Dynamics of Chlorophyll a Content in the Ob River and its Relationship with Abiotic Factors

Seasonal and interannual dynamics of phytoplankton content (chlorophyll a), organic matter, and nutrients, as well as water temperature and water transparency, have been analyzed for the weekly observations performed in the Ob River from February to November, 2012–2015. The period of intensive phytoplankton growth (>10 mg/m3 Chl. a) lasts 70–90 days at a long spring flood in average and high-water years; in the absence of flooding during a dry year it exceeds 110 days. Seasonal phytoplankton dynamics is characterized by two summer maxima in July–August and in September (Chl. a content ~25 mg/m3); during a dry year, there is also a peak in June (up to 37.3 mg/m3). Seasonal dynamics of Chl. a in summer–autumn low water is closely related to the water temperature; in high water and autumn low water it is closely related to water transparency; in summer it is closely related to BOD5; and, in the summer–autumn period, it is closely related with nitrate content. According to the annual average (5.7–11.1 mg/m3) and maximum (19.2–35.0 mg/m3) concentrations of chlorophyll a, the trophic status of the river corresponds to mesotrophic–eutrophic water and the water quality corresponds to the second class, i.e., “clean water.”