An Ecological Study of the River Towy

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Abstract
The mainstream of this unpolluted river measures about 64 miles. The river has soft, nearly neutral water, a rather uniform and stable pebble-gravel bed and provides particularly good fishing for sewin. Trout and salmon occur also but course fish are rare. All observations were made during the summer months. During the dry and sunny summer of 1949 the river had a rich flora including about 130 spp. of algae. Green algae were particularly abundant, predominant types being Oedogonium, Spirogyra and Scenedesmus, and the desmids were well represented. In 1950 the summer was cool and wet and the flora appeared to be dominated by Stigeoclonium, Ulothrix and diatoms. The summer fauna included about 106 spp. Characteristic animals in 1949 were Ephemerella notata, Glossosoma boltoni, Leuctra fusciventris, and the minnow Phoxinus phoxinus, Tanytarsus larvae of the group Rheotanytarsus occurred in great numbers. In July 1950 quantitative sampling of the pebble bed at four stations indicated populations of 319 to 1091 macroscopic animals per 2500 sq. cm. Severe and prolonged flooding later in the summer appeared to have a most destructive effect on the fauna; samples taken in Sept. indicated populations of 40 to 57 animals per 2500 sq. cm. The gut contents of minnows varied greatly with the size of the fish and the habitat from which they were taken; a marked tendency towards a vegetable diet was noticeable, of the 350 fish examined 210 having a stomach contents which was mainly vegetable and only 76 a diet in which animal food was predominant. The most important food of some of the more numerous invertebrate animals was green algae and diatoms. Certain spp., such as Ephemerella notata, seem to favor the filamentous algae while others, such as the Rheotanytarsus larvae, prefer non-filamentous forms.