Phytoplankton Primary Production and Light in Waldo Lake, Oregon

Abstract

ABSTRACT Waldo Lake is located in the Willamette National Forest along the west side of Oregon's Cascade Mountain Range. The lake is regarded as one of the most oligotrophic lakes in the world, chemically similar to distilled water, with very low rates of phytoplankton primary production and concentrations of chlorophyll a. Historically, vertical light transmission was considerable, with 1% of the lake's incident solar radiation penetrating to depths greater than 60 m. The lake remains exceedingly clear, as demonstrated by transmissometer profiles and Secchi-disk readings that range between 35 and 40 m. Deep chlorophyll maxima are usually found between 60 and 100 m where measurable light is less than 1% of surface illumination. Conversely, primary production rates and assimilation numbers are maximum in the lake's upper 20 m. Unusually large assimilation numbers in the upper 20 m suggests that the phytoplankton that occupy this stratum are photosynthetically efficient, capable of high rates of production with minimal chlorophyll. There are indications, however, that downward irradiance of blue, green and red light has shifted toward a condition in which the attenuation of blue light has increased more than green and red light. This may be attributed to an increase in phytoplankton density, expressed as a roughly 20-fold increase in phytoplankton primary production over the past 25–30 years. On this basis, Waldo Lake may have shifted from an ultraoligotrophic condition to a more eutrophic state.