Crater Lake is a pristine oligotrophic lake possessing unique aquatic ecology. Extensive moss mats discovered in 1989 led to a focus of their impact upon the bio-systems of the lake. The identification of two distinct moss mat populations, a productive verdant community and an ancient sediment laden community, raised questions concerning their longevity and growth. The moss mats grew at depths from 25 meters to 140 meters, notably absent in the shallowest 20 meters of the lake. This study examines the influence of UVB, light incidence, and depth on growth rates of the predominant species in the verdant mats. Considerable growth was observed under the effects of all experimental treatments, showing negligible UVB impact and an increasing growth rate with decreasing light incidence. Growth declined in deeper depths (80 m) where light incidence is 1% and chlorophyll/ production maxima occur.