Light intensity and the vertical distribution of algae in tertiary oxidation ponds

Abstract

Studies on the use of oxidation ponds for the tertiary treatment of municipal waste waters had shown that solar heating results in thermal stratification. Under these conditions there is probable short circuiting of flow. Installation of weirs to minimize stratification by forcing an under-over flow pattern was followed by the development of two distinct algal associations as a result of the skimming action of the surface weir. Upstream of the surface weir a thick algal mat developed which consisted of an interlocking complex of cysts of Euglena rostifera Johnson and filaments of Oscillatoria tenuis. Downstream of the skimming weir, the surface of the pond was free of the mat but deep green in appearance. This color was due to a population of a single species, Euglena rostifera. The green color of the clear pond areas was observed to change considerably in intensity in the course of the diurnal cycle. Detailed investigation of this phenomena revealed a vertical migration rhythm of the Euglena associated with the diurnal cycle. Examination of the diurnal migration and associated factors showed the following: 1. (1) The Euglena moved from the bottom sludge, where it settles at night to levels in the pond where light intensity measurements did not exceed 75 c ft −2. 2. (2) Diurnal temperature changes did not affect the vertical migration of the Euglena. 3. (3) Under the algal mat, consisting of Oscillatoria tenuis and encysted Euglena rostifera, there was no vertical migration. Isolation and growth of single cell isolates of E. rostifera showed that optimum growth was at the same light intensity range found in the ponds where the maximum populations were located. At higher light intensities the proportions of chlorophylls a, b, and c decreased and carotenoid pigments increased. Growth in total darkness produced complete loss of chlorophylls but also a sharp increase in carotenoid pigments.