Thermal Stratification

Report
Lake Mixing: Density
Density of Water
1.001
Maximum
3
Density (g/cm )
1.000
0.999
0.998
0.997
0.996
0.995
0
5
10
15
20
o
Temperature ( C)
25
30
Thermal Stratification
Temperature (°C)
0
10
20
0
Depth (m)
5
Epilimnion
10
15
20
Hypolimnion
30
Seasonal Stratification
Thin ice?
Field Trip Results
Temp. (oC)
0
5
10
15
20
0
2
Depth (m)
4
6
8
10
12
14
16
Dollar Bay
Portage
Chemistry-Physics-Biology
Linkage
Dissolved Oxygen (mg/L)
0
2
4
6
8
0
2
Depth (m)
4
6
8
10
12
14
16
Dollar Bay
Portage
Significance of Stratification
Unstratified,
Single CSTR (CMFR)
Stratified
2 CSTRs with
feedback
Upwelling
Nutrient Limitation
The growth of algae and higher aquatic plants in lakes is
regulated by conditions of light and temperature and the
availability of those inorganic nutrients required to support
growth. The element most often in limiting supply is
phosphorus, P.
C
C
C C C
C C
C C
P P
C C C
P
P
P
P
O O
O O
P
P
C C C
O O
O O
O O O O
O
O O O O
C
O
+
O O O O
O
O
C
C
Eutrophication
P
Effects of Eutrophication
1.
2.
3.
4.
5.
6.
7.
Oligotrophic
Low biomass
High diversity
Complex food web
Oxic waters
Cold-water fish present
High aesthetic quality
No taste or odor problems
1.
2.
3.
4.
5.
6.
7.
8.
9.
Eutrophic
High biomass
Low diversity
Simple food chain
Anoxic bottom waters
Cold-water fish absent
Low aesthetic quality
Taste and odor problems
Rough fish abundant
Toxic algae present
Oxygen supply
14
Surface mass transport
Dissolved Oxygen (mg/L)
12
10
Vertical mass transport
8
6
4
2
0
Hypolimnetic
oxygen demand
Onondaga Lake: “most polluted
lake in U.S.A.”
Biogeochemistry: study of the
interactions of biology, geology,
chemistry, physics
Water Reservoirs
Hydrologic Cycle
Biogeochemical Cycles
Carbon Cycle
Another view of the carbon cycle
Photosynthesis
CO2
Organic C
Respiration
Methane
oxidation
Methanogenesis
CH4
Nitrogen Cycle
N2
FIXATION
DENITRIFICATION
NITRIFICATION
NH3/NH4+
DISSIMILATORY NO3
REDUCTION
ASSIMILATION
ASSIMILATION
Organic-N
Human perturbations to N Cycle
Mississippi R.
L. Superior
N2O Emissions:
310 x greenhouse effect of CO2
U.S. Emissions increased 1.1% in 1990s
30% of anthropogenic emissions occur in “coastal” areas
No reliable estimates of emissions from Great Lakes
Gulf of Mexico Hypoxic Zone
July 23-28, 1999, Shelfwide Oxygen Survey
30.0
L. Calcasieu
Atchafalaya R.
Latitude (deg.)
Sabine L.
Mississippi R.
29.5
Terrebonne
Bay
29.0
50 km
28.5
93.5
92.5
90.5
91.5
89.5
Longitude (deg.)
Bottom Dissolved Oxygen Less than 2.0 mg/L
(Rabalais, Turner & Wiseman)
Eutrophication
Eutrophication: the process of becoming or being made eutrophic
Eutrophic: the state of being enriched in nutrients or food sources
In aquatic ecosystems, eutrophication is caused by excessive inputs
of nutrients, both N & P. Generally, freshwaters are P-limited and
coastal estuarine waters are N-limited. The nutrients enhance algal
growth, and this, in turn, may have a cascade of effects on the
ecosystem. These effects may include: algal blooms, growth of
undesirable algal species, oxygen depletion or anoxia in bottom
waters, loss of cold-water fish species, abundance of “rough fish”,
fish kills, unpleasant tastes and odors.
Sources of nutrients
• Point sources
– Sewage treatment plant discharges
– Storm sewer discharges
– Industrial discharges
• Non-point sources
– Atmospheric deposition
– Agricultural runoff (fertilizer, soil erosion)
– Septic systems
Solution: Reduce nutrient inputs
• Agriculture
– Reduce animal density, restrict timing of manure spreading,
buffer strips by streams, reduced tillage, underground
fertilizer application, wetland preservation and construction
• Watershed management
– Buffer zones, wetland filters
• Storm runoff
– Eliminate combined sewer systems (CSO’s)
– Stormwater treatment required (holding ponds, alum)
– Education on yard fertilization
• Erosion from construction, forestry
– Erosion barriers, soil cover, road and bridge stabilization
• Septic systems
– Distance from lake, adequate drainfields
Mitigation strategies
Often there is pressure for quick actions that
will reduce the severity of the symptoms.
Numerous options exist. To understand
these options and choose among them, one
should understand the nutrient cycle within
the aquatic system (lake).
P Cycle
Organic
P
Mineralization
Organic
P
Settling
Epilimnion
Uptake
Dispersion
Inorganic
P
Inorganic
P
Settling
Hypolimnion
Sediment
Inorganic
P
B
u
r
i
a
l
Mineralization
Organic
P
B
u
r
i
a
l
The P cycle may
be manipulated
in several ways
to reduce the
regeneration of
inorganic P and
its transport to
the epilimnion or
to reduce the
algal uptake of P.
Within-lake actions
• Reduce algal growth
– Apply algicide
– Biomanipulation
• Reduce mineralization
– Remove organic P before it is mineralized
• Dredging
• Macrophyte harvesting
• Reduce transport of inorg. P to epilimnion
– Hypolimnetic water withdrawal
Macrophyte harvesting
Lake Phosphorus Cycle
Loading, W
Outflow
Q
Cl
Settling
Burial
dC
V
min 
mout rxn
dt
dC
V
W 
QC 
vsCA
dt
Vollenweider Model
Steady State Solution:
W
C
Q vs A
P loading rate, J,
2
(g P/m yr)
100
W
W
A
J
A
C
 

