Comparative limnology of a deep‐discharge reservoir and a surface‐discharge lake on the Madison River, Montana

SUMMARY. Phytoplankton standing crop, primary production, light penetration, temperature, and various chemical concentrations were measured in a man‐made, deep‐discharge, reservoir and in a natural, surface‐discharge, lake in order to relate limnological conditions in the two bodies of water to their depth of outflow. The quantity and depth distribution of heat stored during the summer varied markedly. The reservoir functioned as a heat trap, whereas heat was readily dissipated from the lake. Salinity increased more in the lower layer of the lake. Throughout the summer, nutrient‐rich water was discharged from the reservoir whereas nutrient‐poor water was discharged from the lake. Phytoplankton standing crops were greater in the lake and were dominated by flagellates and diatoms. In the reservoir, blue‐green algae were predominant. Rates of primary production and respiration were higher in the lake, but estimated algal turnover times were faster in the reservoir. It was concluded that depth of outflow has a direct and predictable effect on certain physical and chemical conditions within these two bodies of water; but effects of discharge depth on phytoplankton were secondary and thus difficult to ascertain.