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Diatoms are a major food source for aquatic grazing snails and insects, thereby contributing to aquatic food webs. Diatoms are microscopic and are seldom visible; an exception is colonies of Didymosphenia geminata. Didymosphenia secrete long extracellular stalks that can form mats several centimeters thick. Mats occur on single rocks or may cover areas of a streambed. Recently, this species has expanded from its original boreal or alpine, oligotrophic habitat and is appearing in blooms on rocky habitats below dams in regulated rivers. In April 2009, Didymosphenia geminata was collected in the Mountain Fork downstream of Broken Bow Lake, where colonies up to 13 cm in diameter were observed. This is the first record of this species in Oklahoma and is the southernmost known occurrence in the United States. Oklahoma is outside the modeled area of potential occurrence and the artificially low temperature maintained for the trout fishery likely enabled the establishment of Didymosphenia. Didymosphenia mats are unsightly, alter benthic communities, and may affect fish growth, condition, and spawning. In response to the establishment of Didymosphenia, public education, a surveillance program, and local ecological studies are recommended.