Devils Lake, in northeastern North Dakota, is a closed-basin lake characterized by large fluctuations in water level and in concentrations of dissolved chemical constituents. A study was conducted to assess spatial and seasonal variability in water-quality conditions in Devils Lake during September 1988 through October 1990.

Specific conductance, which generally increased from west to east in Devils Lake and East Devils Lake, ranged from 3,580 to 20,100 microsiemens per centimeter. pH, water temperature, and dissolved oxygen generally were similar among sites. Devils Lake generally does not undergo thermal stratification during open-water periods but does undergo inverse thermal stratification in the winter. Vertical variability of water-column properties during open-water periods generally was small, but near-bottom dissolved oxygen was less than near-surface dissolved oxygen during summer sampling periods. The potential exists for establishment of near-bottom anoxia during the summer and during the winter.

Concentrations of all major ions generally increased eastward through Devils Lake and East Devils Lake, but sodium, sulfate, and chloride were enriched relative to the other major ions.

Dissolved-solids concentrations varied both spatially and seasonally. Median dissolvedsolids concentrations generally increased from west to east. Dissolved-solids concentrations generally were largest in the winter when ions were concentrated because of ice formation and smallest in the spring when water was diluted because of icemelt, surface-water inflow, and seasonal precipitation. Dissolved-solids concentrations generally increased in the summer and fall when evaporation exceeded surface-water inflow and precipitation. Although the dissolvedsolids concentration of Devils Lake generally fluctuates inversely with water level, the dissolvedsolids mass generally fluctuates directly with water level. During periods of extremely low water levels, dissolved solids may be lost from Devils Lake.

Concentrations of nutrients in the euphoric zone varied spatially and seasonally. Dissolved nitrite and dissolved nitrite plus nitrate concentrations generally were below the detection limit of 0.10 milligram per liter. Total ammonia and dissolved ammonia concentrations generally were below 0.10 milligram per liter, except during winter sampling periods. Total nitrogen concentrations were extremely large and generally were similar to concentrations reported for hypereutrophic systems. Total nitrogen concentrations commonly were between 1.0 and 8.6 milligrams per liter. Total ammonia, dissolved ammonia, and total nitrogen concentrations generally were larger at shallower sampling sites than at deeper sampling sites; sediment-water interactions may influence concentrations of nitrogen species. The ratio of sediment surface area to lake volume for a given part of the lake may partly explain spatial variability in concentrations of nitrogen species.

Total nitrogen concentrations generally were smallest in the fall, increased in the winter, remained the same or decreased slightly in the spring, and increased again in the summer. Phosphorus concentrations generally were smallest in the fall, increased in the winter, decreased in the spring, and increased again in the summer.