Transcript EVPP 550 Waterscape Ecology and Management

EVPP 550
Waterscape Ecology and
Management
Professor
R. Christian
Jones
Fall 2007
Origins of Lakes
•
•
•
•
•
•
Glacial
Tectonic
Volcanic
Solution
Fluviatile
Impoundments
Origin of Lakes - Tectonic
• Epeirogenesis or overall
crustal uplifting
• More complex than graben
• Entire section of the crust
is uplifted
– Caspian Sea: formerly part
of the ocean, cut off by
crustal uplift
– Lake Okeechobee, FL:
similar origin, partially
maintained by daming with
plant material
– Lake Titicaca, Peru
Origin of Lakes – Tectonic
• Earthquake Lakes
– Reelfoot Lake, TNKY
– Major earthquake (8
on Richter scale)
– Caused surface to
uplift in some areas
and subside in
others
– Mississippi R was
diverted into a
subsidence region
for several days
forming Reelfoot
Lake
Origin of Lakes - Tectonic
• Landslide Lakes
– Mountain Lake, VA
• One of two natural lakes
in Virginia
• Formed when landslide
dammed a mountain
valley
• The lake is estimated to
be about 6,000 years
old and geologists
believe it must have
been formed by rock
slides and damming
Origin of Lakes - Volcanic
• Crater/caldera Lakes
– Lake occupies a
caldera or collapsed
volcanic crater/cone
– If cone blows out the
side like Mt. St.
Helens, no basin left
– Ex. Crater Lake, OR
Origin of Lakes – Volcanic Lakes
• Lava dams
– Lava flow dams
an existing valley
– Lake Kivu, Africa
– Meromictic Lake,
contains high
conc of CO2
– Could cause
suffocation if
overturned
Origin of Lakes – Solution Lakes
• Carbonate areas
– Basin created by
dissolution of removal
by groundwater of
CaCO3 and MgCO3
rocks
– Overlying ground
eventually collapses:
“sinkhole”
– May lead to lakes or, if
there are seams of
carbonate, to a “karst”
landscape
– Lakes of Central Florida
Origin of Lakes – Solution Lakes
• Salt collapse basins
– Underground
seepage dissolves
salt lenses, ground
collapses and basin
fills
– Montezuma Well,
AZ
Origin of Lakes – Fluviatile
(river-made)
• Ponding by deltas
– Lake Pepin: WI-MN
• Oxbow Lakes
– Isolated meanders
of an alluvial river
– Lake Chicot, AR
• Pothole Lakes
– Excavated by
streambed erosion
– Grand Coulee
Lakes, WA
Origin of Lakes – Animals
• Humans
– Intentional reservoirs
– Incidental flooding of
basins constructed for
other purposes
• Quarries
• Peat diggings
• Other agents
– Beavers
– Alligators
Origins of Lakes - Reservoirs
•
Purposes
–
Water supply
•
•
–
–
–
–
–
–
Human
Livestock
Irrigation
Flood control
Sediment control
Recreational
Power generation
Navigation
Origin of Lakes – Lake Districts
• Because most of the
factors responsible for lake
origins or localized or
regional, lakes tend to be
clustered in “districts”
– Glacial Lakes: MN, WI,
Ontario, NY, New England
– Oxbow Lakes: lower
Mississippi Valley (AR, MS,
LA, TN)
– “English Lake District”
– Even reservoirs are
clustered due to favorable
geology, physiography,
demand
Morphology of
Lakes
• Parameters related to surface dimensions
– Maximum length
• Distance across water between two most separated
points on shoreline
• Most significant when this corresponds with direction
of prevailing winds
• Less clear in curved lakes
– Maximum width or breadth
• Greatest distance across water perpendicular to axis
of maximum length
Morphology of Lakes
• Parameters related to surface dimensions
– Surface area
• Can be derived from map by planimetry, weighing
or counting squares
• Determines the amount of solar energy entering
the lake and the interface available for heat and
gas exchange with the atmosphere
– Mean width
• Surface area/maximum length
Morphology of Lakes
• Parameters related to surface dimensions
– Shoreline length
• Related to the amount of shallow water available for
littoral organisms as well as the degree of interaction with
adjacent terrestrial system (leaffall)
– Shoreline development index, DL
• Compares the lakes actual shoreline length with that of a
circular lake of the same surface area
• Allows comparison among lakes
• High DL, elongate latkes, river impoundments
• Low DL, calderas, solution basins, simple kettle lakes
Morphology of Lakes
• Parameters requiring
bathymetric or subsurface
dimensions
– Maximum depth, zmax
• Popular and oft-cited datum
• Some ecological significance
– Relative depth, zr
• Ratio of maximum depth to
diameter of a circular lake with
the same area
• Provides a way of comparing
large and small lakes
Morphology of Lakes
• Volume
– Total amount of water in the lake
