Effects of Macrozoobenthos and Zooplankton on the Occurrence of Yangtze Finless Porpoise Via the Bottom-up Force at a Confluence of Rivers

Recent study on the confluence of rivers showed that the fish community has significant effects on the occurrence of Yangtze finless porpoise (YFP). Macrozoobenthos and zooplankton, as food sources for fish, may also have significant effects on the occurrence of YFP via the bottom-up force. To understand the community structures of macrozoobenthos and zooplankton, and further test our hypothesis, field investigations were conducted on a monthly basis between September 2013 and August 2014 at the confluence of the Yangtze and Wanhe rivers and adjacent waters. The results showed that (1) A total of 31 species of macrozoobenthos were identified, and the density and biomass were 107.2 ind./m2 and 6.15 g/m2, respectively. More species were detected at the confluence (17 species) than at the adjacent waters because of the occurrence of a large number of Mollusca specimens. However, species number and density among all the sites had no significant differences. (2) Total species number of zooplankton was 64, with a density and biomass of 20.7 ind./m2 and 0.043 g/m2, respectively. Species number was the highest at the confluence (53 species), and the density and biomass were obviously higher at the confluence than in the other sites in May-July. However, there were no significant differences in species number, density and biomass among all the sites. (3) There were no significant correlations between the species number, density and biomass of macrozoobenthos and the five fish parameters that significantly affect the occurrence of YFP; however, significantly negative correlations were found between the species number, density and biomass of zooplankton and fish species richness. After comprehensive analyses, we concluded that both macrozoobenthos and zooplankton could not significantly affect the occurrence of YFP via the bottom-up force at the confluence.