Direct and Indirect Effects of Solar Ultraviolet Radiation on Attached Bacteria and Algae in Lotic Systems

Abstract

I examined the effects of solar ultraviolet radiation (UVR) on attached bacteria and algal densities in lotic systems in outdoor artificial stream apparatus. Flumes were covered with four types of film for UVR screening treatments, and attached bacterial cell densities and their temporal variations were compared between conditions excluding and including solar UVR. Attached bacterial cell densities were depressed by solar UVR, and both accrual rate and saturated density were significantly lower in the +UVR (full solar radiation) condition than in −UVR and dark conditions. Solar UVR also indirectly affected the rate of algal accrual. Microscopic direct observations of attached bacterial cell density and algae on substrata showed that solar UVR depressed the accrual of attached bacteria and consequently the frequency of sites with high bacterial cell density that can trap suspended algae in the water. The final amount of algal accrual in the +UVR condition was one-fourth of that in the −UVR condition. Therefore, the effects of solar UVR may be more serious in systems where periphyton are frequently removed by floods.