Crater Lake, Oregon -589 m deep and possibly the clearest lake in the world,
Transparency up to 90 m.
Thermocline very deep for its size
No rooted plants.
Mud doesn’t accumulate on the bottom till > 90 m depth
Why is this
lake so
different
from most
lakes?
Some of the most spectacular tectonic lakes are formed in volcanic craters.
Physical features of lakes that determine habitat characteristics
•inflow from the watershed/Catchment
•Water residence time
•Morphometry, Mean depth and volume
•Thermal stratification and physical mixing
•wind./currents/wave action
•Sediment deposition
•Light extinction
How much water flows into lake Beauvais lake in a year from
its watershed?
Assume runoff
coefficient of
0.15 m
Drainage area
=7.9 km2
Lake area
=0.9 km2
How much water
would you
expect flows into
this lake /yr?
Evaporation from
lake surface
exceeds
precipitation by
0.085 mm/yr
How much water
flows out of the
lake?
Assume runoff Drainage area
=7.9 km2
coefficient of
Lakearea
0.15 m
=0.9 km2
P─ E on lake
surface
= ─ 0.085 m/yr
How much water would you expect flows into this lake /yr?
Qi = r * DA = 0.15 m/yr * 7 x 106 m2 = 1.05 x 106m3/yr
What is the net evaporation in a year?
(P-E)*A = ─ 0.085 m * lake area = ─ 0.085m/yr * 9 x 105 m2 = -7.65 x 104 m3/yr
How much water flows out of the lake in a year?
Qo = Qi + (P-E)*A = 1.05 x 106m3/yr + (─ 7.65 x 104 m3/yr) = 9.75 x 105 m3/yr
Definition of water residence time and flushing rate
Chapter 4
Water residence time  w (time units)
How long would it take for the entire volume to drain out of the lake
if no new water wer e entering it.
V
L3
V
w 
, units 3  t , renewal time 
Qo
L /t
Qi
Qo  mean discharge out of the lake
Qi  mean discharge from watershed into the lake
Qo  Qi  P  E A
The approximat e inverse is flushing or renewal rate,
How many times a year can the inflow fill the lake
Qi
L3 / t 1
h  , units 3 
V
L
t
Lake Area = 0.9 km2
Mean depth= 4.3 m
Lake Volume = 3.8 x 106 m3
Water residence time=
Mean renewal rate=
Lake Area = 0.9 km2
Mean depth= 4.3 m
Lake Volume = 3.8 x 106 m3
Water residence time=
Mean renewal rate=
Water residence time
Mean flushing rate
V
3.8 106 m3
w 

 3.9 yr
5 3
1
Qo 9.7 10 m  yr
Qi 1.05 106 m3  yr 1
1
h 

0
.
28

yr
V
3.8 106 m3
How much of the
water flowing into
this lake from its
watershed could
you allocate for
irrigation before
the lake would
gradually begin to
disappear?
Answer
Over 92%
Lake management—the water inflow budget
or what happens when you over allocate?
The Aral Sea in the former Soviet Union—mismanaging the river water inflow
Allocation to desert irrigation > inflow minus evaporation
Fig. 5.19
. Effects
Ecosystem collapse, loss of biodiversity, worsening of water-salt balance in the
agricultural areas, pollution of rivers and drinking water, changing of the regional
climate – all these are new environmental developments in Central Asia.
Calculating volume and mean depth
Mean depth = Volume/surface area
The hypsographic
curve
Area under the
curve = volume
Fig. 7.1 in text
Lakes partition themselves into temperature zones
Thermal stratification in lakes
•In deep lakes only the surface
layers are well mixed and quite
warm, whereas the deeper parts
remain cold.
•The thermocline occurs deeper
in large lakes because wind
energy is transmitted to greater
depths
•Wind energy increases with
fetch
•In small lakes convection also
plays a role in determining
thermocline depth
Fig. 11.8 in text
The seasonal pattern of thermal
stratification in a deep temperate zone lake
Depth-time graph of isotherms
During spring turnover the entire
Water column is 4oC—why 4oC
Same thing happens again in the fall
Vertical thermal profiles
Heat diffuses much faster down the water column in large lakes—wind mixing
Hence the thermocline is
deeper in large lakes
depth of the thermocli ne
 A (km ) 
Zt (m)  ln 

