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The general biology and production ecology of the crustacean zooplankton of Loch Leven, Kinross, has been studied as a contribution to a major International Biological Programme - Freshwater Productivity study. The zooplankton populations were sampled on a routine basis throughout the period September 1971 - January 1974 and data was collected on all the major population parameters of the two co dominant species, Daphnia hyalina var lacustris Sars and Cyclops strenuous abyssorum Sars, including changes in numerical densities of the various age and/or size groups, brood-sizes, body-sizes, horizontal and vertical distributions, cyclomorphotic changes in Daphnia and gut-contents analyses. The numerically subordinate species, Diaptomus gracilis Sars, Leptodpra kindti (Focke) and Bvthotrephes longimanus Leydig, were monitored solely in terms of numerical-densities. Field observations were also made on water temperatures and water transparency; and additional laboratory work involved the assessment of growth-rates at various temperatures and the evaluation of a length-weight relationship for Daphnia, the resultant information being used in conjunction with appropriate population data to calculate biomass and production using a method based on the concept of finite growth-rates. The work somewhat emphasised Daphnia due to its important role in secondary production and because this species had never before been studied in detail at Loch Leven. Cyclops strenuus abyssorum, which exclusively dominated the loch plankton within the years 1966-69, had been previously studied during 1969. The seasonal changes in the above parameters are discussed and compared and contrasted with similar populations from other waters and also with the earlier work at Loch Leven. The annual mean biomass and total annual production for Daphnia were found to be 0.57 gC/m2 and 13.68 gC/m2/year respectively in 1972; and 0.50 gC/m and 15.64 gC/m2/ 2 year in 1973. For Cyclops these were 0.50 gC/m and 4.45 gC/m2/year in 1972; and 0.95 gC/m and 9.45 gC/m / year in 1973. The zooplankton was assessed within the context of the Loch Leven ecosystem, with special emphasis on trophic interactions, and a detailed discussion of the peculiar post-war zooplankton species changes which had occurred at Loch Leven is presented.