Abundance, Population Dynamics and Production of Zoobenthos in Lake Mývatn, Iceland

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Abstract
The macrozoobenthos in Lake Myvatn, sampled from 1.5-4 m depth at 6 stations during 1972-1974 and at 1 station in 1975-1976 produced 20 spp. [Hydra attenuata, Chaetogaster spp., Nais spp., Tubifex tubifex, Peloscolex ferox, Lumbriculus variegatus, Helobdella stagnalis, Procladius islandicus, Macropelopia nebulosa, Ablabesmyia monilis, Arctopelopia griseipennis, Orthocladius oblidens, Pogonocladius consubrinus, Cricotopus sylvestris, C. tibialis, Psectrocladius barbimanus, Chironomus islandicus, Tanytarsus gracilentus, Radix peregra, Pisidium spp.] averaging 64,500 individuals m-2. Three species, Tanytarsus gracilentus (Holmgren), Chironomus islandicus Kieffer and Tubifex tubifex (Muller) dominated the benthos comprising 79.4%, 9.3% and 7.2%, respectively, of total numbers. C. islandicus has a 2 yr life cycle but occasionally part of a generation develops in 1 yr. T. gracilentus has 2 generations a year. Growth of C. islandicus and T. gracilentus was measured and the production calculated. Production of the remaining species was calculated from their mean biomass. The mean production of the total benthos community was 28.4 g ash-free dry wt, .apprx. 670 kJ m-2 yr-1 during 1972-1974. T. gracilentus, C. islandicus and T. tubifex contributed 67.1%, 23.6% and 2.7%, respectively, to total benthos production. Large annual fluctuations both in quantity and production occurred during 1972-1974. These were mainly due to presence or absence of C. islandicus and T. gracilentus. In 1975-1976 both C. islandicus and T. gracilentus disappeared from the station sampled. No satisfactory explanation of these fluctuations was found.