Primary production studies on a new reservior; Bighorn Lake‐Yellowtail Dam, Montana, U.S.A. *

Summary A definite algal succession pattern was established for all 3 years of study. Volume‐based phytoplankton density and chlorophyll concentration decreased down‐reservoir. However, the depth of the euphotic zone increased down‐reservoir as silt settled out. Consequently, the euphotic zone standing crops were greatest mid‐reservoir. Insufficient light penetration was established as the principal limiting factor to primary production in the upper end of the reservoir. Decreased primary production in the lower end of the reservoir did not appear to be due to nutrient limitation. Comparison of sampling periods common to all years of the study showed that estimated net primary production increased 84% in 1970 over 1968.