Artificial Eutrophication of Lake Washington1

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Abstract
Lake Washington has been receiving increasing amounts of treated sewage, and appears to be responding by changes in kind and quantity of biota. In 1933 and 1950 the dominant phytoplankton organisms were Anabaena and various diatoms and dinoflagellates, but in 1955, apparently for the first time, there was a large population of the blue‐green alga, Oscillatoria rubescens, a species which makes nuisance blooms in a number of lakes. A great increase in the hypolimnetic oxygen deficit is taken as evidence of increased productivity; the deficit was 1.18 mg/cm2/month in 1933, 2.00 in 1950, and 3.13 in 1955. There is a fairly close relation between the decrease of oxygen and increase in phosphate concentration in the hypolimnion between measurements, a much less close relation with the chlorophyll concentration in the epilimnion.