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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