source of energy in all carbon containing materials entering the water. Sunlight drives the entire aquatic ecosystem.
What Do Limnologists Measure? How much light falls the on water surface How deeply it penetrates How it affects aquatic organisms
Solar Constant and Nature of Light Solar Constant 1.94 cal/cm 2 /minute If 100% efficiency it would produce 325 tons of plant material/sq.km/hr.!!! ½ this energy lost as it enters the atmosphere. Photosynthetic plants capture some of the energy, but not very efficiently. 99% lost
Light Energy How to measure light waves? Nanometers (nm) 10-9 m 350nm=ultraviolet 400nm=violet 460nm=blue 520nm=green 580nm=yellow 620nm=orange 700nm=red 780nm=infrared
Light Energy The shorter the wave length of light, the greater the Energy. Violet light has nearly 2X the energy of red light. This is why grow lights are usually violet.
Light energy 2 sources of light radiation- Direct from the Sun, and diffuse reflected light from clouds and sky.
Light Energy and Plants Plants use light from barely ultraviolet to barely infrared. Plants reflect and do not use green and yellow light. That is why they look green. Plants most efficient with violet-blue and red-orange light.
Light at the lake surface Part of the energy reaching the lake surface does not enter the water. It is reflected back. How much depends on angle of incidence. Can lose up to 20%. Angle of incidence depends on time of day, and season of the year. Angle less important with cloud cover.
Light Reflecting Off Water
Bouguer s Law When a parallel beam of monochromatic light enters chemically pure water, it is absorbed exponentially. Absorption varies directly with the log of water thickness. No exceptions known. So light decreases at a constant rate with depth of water.
Beer s s Law Absorbing capacity is directly proportional to the number of absorbing entities. A beam of monochromatic light passing through a given distance of a solution is absorbed exponentially according to the concentration of solute.
Beer s s Law Not so clean in nature Light of many wavelengths enter lake at many different angles. Lake water has many different chemical concentrations through out water column.
Transmitted light Transmittance is the % of light transmitted through 1 m of water. Transmittance varies by wavelength. Color of light changes with depth. Blue and violet most prevalent the deeper you go in pure distilled water. WHY?
Light Transmittance Long light rays are absorbed rapidly. 65% of red light absorbed in 1 st meter of water. Almost all infrared (heat) rays are absorbed in the 1 st meter.
Light Transmittance Dissolved materials in the water absorb up to 80% of light. They also reflect and alter the color of light passing through the water column.
Light Transmittance Light reaching the depths of very clear lakes is often a greenyellow mixture. In heavily stained waters, orange may penetrate farthest.
Secchi Disk Used to measure light penetration. Developed by an Italian oceanographer in the 1860 s. Should be measured between 10AM and 2PM from shady side of boat. In extremely clear lakes Secchi visibility can be 40 meters.
Light Needed For Photosynthesis Minimum intensity of subsurface light needed for photosynthesis is about 1% of surface light. Region of surface to a depth where 99% of surface light has disappeared is called the euphotic zone or photic zone. Depth of this zone varies with time of day and from lake to lake. Below this zone primary productivity is considered nil.
Light Needed For Photosynthesis The photic zone can be determined by using a light meter lowered into the water between 10AM and 2 PM.
Light Needed For Photosynthesis Important rule of thumb! the photic zone is usually about 3 (2.7) times secchi disk visibility.
Secchi Disk Use Can be used to estimate chlorophyll content of the phytoplankton community.
Effect Of Ice And Snow On Light Transmittance Clear ice transmits light better than the water beneath it. Why? Particulate and dissolved matter is less in the ice than in the water.
Effect Of Ice And Snow On Light Transmittance Snow blocks light. 99% of light blocked by 18cm (7 in.) of new snow. Reflectance (albedo) from the white surface is a an important factor. Can result in winter kill of fish due to low dissolved oxygen content of the water.
Snow Covered Pond
Opaque Layers In The Water Column 50% of total illumination disappears in 1 st meter. Bougurer s Law and Beer s Law require that water is homgenous. Not always true in the real world. Layers of phytoplankton, zooplankton or mineral concentrations can appear at any depth and distort light penetration.
Color Lake color is determined by a number of factors. Materials in solution and particulate matter both living and non-living are important. Reflections of subsurface objects play a role in shallow ponds. Reddish ponds due to suspended soil particles.
Color Some bodies of water tinged red due to certain species of blue-green algae. Green water is caused by large phytoplankton populations Diatoms and certain dinoflagellates can color the water dark yellow. Or in some cases, orange or reddish orange.
Color Filtration can remove objects causing the apparent color, leaving the so-called true color of the water. The true color ranges from clear blue to dark brown or black. Bluest waters are transparent and ultraoligotrophic. (Crater Lake)
Color Water with dissolved organic material,humic substances, peat or lake sediments have colors ranging from green in some oligotrophic lakes, through yellow in eutrophic lakes to brown or tea colored water in acidic bogs.
Color The Forel Ule scale is a standard color scale used to determine water color.
Light Absorption By Plant And Bacterial Pigments Chlorophyll a is the main pigment involved in algal photosynthesis. It absorbs light at 2 peaks. 670-680 nm (redorange) and 435 nm (blue-violet). Thus photosynthesis can occur at many levels. Red light in shallow water, blue in deeper water.
Light Absorption By Plant And Bacterial Pigments Other plant pigments sensitive to various wavelengths of light are also present and contribute to photosynthesis. Although not as important as chlorophyll a. The result is that energy from light waves ranging from 400 to 700nm can be used for photosynthesis (primary production).
Light Penetration And Aquatic Plant Growth Depth of plant growth usually determined by depth of photic zone (3X secchi reading) Phytoplankton or turbidity can become so numerous as to block most submerged, rooted aquatic plant growth.
Light Penetration And Aquatic Plant Growth Lake Tahoe deepest rooted aquatic plants. 136 feet. No angiosperms below about 26 feet probably due to water pressure. However, Potemogeton and Elodea have been found 30 and 36 ft. respectively.
Light And Aquatic Animals In some species of zooplankton, diminishing light promotes upward swimming and brightening light causes them to swim downwards.
Light And Aquatic Animals Many species of zooplankton migrate up and down through the water column depending on time of day and corresponding light intensity. UV radiation in surface water can kill organisms that can not swim to deeper more protected areas.
Light And Aquatic Animals Daily migration also reduces predation on some species.
Light And Aquatic Animals Some species of zooplankton have pigments that protect them from harmful UV radiation found in shallow water.
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