POPULATION DYNAMICS,LIFE CYCLES,SECONDARY PRODUCTION AND TROPHIC BASIS ANALYSISOF TWO MAYFLIES IN HEIZHUCHONG STREAM

Mayfly plays an important role in material cycle and energy flow in riverine ecosystem.During June 2003 to June 2004,an investigation on production and its trophic basis of dominant species of mayfly was carried out in a second-order river of the Hanjiang River Basin.There were six types of habitats to sample.Station 1 (S1)was in the middle of the stream with big round rocks in the upper reach of the stream,station 2 (S2)in the inner part of a weir with cobble substratum,station 3 (S3)in the middle of a peacefully flowing section with gravel-sand bottom,station 4 (S4)at the side of the riparian with aquatic macrophytes and snags,station 5 (S5)in the middle of the riffle and station 6 (S6)at the lower reach with a sewage outlet ca 100 meters up-per.At each station,two quantitative samples were taken with a 167μ m D-frame kick net or a Surber net,and the samples weresieved with a 167μ m net,sorted in a dish with naked eye, fixed in 10% formalin for later processing.The life cycle of Lep-tophlebia sp.and Ephemera sp.was analyzed by the monthly size frequency method,and then cohorts and annual production wereestimated.The results showed that Leptophlebia sp.and Ephemera sp.both developed two generations per year.The pupation ofLeptophlebia sp.mainly occurred in fall and winter; while pupation of Ephemera sp.mainly in summer and winter; the abundanceand biomass of the Leptophlebia sp.reached theirhighest peaks (913 ind/m2,0.9920g/m2)in October and August, respectively,while the abundance and biomass of the Ephemera sp reached theirs (107ind/m2,2.3639g/m2)in June and March.The cohortproduction and cohort P/B ratio of the Leptophlebia sp.population were 19.0181 g/m2· a and 5.7,the annual production and P/B ratio were 38.0362 g/m2·a and 11.4.The cohort production and cohort P/B ratio of the Ephemera sp.population were38.0159g/m2·a and 5.9,the annual production and P/B ratio were 76.0318g/m2·a and 11.8.Gut content analyses showed thatboth Leptophlebia sp.and Ephemera sp.consumed a large portion of amorphous detritus, constituting 81.7%and 94.6% of theirdiets and contributing 59.2% and 84.9% to the secondary production.Diatoms were the second important food type of Lep-tophlebia sp.,averaging 16.8%of themass and contributing 36.4%to its production,while diatoms merely comprised 1.5%ofthe diet of Ephemera sp.and contributed 4.1%to the production.