– Most easily derived from hypsographic curve
– Hypsographic curve: Plot of Area vs. Depth
Morphology of Lakes
• Volume
– Hypsographic curve: Plot of Area
vs. Depth
– Can derive total water volume or
volume of specific strata
Morphology of Lakes
• Mean Depth, z bar
– zbar = V/A
– One of the most important and meaningful
morphometric parameters
– A general index of lake productivity
• ↑ zbar  ↑ volume/area, dilution of incoming solar
energy, ↑ volume unlit
• ↓ zbar  ↓ volume/area, concentration of incoming
solar energy, ↓ volume unlit
Morphology of Lakes
• Deepest lakes are grabens; calderas and some
glacial lakes can also be deep
• Grabens have the greatest volume
Morphology of Lakes
• Glacial scour lakes can be large, but not necessarily
deep
• Note that drift basins are neither large nor deep, but are
very numerous
Morphology of Lakes
• Hydraulic retention time, Tr
– Average time spent by water in the lake
– “residence time”
– Tr = Volume/Outflow rate
– Varies greatly, some lakes have no outlet
– Superior 184 yrs
– Tahoe 700 yrs
– Some reservoirs have Tr of only a few days or
even hours
Morphology of Lakes
• Elements also have retention times
– If very soluble and not biologically active (Cl),
elemental retention time ≈ hydraulic retention
time
– If associated with particles or biologically
reactive (P), elemental retention time >>
hydraulic retention time
Light in Lakes
• Sun is virtually the only source of enerby
in natural aquatic habitat: photosynthesis
and heat
• Solar constant
– Rate at which radiation arrives at edge of
Earth’s atmosphere
– ≈ 2 cal/cm2/min
– More than half of this is lost coming through
the atmosphere
Light in Lakes
• Absorption by
different
chemicals in
atmosphere
• Water and ozone
(O3) are
especially
important
• Ozone is the
most important in
the UV range
Light in Lakes
• Spectrum of light,
wavelength, λ
– Ultraviolet: < 400 nm
– Visible: 400-750 nm
– Infrared: > 750 nm
• Light waves may
also be
characterized by
their frequency, ν
– ν = c/λ, where c =
speed of light
Light in Lakes
• Light may be considered to be made up as
particles called photons
• Energy (E) content of a photon is related to its
frequency
• E = hν where h=Planck’s constant
• Therefore higher frequency (shorted wavelenth)
radiation has more energy per photon
• Light is often quantified as photon flux density
• Moles/m2/sec; 1 mole of photons = 1 Einstein
Light in Lakes
• Losses of
Radiant
Energy
– Absorptive
compounds in
atmosphere
– Cloud cover
– Reflection at
Lake’s
surface
Light in Lakes
• Scattering and Absorption
– Physically different processes, but usually
hard to separate
– Scattering
• deflection of photons by particles
• Includes both side scattering and back scattering
• Best measured by “turbidity”
– Absorption
• Conversion of photon to another form of energy
• Usually heat, but sometimes chemical (ex psyn)
Light in Lakes
• Attenuation
– Disappearance of water with depth in a lake
– Due to a combination of scattering and
absorption
– Approximated by the Beer-Bouguer Law
• In a homogeneous medium a constant proportion
of photons and their energy is absorbed
(disappears) with each linear unit of medium
Light in Lakes
• Attenuation
– Mathematical statement of Beer-Bougher Law
• I(z) = I(0) x e-kz
• where
– I(z) is Irradiance (light) at depth z
– I(0) is Irradiance (light) at the surface minus reflection
– k is the coefficient of attenuation
• The rate of light attenuation for each unit of depth
is e-k
Light in Lakes
• K, the rate of light
attenuation is due
to
– Water, kw
• Not very large
• Greatest for longer
wavelengths (red)
• Least for short
wavelengths (blue)
• Explains why in clear
water objects have a
bluish cast
Light in Lakes
• K, the rate of light
attenuation is due
to
– Dissolved material
– Particulate material
• Net result is to shift
wavelength of max
penetration from
blue toward green
as attenuation
increases
Light in Lakes
• K, the rate of light attenuation is determined by
plotting ln I(z) vs z
• Slope is –k, in this case -3.78 m-1
Light in Lakes
• Light attenuation in
lakes is also
approximated by
determining Secchi
disc depth, zSD
• Secchi disc depth has
been shown to be
related inversely to light
attenuation coefficient
• One equation
commonly used is:
• K = 1.7/zSD
Light in Lakes
• Photic zone
– Lower limit defined by 1% of surface light
– Depth at which I(z)/I(0) = 0.01
– zPZ = - ln 0.01 / k
– zPZ = 2.7 zSD