Massive floating and benthic mats of the filamentous blue-green alga,Lyngbya wollei, occlude small lakes and shallow coves of large reservoirs, especially in the southeastern United States.In situstanding crops (up to 6.66 kg fresh weight m−2), seasonal vertical distribution (surface, benthic and mid-depth suspended) and metabolic characteristics ofL. wolleimats in Marten's Pond, S.C. are described and compared with midsummer infestations in three other southeastern impoundments. The dense floating mats which are the most visible manifestation of infestations limit irradiance available to subsurface filaments; less than 2% of incident irradiance penetrates surface mats. Surface filaments containing minimal chlorophyll a and abundant myxoxanthophyll are photosynthetically inactive, whereas subsurface filaments having greater concentrations of phycobilins and chlorophyll a are most active in photosynthesis.L. wolleiis perennial because virtually all of the summer biomass overwinters on the lake sediments. The bulk ofL. wolleibiomass (> 64%) in Marten's Pond remained subsurface throughout the year.L. wolleipersists and grows in this subsurface environment due to the abundant phycobilin pigments, adapted for interception of a broad spectrum of low intensity light and the low photosynthetic light compensation point (32 μ Einsteins m−2sec−1) of healthy filaments.