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Wetland Symposium

Spatial and Temporal Variability in Chemical Characteristics and Water-Column Biota in the Cottonwood Lake Area Wetlands, North Dakota


JAMES W. LaBAUGH AND GEORGE A. SWANSON

U.S. Geological Survey, Water Resources Division, Mailstop 413, Building 53, Lakewood, CO 80225; U.S. Fish and Wildlife Service, Northern Prairie Wildlife Research Center, Route 1, Box 96C, Jamestown, ND 58401

Two seasonal wetlands (T3, T8) and three semipermanent wetlands (P1, P8, P11) were examined to determine the role of ground-water interaction and hydrological variability on chemical and biological characteristics of the wetlands. In some of the years between 1984 and 1992, the seasonal wetlands did not contain water and on occasion the semipermanent wetlands became dry. Specific conductance was lower in seasonal wetlands, (<0.2 mS•cm-1 [T8]), and greatest in semipermanent wetlands (maximum of 43 mS•cm-1 [P11]). Major ion concentration and relative abundance of major ions were related to each wetland's position in the ground-water flow system. Within individual wetlands, specific conductance varied considerably, up to tens of mS•cm-1 in P11, in response to changes in hydrological conditions.

In contrast to major ion concentrations, both total and ortho phosphorus concentrations were greater in the seasonal wetlands than in the semipermanent wetlands. Vegetation in the shallow-marsh zone of the seasonal wetlands and leaching from standing dead plants in that zone provided sources of phosphorus not always present in the open-water zones of the semipermanent wetlands. There was considerable seasonal variability in phosphorus within each wetland, but no distinct pattern was evident. Resuspension of sediment also contributed to seasonal variability. The importance of sediment as a source of particulate and dissolved phosphorus was evident in P11, where, following desiccation, much sediment was lost due to deflation by wind, causing phosphorus concentrations to be lower than in previous years, when the wetland did not become dry.

Clear distinctions between wetlands could not be made based on concentration of total nitrogen, which often varied more spatially within a wetland than temporally over a season. Dissolved inorganic nitrogen usually was a small fraction of nitrogen content of water in the wetlands. Most of the dissolved inorganic nitrogen in the wetlands was ammonia. Nitrate and nitrite commonly were below detection levels, particularly in wetlands T3 and T8. The differences between ammonia and nitrate plus nitrite suggest nitrification was inhibited in the wetlands.

In the semipermanent wetlands, the amount of algae in the water column, measured as chlorophyll-a, declined after spring thaw in April, usually remained small through mid-summer, and increased in September or October. No seasonal pattern was evident in the seasonal wetlands. Between 1984 and 1992, values ranged from less than 1 mg•m-3 in all five wetlands to 380 mg•m-3 in wetlands T3 and P11. Large values of chlorophyll-a in the semipermanent wetlands usually coincided with periods when migrating waterfowl were present. Years with smaller values were those in which the wetlands occasionally became dry. The algal taxon representing the largest fraction of algal biovolume varied within and among wetlands. Often the most abundant taxa were crytophyta in wetlands P1 and P8, chlorophyta and diatoms in wetland P11, and various taxa in wetlands T3 and T8. Chrysophyta were occasionally abundant only in T3 and T8.

Copepods were the most numerous microinvertebrates in all five wetlands, which had markedly different species composition. Microinvertebrates populations larger than 2,000 individuals/liter characterized the seasonal wetlands. In the semipermanent wetlands, populations were commonly less than 1,000 individuals/liter. Seasonal and annual changes in microinvertebrate abundance were not related to algal abundance except in T8, suggesting that in these wetlands bacteria may have been more important than algae as a food resource for microinvertebrates.


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