Limnology 101

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Limnology 101
Dan Obrecht MU Limnology obrechtd_at_missouri.edu
The Missouri Department of Natural Resources
Region VII, US Environmental Protection Agency,
through the Missouri Department of Natural
Resources, has provided partial funding for this
project under Section 319 of the Clean Water Act
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Limnology is the science dealing with the
physical, chemical, biological and meteorological
study of inland waters.
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Lake Types

• Glaciated Lakes
• Reservoirs
• Oxbows

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Glaciated Lake
Reservoir
Oxbow
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How do the lake types differ?

• Depth
• Residence Time
• Lifespan

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Depth

• Glaciated Lakes Deep relative to area
• Reservoirs Vary, but often shallow for a given
  size relative to glaciated lake
• Oxbow Shallow

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Why is depth important?

• Shallow lakes may mix sporadically throughout the
  summer, leading to more internal loading of
  nutrients.
• A deeper lake has a larger volume of water, which
  influences hydrology.

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Residence Time is the theoretical time it takes
water to move through the waterbody. It is the
reciprocal of Flushing Rate.
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Residence Time

• Glaciated Lakes Moderate to high residence
  times
• Reservoirs Generally low to moderate residence
  times
• Oxbows Varies with connectivity to river,
  generally low due to low volumes

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Why is Residence Time important?

• Residence time can be used to gauge inputs
  relative to lake volume. A lake with a long
  residence time has low inputs, while a lake with
  a short residence time has greater inputs
  relative to lake volume.

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• Residence time also indicates how much
  sedimentation can occur. A long residence time
  translates to more material settling out to the
  bottom of the lake. A short residence time means
  this sedimentation does not occur.

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Lifespan

• Over time, lakes become shallower as organic and
  inorganic matter settles to the bottom. As the
  lake becomes shallower, it also becomes more
  productive accelerating the rate of
  sedimentation. At some point the lake becomes a
  wetland.

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Lifespan

• Glaciated Lakes High lifespan
• Reservoirs Short to moderate lifespan
• Oxbow Short lifespan

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Phosphorus and Nitrogen (Causal Variables)
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Chlorophyll (photosynthetic pigment in algae) is
a Response Variable
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Missouri Lakes Phosphorus vs Algal Biomass
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Missouri Lakes Nitrogen vs Algal Biomass
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Algae arent all bad!

• Base of the food web
• Source of dissolved oxygen

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Impairments associated with excess algae

• Loss of aesthetic beauty
• Decreased recreation
• Taste and odor problems
• Increased cost to treat for drinking water
• Impacts on dissolved oxygen
• Toxins

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Transparency as measured by the Secchi Disk is
another response variable.
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Suspended soil materials also influence water
clarity in Missouris lakes.
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Where do the nutrients come from?
According to the EPA, the three top sources of
pollution impairing lakes and reservoirs in the
USA are
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Agriculture
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Municipal Point Sources
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Urban Runoff
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Miscellaneous Terms
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Trophic State Classification

• Oligotrophic low nutrients, low algal biomass,
  high clarity, dissolved oxygen throughout water
  column
• Mesotrophic moderate nutrients and algal
  biomass, some clarity

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Trophic State Classification

• Eutrophic rich in nutrients and algal biomass,
  turbid, loss of dissolved oxygen in lower layer
  during summer stratification
• Hypereutrophic very nutrient rich, algal
  biomass levels that have a negative impact on
  lake use

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Eutrophication The process of lake aging, in
which productivity increases overtime as the lake
becomes shallower.