Water Clarity Testing

Note:  This activity is often presented in combination with the Plankton Net Trawl
 
The Secchi disk test, is a simple measurement of water clarity - the physical conditions of the water.  Your instructor will provide a brief introduction of the Secchi disk, and will ask team members to discuss what the Secchi disk directly measures (clarity of water) and what that observation infers (how far sunlight travels in the water, and concentrations of plankton and suspended sediment, generally.)  Teams will be asked to make a prediction of how far down, the Secchi disk might travel before it diissapears. 
 
Procedure: 
Two Secchi disks will be available, so teams may divided into pairs for this activity.  One person will lower the disk and one person will hold the reel.  Two set of hands on the line at all times.  .
  • Each pair will choose a location to work from on the boat (may be opposite sides). They will need to consider wind direction- disk should not drift under boat, and line should not play out at more than a 15 degree angle in any direction. 
  • Teams will lower the disks slowly into the water, looking straight down and noting the change in appearance of the disk as it descends into the water.
  • Line is marked in half-meter intervals; pairs will keep track of how many marks go under the surface.
  • When the disk disappears, teams will carefully raise and lower the disk to come to consensus on the exact depth at which is no longer observable.  This will be the Secchi Disk Depth.  Test may be repeated as needed.
  • Disks will then be retrieved, with lines carefully rewound, and stowed for the next team.

 

Observations/Discussion
Teams will record their findings on the lab sheet.
  • Be sure to note time, field conditions
  • Each team should record both individual readings for comparison
Are all readings the same?  Why/why not?    What factors might influence how well you can observe the disk?  What factors impact actual water clarity?  Are they different?
  • Consider reflected sunlight and waves, which impact your observation, not the actual clarity of deep water. 
  • Is recording an average Secchi depth appropriate?
What is ACTUALLY being measured?   Emphasis- we are not assessing depth or “pollution” levels.  This is a measure of light penetration. 
  • We will figure out that light goes down from the water surface to the Secchi disk, reflects off the disk, and returns to the surface.  Thus, we know light can travel at least twice as far as the direct Secchi reading.  Because our eyes are not highly sensitive to light, a rule of thumb is that light attenuation limits plant growth at a depth about three times the Secchi reading.
  • By completing the Secchi test, we are gaining an understanding of an environment that we cannot directly observe- we know where the light stops, and where photosynthesis stops- also, water heating is greatly diminished below the sunlit zone… which provides clues as to where we want to di other tests- plankton trawl and/or water chemistry sampling.
What factors INFLUENCE clarity of water?  (biological organisms or silt/sediment are the main ones).  What events might cause either of these factors to change?  What factors ARE EFFECTED by clarity?  Do we expect Secchi Clarity to change over the year?  Why?
 
NEXT ACTIVITY:  Plankton Trawl.
 
Further Applications
  • Seasonal cycles and responses to specific weather events. 
  • Trophic Status indicator and applications. 
  • Historical patterns
 
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