Q vs A A Q v
q vs
s
A
10
1
1
10
100
1000
0.1
Hydraulic Loading rate, q, (m 3/m2-yr)
log( J ) log C log(q vs )
Terms to know:
Epilimnion
Hypolimnion
Thermocline
Metalimnion
Oligotrophic
Eutrophic
Mesotrophic
Oxygen sag curve
Critical point
Oxygen deficit
Saturation
Reaeration
Deoxygenation
Denitrification
Nitrification
Acid rain
Mineralization
Limiting nutrient
Liebig’s Law
Sulfate reduction
Nitrogen fixation
Hydrologic cycle
Evapotranspiration
Biogeochemical cycle
Micronutrient
Macronutrient
Review of previous terms:
Biotic
Abiotic
Atmosphere
Hydrosphere
Lithosphere
Biosphere
Ecosphere
Ecology
Species
Population
Community
Organism groups:
viruses
bacteria
algae
fungi
protozoa
rotifers
microcrustaceans
macrophytes
macroinvertebrates
fish
Producers
Herbivores
Omnivores
Food chain
Microbial loop
Consumers
Carnivores
Trophic level
Food web
Decomposers
Photosynthesis
Chlorophyll
Respiration
Redox
Reduction
Oxidation
Electron donor
Electron acceptor
Aerobes
Obligate vs. facultative
Anaerobic respiration
Aerobic respiration
anoxic
Anaerobic
Fermentation
Autotroph
Heterotroph
Biomass
Productivity
Primary production
Secondary production
Lithotrophs
Photoautotrophs
Photoheterotrophs
Chemoheterotrophs
Chemoautotrophs

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