Arbor Lake Water Quality

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Arbor Lake Water Quality

• External Internal Nutrient Loading

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IOWATER Monitoring Sites

• This map shows the locations and names of sites
  being monitored by IOWATER volunteers.
• Water samples have been analyzed monthly from the
  principal inlets and the one outlet since October
  of 2003.
• Hazel Creek Wood Creek enter the lakes north
  end while Hobo Drain Hobo Creek enter the east
  arm of the lake.

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External Phosphorus Loading

• Phosphorus concentrations (mg/L) are shown for
  each of the principal inlets and the single
  outlet. The total range is shown by the vertical
  bar and the 25th and 75th percentiles are
  represented by the box.

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Weighted Avg. Phosphorus Load

• Table I shows the average Phosphorus
  concentrations and average flow rates of the
  principal inlets and the single outlet.
• From this, a weighted average percent P. load can
  be computed for each inlet, and the sum of the
  inlets can then be compared with the outlet.
• Five times as much Phosphorus enters the lake as
  exits the lake.

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Table I - External Phosphorus Load
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Lake Stratification

• In Summer the lake is stratified into a mixed
  layer of warm water on top of a cooler layer with
  a thermal gradient.
• Algal blooms in the photic zone deplete
  dissolved nutrients.
• Algae die and sink into the thermocline where
  bacterial decomposition releases nutrients and
  depletes oxygen.
• The result is a concentration of Phosphorus in
  the lower stratum of the lake.

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Deep Sampling

• In the summer of 2006 the deeper part of the lake
  was sampled using a 10 ft. length of PVC pipe
  inserted through a styrofoam buoy and tethered to
  a canoe.
• Water was withdrawn from the pipe using a vacuum
  pump designed to siphon gas from the tank of a
  car.
• The volume of the pipe was computed to be 0.6
  Liter. Several volumes were removed prior to
  taking the sample.
• The resulting data confirms that nutrient
  enrichment accompanies oxygen depletion in a
  stratified lake.

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Lake Volumes

• The hypsograph is derived from the bathymetric
  map and is used to compute volumes shown in Table
  II.
• 75 of the lake volume lies above 2.5 meters
  depth.

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Table II Lake Volume