A Modeling Analysis
of the Clear Water Phase
CE5504 - Surface Water Quality Modeling
Case History
A widespread phenomenon, following a pattern …
Chlorophyll
A distinct period of clear
water typical of spring algal
succession in many
temperate mesotrophic
and eutrophic lakes.
Lampert, W. et al. 1986. Phytoplankton control by grazing
zooplankton: A study on the spring clear water phase.
Limnology and Oceanography.
Secchi Disc
The PEG “Model”
“ … it is assumed that events in the plankton
are neither random not chaotic but that there is
a seasonal development which is a predictable
consequence of previous events.”
Sommer et al. 1986
The Plankton Ecology Group, a working body
associated with SIL, sought to construct a word
model of the seasonal events which occur in the
phytoplankton of an idealized ‘standard’ lake.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Auer et al. 1990
PEG 1. Towards the end of winter, nutrient availability and increased light
permit unlimited growth of the phytoplankton. A spring crop of small, fastgrowing algae such as Cryptophyceae and small centric diatoms develops.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Auer et al. 1990
Spada et al. 2004
PEG 4. Herbivore populations increase exponentially up to the point at which
their density is high enough to produce a community filtration rate, and thus
cropping rate, that exceeds the reproduction rate of the phytoplankton.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Auer et al. 1990
PEG 5. As a consequence of herbivore grazing, the phytoplankton biomass
decreases rapidly to very low levels. There then follows a ‘clear-water’
equilibrium phase which persists until inedible algal species develop in
significant numbers.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Spada et al. 2004
PEG 6. Herbivorous zooplanktonic species become food-limited and both
their body weight per unit length and their fecundity declines. This results in
a decrease in their population densities and biomasses.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Spada et al. 2004
PEG 7. Fish predation accelerates the decline of herbivorous planktonic
populations to very low levels and this trend is accompanied by a shift
towards smaller average body size amongst surviving crustaceans.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Auer et al. 1990
PEG 8. Under conditions of reduced grazing pressure and sustained non-limiting
concentration of nutrients, the phytoplankton summer crops start to build up. The
composition of the phytoplankton becomes complex both due to the increase in
species richness and to the functional diversification into those species available
to filter-feeders and those only consumed by specialist feeders.
… well described by the Phytoplankton Ecology Group (PEG) Model
Data for Onondaga Lake
Auer et al. 2004
PEG 10. From this time onward, the algal growth becomes nutrient-limited
and this prevents an explosive growth of ‘edible’ algae. Grazing by
predator-controlled herbivores balances the nutrient-limited growth rate of
edible algal species.
modeling … from words to equations
PEG 1…, PEG 4…, PEG 5…, PEG 7…, PEG 8…
dc
V
 ...
dt
AQUATOX 2.0
Park, R.A., Clough, J.S. and M. Coombs Wellman. 2004. AQUATOX:
Modeling Environmental Fate and Ecological Effects in Aquatic
Ecosystems. Release 2. U.S. Environmental Protection Agency,
Office of Water, Washington, DC.
modeling … organisms
edible algae:
flagellate greens
small diatoms
cryptomonads
www.biology.mcgill.ca
ceaspub.eas.asu.edu
serc5.si.edu
inedible algae:
large diatoms
large greens
cyanobacteria
dinoflagellates
www.microscopy-uk.org.uk
biodidac.bio.uottawa.ca
www.bio.mtu.edu
modeling … organisms
cladoceran:
Daphnia
prefedible = 1
prefinedible = 0
planktivore:
gizzard shad
prefcladoceran = 1
modeling … species composition
100%
lg
gr
80%
60%
dino
crypto
cyano
40%
20%
sm
diat
Data for Onondaga Lake
UFI; Cliff Siegrfried
0%
100%
80%
inedible
60%
40%
edible
20%
0%
modeling kinetic coefficients
edible algae
inedible algae:
Kp = 0.005 mg/L
Topt = 16 °C
C0 = 0.4 mg/L
Kp = 0.025 mg/L
Topt = 25 °C
C0 = 0.05 mg/L
Values for other coefficients used in the calibration process
(Pmax, Kresp, Kmort, Ksettle) were identical for both groups.
modeling … nutrients
1.0
0.3
Nitrogen (mgN∙L-1)
0.2
0.6
0.4
P
0.1
Set initial conditions and loads to
achieve saturation.
0.2
0.0
0.0
A
M
J
J
A
S
Phosphorus (mgP∙L-1)
N
0.8
modeling … temperature
Temperature
(ly∙d-1)
Light (°C)
Incident
500
20
400
15
300
10
200
5
100
Latitude-specific seasonal pattern.
