[en] The possibility that pollution might deplete the stratospheric ozone layer and intensify solar uv at the earth's surface focuses attention on the role of solar uv in the various ecosystems at the earth's surface. Previous studies suggested that solar uv might contribute to bacterial die off in wastewater and the studies reported here were directed toward elucidating the action of solar uv in natural waters. It was assumed that solar uv action on aquatic ecosystems can be evaluated (using proper models) on the basis of the following four independently measurable quantities: (1) the intensity of solar uv at the water surface; (2) the attenuation of the uv in the water column, (3) the position of the critical organism in the water, and (4) the sensitivity of the individual organisms to solar UV exposure. These four factors were investigated on a continuing basis and the results of measurements were utilized along with special field and laboratory experiments to assess uv-B actions