Variation pattern of crustacean zooplankton in a bigheaded carps-dominated reservoir

The relationship between crustacean zooplankton and (planktivorous) fish has not been well clarified in bigheaded-carps-dominated reservoirs, for most of the past studies to explore that kind of knowledge above are often obtained from the studies in temperate lakes and/or shallow lakes. The limitation of these studies is that crustacean zooplankton lack of either large grazers such as the bigheaded carps or the vertical migrating space for zooplankton to avoid predation by planktivorous fish. Reservoirs are more and more important drinking water source for human consumption, so a better environmental management of reservoirs will rely on a better understanding on the relationship between zooplankton and fish in bigheaded carp dominated reservoirs. To help reach that goal, we investigated the structure and variation of zooplankton and studied the relationship between zooplankton and fish in Tangpu reservoir, Zhejiang province, China. Our results showed that the species of crustacean zooplankton were richer. A total of 59 species of crustacean zooplankton were detected, including 33 species of copepods and 26 species of cladoceran, and dominated by larger species. The density of crustacean zooplankton varied between 15.5–68.5 ind./L with an annual average density of 24.9 ± 14.4 ind./L, it is higher in this planktivorous carps dominated deep reservoir than in many carp-less-dominated reservoirs or in shallow lakes. We also found that zooplankton and fish had a differential vertical distribution, i.e., zooplankton occurred 5 m–10 m but fish occurred 0.5 m–10 m in many months of the year. This could well explain why zooplankton were not reduced and not dominated by small species in bigheaded carps dominated reservoir, so we concluded that in planktivorous carp dominated deep reservoir ecosystems, zooplankton might not compete directly with planktivorous carps for phytoplankton, instead by distributing or migrating to deeper water column, they may survive and thrive on the phytoplankton in deeper column and the detritus resulted from the feces of planktivorous fish, thus might be able to coordinately improve water quality with planktivorous fish.