Reconstruction of the past plankton community in Lake Biwa using Daphnia resting eggs

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To clarify changes in the plankton community for the past 100 years in lake Biwa, we performed paleolimnological analyses on cladoceran remains and genetic analyses on daphnia resting eggs in die lake sediments. Dating of the lake sediments was determined using 137Cs and 210Pb. For genetic analyses, we developed a new method to extract DNA efficiently from the resting eggs of Daphnia species. Identification of Daphnia species was then made based on sequences of mt DNA 12S rRNA and nuclear DNA-ITS regions with PCR-RFLP method.The results revealed historical changes in the zooplankton community. Before 1900, D. mendotae which have been thought as endemic species in Norm America were abundantly occurred and sole Hyalodaphnia species in this lake. After 1990, however, D. galeata was occurred and produced hybrids with D. mendotae. Thus, at present, Hyalodaphnia populations in Lake Biwa compose of D. mendotae, D. galeata and their hybrid. Detailed analysis of the present population showed th … More at D. mendotae were most dominant Hyalodaphnia species and that existence balance of these species (and hybrid) deviated from the Hardy-Weinberg equilibrium, suggesting that D. mendotae and D. galeata are isolated as single species within the lake. Other than these species, D. pulicaria occurred suddenly and abundantly in 1999. Genetic analyses showed that this population is genetically most close to those in lakes at west coast of North America, suggesting that D. pulicaria occurred in Lake Biwa is a recent invader from North America.Paleolimnological analyses on cladoceran remains further revealed that population density of Daphnia species (mainly D. mendotae) has increased, they did not produce resting eggs for the last 20 years. Experimental analysis indicates mat this decrease in resting egg production is not due to changes in water quality. Since Daphnia became to appear abundantly even in winter since 1980, this decrease in resting egg production seems to be due to a results of natural selection caused by changes in food and water temperature regimes in winter. These results suggest that the zooplankton community in Lake Biwa has largely changed for the past 100 years and has received frequently various type of environmental disturbances including eutroophication, species invasions and shift in water temperature regimes. Less