THE CAUSE OF A METALIMNETIC MINIMUM OF DISSOLVED OXYGEN1

Coincident with a condition of rapidly increasing eutrophication, Lake Washington, a large, deep lake, near Seattle, is exhibiting an increase in the magnitude of a metalimnctic depletion of dissolved oxygen. Possible causes of this depletion are examined, and the conclusion is that the respiration of a metalimnctic population of non-migrating copepods is the prime agent. Calculations arc prcscnted in support of this conclusion, and chemical properties of the notch arc compared with those of a hypolimnetic notch due to morphomctric causes, to provide further cvidcnce. The recent increase in magnitude of the phenomenon is considered to be due to the increase in numbers of copepods. On August 9, 1913, Kemmerer, Bovard, and Boorman ( 1923) made the first recorded determinations of dissolved oxygen in Lake Washington. At that time nearby Seattle was small (population about 250,000), and this large lake ( 8760 ha; mean depth 33 m; maximum depth 66 m) was oligotrophic. Today, Seattle is considerably larger (population about 75O,OOO), and Lake Washington is eutrophic. One of the most interesting changes associated with this sewage-generated eutrophication of the lake ( Edmondson et al. 1956) has been the continued increase in size of a metalimnetic minimum of dissolved oxygen. Most of the data to be presented concerning the minimum have been gathered as part of a study of this domestic eutrophication. Figure 1 illustrates the change over the last 25 years. The minimum usually appears at the beginning of June and grows progressively stronger until it disappears toward the end of October. The lowest dissolved oxygen concentration ever observed in the notch was 2.2 mg/L at 15 m on October 1, 1957. Although depletion proceeded at a faster rate in 1958 than in 1957, the lowest figure observed in 1958 was 2.9 1 This work has been supported by grants from the National Institutes of Health (Grant No. RG 4623, ) and the National Science Foundation (Grant No. G 6167). Their help is gratefully acknowledged as arc the suggestions and criticisms of Dr. W. T. Edmondson with whom I have been associated in this investigation. 2 Present address : The Johns Hopkins University, Dcpartmcnt of Sanitary Engineering and Water Resources, Baltimore, Maryland. mg/L at 11 m on August 18. This failure to fall below the 1957 value probably results from the windier conditions in 1958. The relatively rapid rate of increase of magnitude of the notch in recent years reflects the general pattern of the eutrophication of the lake. This phenomenon of progressive increase of an oxygen notch has been exhibited by other fertilized lakes (Ohle 1951; Thomas 1958)) but no satisfactory explanation has been put forward. The data on Lake Washington support an early hypothesis of Minder ( 1923 ) , and may help in understanding these other situations. CAUSES OF OXYGEN MINIMA There arc three possible causes of midwater oxygen minima: 1) interposition of water masses having low dissolved oxygen content between masses containing more dissolved oxygen; 2 ) morphological features of the basin associated with horizontal water movement; and 3 ) oxygen-consuming events occurring in situ. The first of these is relatively rare in lakes, although it probably occurs in the sea. Ellis ( 1940) concluded that such a situation occurred in Elephant Butte Reservoir, New Mexico, as a result of the interaction of three major factors: 1) flows produced by the draw-off at the foot of the dam; 2) disturbances caused by the shape of the impoundment, and deflections by a submerged obstruction; and 3) variability in the composition and volume of the waters entering the lower lake from the upper lake.