Artificial substrates colonized by diatoms from Yaquina Estuary, Oregon, were exposed to solar visible radiation and three levels of ultraviolet radiation (UV-B, 290-320 nm). Flow-through microcosms were constructed inside a glasshouse to serve as chambers for the artificial substrates. The artificial substrates were sampled during three spring/summer experiments. Chlorophyll a concentration, biomass (ash-free dry weight), primary productivity (radiocarbon uptake), and community composition were the parameters measured biweekly for each four-week experimental period. The results indicated that daily exposure to enhanced levels of UV-B radiation was associated with a decrease in species diversity of benthic diatom assemblages during all experiments after four weeks of growth.