Trophic status of three large Missouri River reservoirs

Probability-based surveys conducted between 2001-2004 characterized the three large reservoirs of the Missouri RiverLake Oahe, Lake Sakakawea, and Fort Peck Lakeas mesotrophic to eutrophic, phosphorus (P) limited, and generally supporting cold water habitat (bottom waters <15°C and dissolved oxygen [DO] concentrations >5 mg/L) in midsummer. Riverine zones were shallower, warmer, more eutrophic, and had lower DO and higher suspended matter concentrations than lacustrine zones. Similar, although more variable, differences were found between bays and open-water areas. Between sampling years, water levels decreased in each reservoir. In the first year of sampling, area-weighted mean reservoir trophic status index based on chlorophyll (TSI _chl) was about 37 in all three reservoirs. Sixty percent of Oahe and Sakakawea and 40% of Fort Peck had TSI_chl > 50. Trophic status index based on Secchi depth (TSI_SD) averaged about 50 in each reservoir across years. Because mean TSI_chl < TSI_SD, light attenuation was considered to be silt, not algae, dominated. Trophic status index based on total P (TSI_TP) and the ratio of N:P concentrations indicated that the reservoirs were very P limited. Mean bottom temperature and DO concentration in Oahe were unchanged between years at about 19C and 7.5 mg/L, respectively. Bottom temperatures in Sakakawea increased (from 15°C to 21°C) and DO concentration decreased (from 7.3 mg/L to 6.0 mg/L) with lower water levels. In Fort Peck, bottom temperature remained about 18C, but DO concentration fell from 7.23 mg/L to 4.96 mg/L. Results show that surveys successfully characterized important environmental conditions throughout these large reservoirs.