DIEL STUDY OF LEMOLO LAKE, DOUGLAS COUNTY, OREGON

As a component of a program of environmental studies associated with r elicensing of the North Umpqua Hydroelectric Project, limnological dat a were routinely collected in Lemolo Lake, a 419 acre reservoir in Dou glas County, Oregon, from 1992-1994. In addition to the periodic data, an intensive, short term study was conducted to provide information o n diel changes in the reservoir during a period of heightened primary production. This paper describes the results of the diel study. Data c ollected over a 24-hour period consisted of in situ profiles, continuo us monitoring, and sample collection in both the reservoir and the Nor th Umpqua River approximately 800 meters downstream of the dam. Data c ollected in the reservoir were indicative of eutrophic conditions, and nitrogen limitation. Chlorophyll a concentrations for samples collect ed August 11 and 12, 1993 were 28.4 and 33.4 mu g/L, respectively; as compared to 1 mu g/L in July. Phytoplankton biomass was dominated by t he blue-green algae Anabaena. Secchi depth was markedly reduced during the study period, and epilimnetic pH remained near 9.0. This study hi ghlighted the existence of a surface layer in Lemolo Lake which can be come highly productive during the summer months. Withdrawal of hypolim netic water for power generation and instream flow creates lentic cond itions on top of a more lotic hypolimnion. Overall retention of the re servoir is short (15 days), but slow growing cyanobacteria and large b odied cladocerans are indicative of longer retention of Lemolo Lake su rface waters. The ability to remain buoyant in the water column, and t o thrive in a nitrogen poor system, are particularly important advanta ges for Anabaena in Lemolo Lake.