Annual variation in biomass and the community structure of crustacean zooplankton over 5 years in Lake Toya, Japan

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Biomass and population dynamics of crustacean zooplankton were investigated in Lake Toya, oligotrophic caldera lake in Japan, during five years from 1992 to 1997. Zooplankton biomass increased in summer with increasing water temperature every year, and varied annually even if temperature and food conditions did not change so much. In 1992 and 1993, zooplankton biomass reached up to > 4 g dry-wt m^[-2], while it declined below 1 g dry-wt m^[-2] after 1994. This extreme change of the zooplankton biomass depends on succession of three dominant species from large ones, such as Daphnia longispina and Cyclops cf. sibiricus, to small ones, such as Eubosmina longispina and Bosmina longirostris. Analyzing population dynamics of the dominant crustaceans demonstrates that the year-to-year variation in the abundance is related to the death rate but not birth rate. Since chlorophyll a concentrations within the euphotic zone and the birth rates of each species were rather higher in low zooplankton biomass years, the higher death rates after 1994 is not considered to be attributed to food shortage. Two planktivorous fish, lacustrine sockeye salmon (Oncorhynchus nerka) and pond smelt (Hypomesus transpacificus nipponensis), were artificially introduced every year for the sake of commercial fisheries in the lake. Although the two fishes are expected to be the most important predator for the large crustaceans, D. longispina and C. cf. sibiricus, which are actually fed on by the two fishes, the annual catch of the fishes was positively correlated to the average biomass of the crustaceans during summer season. This conflicting result is considered to be attributed to less catch of the fishes with fishing regulation for a gill net used by fishermen due to lowering the growth rate of the fishes in the low zooplankton biomass years. Proliferation of young and/or small fishes, including young-of-the-year pond smelt, which cannot be caught with the gill net may play an important role for eliminating the large crustaceans. Our results suggest that excessively artificial introduction of planktivorous fish, especially pond smelt, limits the zooplankton production in oligotrophic lake such as Lake Toya, subsequently decreasing the growth rate of the fish, and that this negative feed-back finally enhances lowering the annual fish catch.

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  • Limnology

    Limnology 14 (1), 59-70, 2013-01

    Springer Japan

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