Northern Prairie Wildlife Research Center
Classification of Natural Ponds and Lakes
in the Glaciated Prairie Region
Plant Species Composition and Differences in Salinity of Surface Water
Important differences in species composition of the
characteristic plant associations within zones are
correlated with differences in average salinity of surface
water. Distinctive associations of plants may be
classified as fresh, slightly brackish, moderately
brackish, brackish, subsaline, or saline, although
measurements of specific conductance (micromhos/cm3)
used to indicate differences in salinity of water were
found to fluctuate widely within many ponds or lakes.
Reduction in specific conductance appeared to be
related to dilution caused by increasing water depth and
occasionally to periodic overflow. Rising water levels
result from accelerated inflow from surface runoff in
combination with precipitation directly on ponds. An
increase in specific conductance was usually associated
with losses in water depth caused by evapotranspiration, or with a greater inflow from ground-water seepage or springs resulting from rising water tables. Many of the
plant associations that are indicative of average
differences in salinity also persist temporarily over
widely overlapping ranges of salinity. Since unstable
water conditions are characteristic of most prairie ponds
and lakes, plant associations proved to be more reliable
indicators of average salinity than single measurements
of specific conductance.
Fluctuations in specific conductance are less
pronounced during average or normal water conditions
than during periods of excessive water depth or extreme
drought. The approximate normal and extreme ranges in
specific conductance (micromhos/cm3) of surface water
in plant communities that are indicators of differences in
average salinity are as follows:
| Plant Community | Normal Range | Extreme Range |
| Fresh | <40-500 | <40-700 |
| Slightly brackish | 500-2,000 | 300-2,200 |
| Moderately brackish | 2,000-5,000 | 1,000-8,000 |
| Brackish | 5,000-15,000 | 1,600-18,000 |
| Subsaline | 15,000-45,000 | 3,500-70,000 |
| Saline | 45,000-100,000+ | 20,000-100,000+ |
The influence of salinity and other environmental
factors, including grazing, mowing, and burning, on
vegetation of prairie ponds and lakes is treated in more
detail by Stewart and Kantrud (in press).
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U.S. Geological Survey