00
AA
M
M
JJ
J J
A A
S
S
modeling … optics
Secci disc transparency is estimated from the model-calculated
extinction coefficient, based on a polynomial published by Effler et al. (1996):
1
d
SD  a0  a1 k  a2
k 
1 2
d
 a3
k 
1 3
d
 a4
k 
1 4
d
The extinction coefficient is calculated as the sum of partial extinction
coefficients provided by Effler et al. (1996):
kd  kw  kgelbstoff  kchlorophyll  k fss
kw  kgelbstoff  0.39 m1
kchl
m2
 0.011
Chl
mg Chl
k fss
m2
 0.344
mg fss
fss
(0.65 m1; Effler et al.1996)
( Effler et al.1996)
( Effler et al.1996)
modeling … transparency
8
Secchi Disk (m)
6
4
2
Set detritus to achieve ‘clearwater’
transparency.
0
A
M
J
J
A
S
Maximum Secchi Disc (m)
modeling … target data sets
0
return of
large-bodied
Daphniids
1
2
relapse
3
4
5
6
7
8
1975
1980
1985
1990
1995
2000
2005
Data for Onondaga Lake
Spada et al. 2004
Oh … now we get to see the snake eat the rabbits!
1985 data set ... chlorophyll and transparency
60
40
20
0
M
J
J
A
S
O
N
7
6
Secch disk (m)
Chlorophyll (µg/L)
80
5
4
3
2
1
0
M
J
J
A
S
O
N
1985 calibration … chlorophyll
dominant
Chlorophyll (µg/L)
80
60
40
20
0
M
J
J
A
S
O
N
1985 calibration … transparency
7
Secch disk (m)
6
5
4
3
2
1
0
M
J
J
A
S
O
N
1985 calibration … zooplankton
absent in 1985
1999 data set … chlorophyll and transparency
60
40
20
0
A
M
J
J
A
S
O
7
6
Secchi disc (m)
Chlorophyll (µg/L)
80
5
4
3
2
1
0
A
M
J
J
A
S
O
1999 data set … daphniids
1.2
Daphniids (mg/L)
1.0
0.8
0.6
daphniids
0.4
0.2
0.0
A
M
J
J
A
S
O
Gmax = 1.6 g/g·d
Topt = 20 °C
C0 = 0.01 mg/L
Other coefficients used
in the calibration process
included Kresp and Kmort.
1999 calibration … daphniids
1.4
Daphniids (mg/L)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
A
M
J
J
A
S
O
1999 performance … chlorophyll
Chlorophyll (µg/L)
80
60
40
20
0
A
M
J
J
A
S
O
1999 performance … transparency
7
Secchi disc (m)
6
5
4
3
2
1
0
A
M
J
J
A
S
O
1999 performance … species composition
100%
80%
60%
model
40%
20%
0%
100%
80%
60%
40%
20%
0%
data
2003 simulation …planktivore added
3
10
9
8
7
2
6
5
4
1
3
2
1
0
0
A
M
J
J
A
S
• cladocerans essentially eliminated
• much slower attenuation of edible forms
• dominated by edible algae
2003 simulation …planktivore added
35
6
30
5
25
4
20
3
15
2
10
1
5
0
0
A
M
J
J
• absence of clear water phase
A
S
What about the rabbits that got away?
… rabbits running, Part 1: time-variable TSS
7
Secchi disc (m)
6
time-variable TSS
5
4
3
2
TSS = 3 mg/L
Neither level, held constant
permits successful simulation of
transparency over the season.
1
0
7
Secchi disc (m)
6
5
4
3
2
1
0
The base case TSS is ramped
down from 3 mg/L to 0.3 mg/L
during the clearing event,
TSS = 0.3 mg/L
This suggest that non-specific
grazing (tripton consumption)
may play a role in the magnitude
of the transparency increase
observed in clearing events.
… rabbits running, Part 2: Aphanizomenon
… the model fails to capture a late June crash in chlorophyll
Chlorophyll (µg/L)
80
60
40
20
0
A
M
J
J
A
S
O
… rabbits running, Part 2: Aphanizomenon
100%
80%
60%
40%
20%
0%
the population crashing was a genus of cyanobacteria,
Aphanizomenon, an inedible form. The crash freed resources
for two edible groups (small diatoms and cryptomonads) …
… rabbits running, Part 2: Aphanizomenon
and a second daphniid peak resulted …
1.4
Daphniids (mg/L)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
A
M
J
J
A
S
O
… rabbits running, Part 2: Aphanizomenon
… bottoming out the chlorophyll
Chlorophyll (µg/L)
80
60
40
20
0
A
M
J
J
A
S
O
… rabbits running, Part 2: Aphanizomenon
… an yielding a second clearing event
7
Secchi disc (m)
6
5
4
3
2
1
0
A
M
J
J
A
S
O
… we have our ways to deal with running rabbits
“Pay no attention to the
modeler behind the curtain.”
Conclusions
The PEG word model and the AQUATOX software
package provide a framework for the successful
simulation of the clearing event phenomenon and
for phytoplankton-transparency relationships in
non-clearing event years.
Certain features of the simulation, particularly the
demise of cyanobacteria populations and the role
of non-specific grazing in driving clearing events,
are not well understood … reminding us that
nature remains the master modeler.