Download Light and the Aquatic Ecosystem

Light and the
Aquatic
Ecosystem
Why is light important?
z Ultimate
source of energy in all carbon
containing materials entering the water.
z Sunlight drives the entire aquatic
ecosystem.
What Do Limnologists
Measure?
z How
much light falls the on water surface
z How deeply it penetrates
z How it affects aquatic organisms
Solar Constant and Nature of
Light
Constant – 1.94 cal/cm2 /minute
z If 100% efficiency – it would produce 325
tons of plant material/sq.km/hr. !!!
z ½ this energy lost as it enters the
atmosphere.
z Photosynthetic plants capture some of the
energy, but not very efficiently.
z 99% lost
z Solar
Light Energy
z How
to measure light waves?
z Nanometers (nm) 10-9 m
z 350nm=ultraviolet 400nm=violet
460nm=blue 520nm=green
580nm=yellow 620nm=orange
700nm=red
780nm=infrared
Light Energy
z The
shorter the wave length of
light, the greater the Energy.
z Violet light has nearly 2X the
energy of red light.
z This is why grow lights are
usually violet.
Light energy
z2
sources of light radiation- Direct from the
Sun, and diffuse reflected light from clouds
and sky.
Light Energy and Plants
z
z
z
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
z Part
of the energy reaching the lake surface
does not enter the water. It is reflected
back.
z How much depends on angle of incidence.
Can lose up to 20%.
z Angle of incidence depends on time of day,
and season of the year.
z Angle less important with cloud cover.
Light Reflecting Off Water
Bouguer’s Law
z When
a parallel beam of monochromatic
light enters chemically pure water, it is
absorbed exponentially.
z Absorption varies directly with the log of
water thickness. No exceptions known.
z So light decreases at a constant rate with
depth of water.
Beer’s Law
z Absorbing
capacity is directly proportional
to the number of absorbing entities.
z A beam of monochromatic light passing
through a given distance of a solution is
absorbed exponentially according to the
concentration of solute.
Beer’s Law
z Not
so clean in nature
z Light of many wavelengths enter lake at
many different angles.
z Lake water has many different chemical
concentrations through out water column.
Transmitted light
z Transmittance
is the % of
light transmitted through 1 m
of water.
z Transmittance varies by
wavelength.
z Color of light changes with
depth.
z Blue and violet most prevalent
the deeper you go in pure
distilled water . WHY?
Light Transmittance
z
z
z
Long light rays are
absorbed rapidly.
65% of red light
absorbed in 1st meter of
water.
Almost all infrared
(heat) rays are absorbed
in the 1st meter.
Light Transmittance
z Dissolved
materials in the water absorb up
to 80% of light.
z They also reflect and alter the color of light
passing through the water column.
Light Transmittance
z Light
reaching the
depths of very clear
lakes is often a greenyellow mixture.
z In heavily stained
waters, orange may
penetrate farthest.
Secchi Disk
z Used
to measure light
penetration.
z Developed by an Italian
oceanographer in the 1860’s.
z Should be measured between
10AM and 2PM from shady
side of boat.
z In extremely clear lakes Secchi
visibility can be 40 meters.
Light Needed For Photosynthesis
z Minimum
intensity of subsurface light
needed for photosynthesis is about 1% of
surface light.
z Region of surface to a depth where 99% of
surface light has disappeared is called the
euphotic zone or photic zone.
z Depth of this zone varies with time of day
and from lake to lake.
z Below this zone primary productivity is
considered nil.
Light Needed For Photosynthesis
z The
photic zone can be determined by using
a light meter lowered into the water
between 10AM and 2 PM.
Light Needed For Photosynthesis
z Important
rule of
thumb! – the photic
zone is usually about 3
(2.7) times secchi disk
visibility.
Secchi Disk Use
z Can
be used to estimate chlorophyll content
of the phytoplankton community.
Effect Of Ice And Snow On Light
Transmittance
z Clear
ice transmits light better than the
water beneath it. Why?
z Particulate and dissolved matter is less in
the ice than in the water.
Effect Of Ice And Snow On Light
Transmittance
z Snow
blocks light. 99% of light blocked by
18cm (7 in.) of new snow.
z Reflectance (albedo) from the white surface
is a an important factor.
z 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
of total illumination disappears in 1st
meter.
z 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.
z 50%
Color
z Lake
color is determined by a number of
factors.
z Materials in solution and particulate matter
both living and non-living are important.
z Reflections of subsurface objects play a role
in shallow ponds.
z Reddish ponds due to suspended soil
particles.
Color
z Some
bodies of water tinged red due to
certain species of blue-green algae.
z Green water is caused by large
phytoplankton populations
z Diatoms and certain dinoflagellates can
color the water dark yellow. Or in some
cases, orange or reddish orange.
Color
z Filtration
can remove objects causing the
apparent color, leaving the so-called true
color of the water.
z The true color ranges from clear blue to
dark brown or black.
z 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
z The
Forel – Ule scale is a standard color
scale used to determine water color.
Light Absorption By Plant And
Bacterial Pigments
z Chlorophyll
a is the main pigment involved
in algal photosynthesis.
z It absorbs light at 2 peaks. 670-680 nm (redorange) and 435 nm (blue-violet).
z Thus photosynthesis can occur at many
levels. Red light in shallow water, blue in
deeper water.
Light Absorption By Plant And
Bacterial Pigments
z Other
plant pigments sensitive to
various wavelengths of light are also
present and contribute to
photosynthesis. Although not as
important as chlorophyll a.
z 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
z Depth
of plant growth usually determined
by depth of photic zone (3X secchi reading)
z Phytoplankton or turbidity can become so
numerous as to block most submerged,
rooted aquatic plant growth.
Light Penetration And Aquatic
Plant Growth
z 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
z In
some species of zooplankton,
diminishing light promotes upward
swimming and brightening light causes
them to swim downwards.
Light And Aquatic Animals
z Many
species of zooplankton migrate up
and down through the water column
depending on time of day and
corresponding light intensity.
z UV radiation in surface water can kill
organisms that can not swim to deeper more
protected areas.
Light And Aquatic Animals
z Daily
migration also
reduces predation on
some species.
Light And Aquatic Animals
z Some
species of zooplankton have pigments
that protect them from harmful UV
radiation found in shallow water.
The End