Dependence of Long-Term Dynamics of Zooplankton in the Ob River on Interannual Changes in Hydrological and Hydrochemical Parameters

Here we summarize a long-term study on qualitative and quantitative composition of zooplankton (Cladocera, Copepoda, Rotifera) in the Ob River. We carried out these investigations at 13 sampling stations of the Middle and Low Ob in the years 1994, 1996, 1999, 2001, 2002 and 2009. It was found that the species richness of all zooplankton and abundance of cladocerans and rotifers is significantly determined by the temperature conditions of the month preceding sampling. In contrast, among other factors, we revealed that pH decreases as well as phosphate and nitrate concentrations increase zooplankton abundance. Dissolved oxygen and oxidizable organic substances (BOD5) were positively correlated to copepod population levels (according to abundance and biomass); on the other hand, an increase in difficult-to-oxidize substances (COD) inhibited their development. During this study, we found that high water levels had a positive influence on zooplankton richness in river itself probably due to being downstream from Ob River floodplain lakes.