 0.043 
2
2.35
Table 11.2 in text
Fig. 12.7 in text
In small lakes mixing is more determined by convection currents driven by
solar heating and is determined by how deep light penetrates
Middle of thermocline
Top of the thermocline
In very large lakes horizontal
thermal shear zones occurs
at river mouths
A thermal bar
Important habitat feature for
many fish species in spring.
Waves- the gravitational response to wind disturbance
The bigger the
wind fetch the
bigger the wave
oscillation
The velocity in
these oscillations
attenuates sharply
with depth
Wave energy and
slope together
determine the
depositional zone
boundary
Fig. 12.3 in text after Rasmussen and Rowan (1997)
Log DBD(m)=─ 0.107 + 0.742 Log F (km) + 0.0653 slope (%)
At depths > depostional
Boundary depth
fine mud accumulates
An undisturbed sediment
core containing varves from
the deposition zone of a
deep lake
The varves can be used to
calculate dates along the
core profile
Paleolimnology--Pollen stratigraphy in lake sediment cores
Cores can be dated with radioisotopes
137Cs
(half-life 30 yr) is
found in fallout from
bomb tests
The most commonly used isotope is 210Pb, half-life 22 yr
The Uranium 238 decay series
238U
226Ra
222Rn
218Po
210Pb
The littoral zone, what determines its outer boundary?
The transparency of lake water is measured by its
extinction coeficient
The extinction coefficient k increases with:
•the concentration of organic matter (colour) of the
water
•the amount of suspended matter
eg, phytoplankton, fine suspended particles, eg
clay
Light extinction --Light enters from above and its intensity (I) is sharply
attenuated with depth (z)—absorption by water or solute molecules or
scattered by particles
Section 10.6
Iz
z 50%
z 10%
z 1%
Iz  I 0e  kz where I is light intensity, and z  depth,
k is the extinction coefficien t in fraction/m
This equation can be rearranged to
give (take ln of both sides)
Photic ln I 0  ln Iz
zone k 
z
What fraction of light remains by depth z?
Iz
 e  kz
I0
What fraction of light is absorbed every z m
1  e  kz
z
Page 144 in text
In general Photosynthesis exceeds respiration above the 1% light level
and rooted plants can grow down to about the 10% light level
Iz
z 50%
z 10%
At what depth z1% will the light intensity be 1% of I 0
Iz
 e  kz so 0.01  e  kz and  ln 0.01  kz...
Photic I 0
zone  z  4.6 ,
k
for the depth at which light intensity is 10% of I 0
2.3
,
k
for the 50% light level
z
z 1%
z
z
Page 144 in text
0.69
k
Consider t he following example problem
If the light intensity at 2 m depth is 50% of the surface intensity
solve for k , the extinction coefficien t
Since
Iz
Iz
Iz  I 0e  kz , then
 e  kz and  0.5
I0
I0
then
e  kz  0.5
then take the natural log (ln) of both sides
 kz  ln 0.5 or kz  0.69
since in this problem z  2, we have
2k  0.69, or
k  0.345
This means that 34.5% of the incident light is absorbed in each m of depth
What fraction of the incident light woul d reach 4m?
I4
 e ( 40.345)  0.25
I0
25% of the light woul d reach 4 m
What would be the depth of the photic zone?
z  4.6 / k
z  4.6 / 0.345  13.3m
The extinction coefficient k increases with:
•the concentration of organic matter (colour) of the water
•the amount of suspended matter
eg, phytoplankton, fine suspended particles, eg clay
Differential absorption by wave length gives water colour
•Red light is absorbed much more than blue in distilled water
•Deep clean water appears blue because most back-scatter
from depth is blue; shallower waters will back-scatter a mix of
blue and greens so such lakes appear blue-green
•Organic matter absorbs blue the most—appears yellow/brown
When a lake is rich in humic matter (tea) the organic matter
absorbs most of the blue, and green end of the spectrum,
•Fine colloids of calcite in water absorb blue mostly—water looks
green
•Suspended clay/silt scatter all wave lengths so water appears
milky (no colour)
•A dense phytoplankton bloom appears green because of
chlorophyll in the algal cells
The electromagnetic spectrum
Pure water absorbs preferentially the longer wave lenghts
—at depth short wavelenths predominate-everything gradually looks blue
Incoming spectrum—white light all colours present
10-20 m depth water blue-green
50 -100 m water blue
Increasing depth
5-10 m depth water greenish
Clean shallow lakes usually appear bluish-green
Deep lakes appear blue because back scattering from deep water is mainly blue
Longer wave lengths have been absorbed already at shallower depths
Water from swamps like these
appears brown because of its high
content of dissolved organic matter which absorbs strongly
at the blue end of the spectrum
Glacier meltwater full of
suspended particles looks milky
white since all wavelengths are
absorbed or back-scattered.
This pond has a dense phyto-plankton bloom, and the green colonial algae make
the water look green
The action spectrum for photosynthesis—blue and red work best
green, yellow and brown are least useful
Based on the absorption spectrum for photosynthetic pigments, would
you expect to find algae or plants growing near the lower boundary
of the photic zone is
(a) A clear lake with little organic or particulate matter in the water
(b) A brown-water humic lake
Consider what you know about the spectral composition at depth in
each of these two types of lakes.
Where does the exponential equation come from. Another way of
writing it is as a rate equation. The rate of change of light intensity with
depth decreases as a linear function of the light Intensity
Section 10.6
Iz
z 50%
z 10%
z 1%
z
Photic
zone
dI
  kI , wher e Iz  I 0e  kz
dz
Show that the equation on the right
satisfies the one on the left
Take the derivative of I 0e  kz with
respect to z
 I0

 ke kz
  kI 0 e  kz
  kI
Practice questions
Explain how flow processes contribute to habitat diversity in
rivers and streams.
Outline some examples of human activities that impact riverine
habitats.
Explain why these activities can put aquatic species at risk.
What is a proglacial lake?
Explain how they form and disappear on the landscape and
why they are important in determining the distribution of
aquatic species?
“Wind streaks” and Langmuir spirals
Fig. 12. 13
Larger scale gravitational responses to wind action—The seiche
Fig. 12.15
The oscillation of the
thermocline during a
seiche
Fig. 12.17
The oscillation of the thermocline produced by internal waves during a large seiche
Fig. 12.18