The Effect of Photosynthesis on the
Oxygen Balance in a Midwestern Stream
CLIFFORD A. MERRITT, Graduate Student
DONALD B McDONALD,   Associate  Professor
WAYNE L. PAULSON, Associate Professor
Department of Civil Engineering
University of Iowa
Iowa City, Iowa
INTRODUCTION
In considering the oxygen balance of surface waters, photosynthesis must be
accounted for. During the daylight hrs chlorophyll-bearing organisms may release
more oxygen than is needed to meet their own respiratory needs, the excess being
available to the remaining aquatic environment. However, during nighttime
conditions, these same organisms continue respiration which can amount to a significant oxygen sink.
It has been recognized that photosynthetic organisms can significantly influence the oxygen resource of a stream and in recent years a number of studies
dealing with methods to quantify the oxygen contributed by photosynthesis have
been undertaken. Goldman and Carter (1), in a Lake Tahoe study, used the
carbon-14 method. By measuring the rate of uptake of this isotope, rates of
photosynthesis were detected. Although a general lack of correlation between
photosynthesis and chlorophyll measurements has been observed, Bailey (2), in a
study of the Sacramento-San Joaquin Estuary, used a fluorometric chlorophyll
technique and obtained good correlation between chlorophyll concentrations and
dissolved oxygen measurements. Odum (3) presented a diurnal curve analysis for
the stream environment. By measuring the diurnal curve, and correcting for respiration and reaeration, the photosynthetic curve was obtained. O'Connell and
Thomas (4) studied the Truckee River below the sewage treatment plant outfall of
Reno, Nevada. In this study the bulk of the photosynthetic activity was carried
out by a large benthal population. The photosynthetic component was obtained
by first using a test chamber containing the benthal organisms and secondly by
measuring all other factors and computing the photosynthetic effect by the finite-
difference method. Hull (5), in a study of the Baltimore Harbor, used the light
and dark bottle method. He found that planktonic algae contributed a significant
amount of oxygen to the environment. Camp (6), in a study of the Merrimack
River in Massachusetts, also used the light and dark bottle method to evaluate
photosynthesis. Symons (7), in studies of various impoundments, has presented
limitations of the light and dark bottle technique. These are: 1) A high surface-
to-volume ratio which is not normal in the natural environment; 2) A relatively
long time is needed to detect DO changes, and results may not be applicable due
to the changing external environment; 3) Little or no mixing in the bottles interferes with the flow of food to and waste products away from the organisms and
tends to give conservative results; and 4) Samples withdrawn from the water may
cause damage to algae due to sunlight exposure. This can be avoided by filling
the bottle at night.
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