Dynamics of plankton runoff in the Upper Ob

River runoff is usually understood as runoff of water, suspended solids, and sediments. However, river runoff includes both abiotic and biotic components. Drifting living organisms appear in the riverbed or enter there from floodplain reservoirs and tributaries. They exist in runoff constantly (e.g. phyto- and zooplankton) or at certain stages of their life cycle (e.g. drifting larvae of benthic invertebrates). The quantitative assessment of volumes of river-transported biotic components as well as studying their dynamics contribute to the knowledge about the features of structural-functional organisation of aquatic ecosystems and their productivity, in particular. Nevertheless, modern studies on biotic runoff are scanty; they are mainly focused on studying river estuaries and assessing water and substances carryover to the World Ocean. Short-term studies of river runoff in the Upper Ob basin were implemented only in 1962-1963. It is obvious that these investigations can hardly give an idea of seasonal and long-term trends in flow changes as well as factors crucial for these dynamics. The aim of the work is to study the intra-annual dynamics of phytoplankton (in terms of chlorophyll content) and zooplankton runoff in the Upper Ob as well as to identify longterm fluctuations in annual plankton runoff. In 2001-2002, the research on the Ob River was carried out every ten days, including the freeze-up period, in 2012-2018 - weekly during the open water period, and 2-3 times per season during freeze-up. Sampling was made by means of scooping from the surface water layer at three stations across the river. Phytoplankton was concentrated on membrane filters with a pore diameter of 0.8 microns. To analyse the chlorophyll content in acetone extract, we employed the spectrophotometric method. Zooplankton was concentrated by straining 100 liters of water through a small Apstein net (a mesh size is 62 microns) concurrently with the measurement of water temperature and transparency using a Secchi disk. For runoff quantification, we applied the data on daily water discharge. These data were preliminary adjusted with due regard for our own measurements. At the observation section, we revealed some differences in water characteristics at the site near the right bank, especially during flood recession. For instance, flows from the vast right-bank floodplain formed a water flow along the right bank during these periods. Therefore, instead of arithmetic averages for three stations, the values weighted average by the site width were calculated with consideration of the share of the right-bank flow in the total one. To calculate runoff of components, their concentrations (biomass) were restored (for every day between sampling days) based on linear interpolation. To identify the factors determining features of long-term dynamics of biotic runoff, some hydrometeorological characteristics were calculated for each year of the study (average air temperature, annual water runoff, average water discharge, duration of spring floods, and significant rain floods). In the course of investigations, daily water runoff during the year had mainly two peaks during the first (April) and second (late May-June) waves of spring-summer flood. Seasonal dynamics of phytorunoff (in terms of chlorophyll a content) had several maxima. The first occurred in April during active snowmelt and intensive surface flush, The Authors declare no conflict of interest.

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