Lake Mixing: Density

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Lake Mixing Density
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Thermal Stratification
Epilimnion
Hypolimnion
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Seasonal Stratification
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Thin ice?
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Field Trip Results
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Chemistry-Physics-Biology Linkage
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Significance of Stratification
Unstratified, Single CSTR (CMFR)
Stratified 2 CSTRs with feedback
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Upwelling
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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.

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Eutrophication
P
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Effects of Eutrophication

• 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

• Oligotrophic
• Low biomass
• High diversity
• Complex food web
• Oxic waters
• Cold-water fish present
• High aesthetic quality
• No taste or odor problems

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Oxygen supply
Surface mass transport
Vertical mass transport
Hypolimnetic oxygen demand
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Onondaga Lake most polluted lake in U.S.A.
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Biogeochemistry study of the interactions of
biology, geology, chemistry, physics
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Water Reservoirs
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Hydrologic Cycle
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(No Transcript)
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Biogeochemical Cycles
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Carbon Cycle
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Another view of the carbon cycle
Photosynthesis
CO2
Organic C
Respiration
Methane oxidation
Methanogenesis
CH4
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Nitrogen Cycle
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Human perturbations to N Cycle
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Mississippi R.
L. Superior
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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
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Gulf of Mexico Hypoxic Zone
July 23-28, 1999, Shelfwide Oxygen Survey
30.0
Atchafalaya R.
L. Calcasieu
Sabine L.
Mississippi R.
29.5
Terrebonne Bay
Latitude (deg.)
29.0
50 km
28.5
90.5
89.5
93.5
92.5
91.5
Longitude (deg.)
Bottom Dissolved Oxygen Less than 2.0 mg/L
(Rabalais, Turner Wiseman)
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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.
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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

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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 (CSOs)
• 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

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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).
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P Cycle
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.
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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

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Macrophyte harvesting
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Lake Phosphorus Cycle
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Vollenweider Model
Steady State Solution
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Terms to know
Epilimnion Hypolimnion Thermocline Metalimnion Oli
gotrophic Eutrophic Mesotrophic Oxygen sag
curve Critical point Oxygen deficit Saturation Rea
eration Deoxygenation
Denitrification Nitrification Acid
rain Mineralization Limiting nutrient Liebigs
Law Sulfate reduction Nitrogen fixation Hydrologic
cycle Evapotranspiration Biogeochemical
cycle Micronutrient Macronutrient
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Review of previous terms
Biotic Abiotic Atmosphere Hydrosphere Lithosphere
Biosphere Ecosphere Ecology Species Population Com
munity
Organism groups viruses bacteria algae fungi prot
ozoa rotifers microcrustaceans macrophytes macroin
vertebrates fish
Photosynthesis Chlorophyll Respiration Redox Reduc
tion Oxidation Electron donor Electron
acceptor Aerobes Obligate vs. facultative Anaerobi
c respiration Aerobic respiration anoxic
Anaerobic Fermentation Autotroph Heterotroph Bioma
ss Productivity Primary production Secondary
production Lithotrophs Photoautotrophs Photohetero
trophs Chemoheterotrophs Chemoautotrophs
Producers Consumers Herbivores Carnivores Omnivore
s Trophic level Food chain Food web Microbial
loop Decomposers