In order to make the data comparable from year to year, only data collected between June and August of each summer have been included in these tables and figures. The interpretation of these data needs to consider four major sources of error and uncertainty:
(a) data are available for only a limited number of years;
(b) water quality data reflect both short term, e.g., changes in annual precipitation, and long term, e.g., changes in land use, trends;
(c) different analytical techniques were used in various studies; and
(d) the lakes differ in area, mean depth and other physical characteristics.
The importance lake characteristics for interpreting water quality data can be seen by examining data for West Lake Okoboji and East Lake Okoboji. Their watersheds are roughly the same size and are estimated to deliver about the same mass of nutrients (total nitrogen and total phosphorous) into each lake annually. The volume of water in West Lake Okoboji, however, is much larger than in East Lake Okoboji. Consequently, nutrients entering West Lake Okoboji are diluted more than those entering East Lake Okoboji. Because East Lake Okoboji is a relatively shallow lake, nutrients that have accumulated in its sediments, especially phosphorous, can more readily re-enter the water column (internal recycling) when its sediments are resuspended in the water column by water currents caused by high winds or boat traffic. Consequently, the higher nutrient and chlorophyll levels and lower Secchi Disc readings in East Lake Okoboji are largely due to its smaller water volume and shallower depth.
NOTES: The best overall indicator of water quality is the Secchi Disc depth. This is the maximum depth at which a standard Secchi Disc (a 20 cm diameter white and black disc) can no longer be seen in the water column when viewed from above on a sunny day. The greater the depth (given in meters, 1 m is a little over 3 ft), the better the water quality (clarity). Chlorophyll A is a measure of the abundance of algae in the water column: the higher the Chlorophyll A, the lower the Secchi Disc reading.
Historic data for East Lake Okoboji collected by Gary Phillips and his students at Iowa Lakes Community College:
(a) Chlorophyll A Data
(b) Total P Data
Complete data for West Lake Okoboji:
(a)
Chlorophyll A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
CLAMP data for East and West Lake Okoboji:
(a)
Chlorophyll A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
CLAMP data for Little and Big Spirit Lakes:
(a)
Chlorophyll
A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
CLAMP data for Upper and Lower Gar and Minnewashta:
(a) Chlorophyll
A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
CLAMP data for other lakes:
(a) Chlorophyll
A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
Data for all lakes in the region:
(a)
Chlorophyll A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
Data for all the lakes in tabular form:
(a)
Chlorophyll A Data
(b)
Total P Data
(c)
Total N Data
(d)
Nitrate Data
(e) Secchi
Disc Data
For more information on CLAMP water quality data, contact the Lakeside Administrative office.
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