Dynamics of plankton runoff in the Upper Ob
Authors:   Institute of Water and Environmental Problems of the Siberian Branch of the Russian Academy of Sciences, Kotovshchikov Anton V. , Burmistrova Olga S. ,   Institute of Water and Environmental Problems of the Siberian Branch of the Russian Academy of Sciences, Dyachenko Alexander V. ,   Institute of Water and Environmental Problems of the Siberian Branch of the Russian Academy of Sciences
Published: October 2024

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 structuralfunctional 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, when mainly non-planktonic species and algal detritus prevailed. The second peak (from late May to early June) was usually owing to large volumes of water runoff, but not high phytoplankton concentrations. The third maximum took place almost annually during the greatest warming of water (from the end of June to the end of July), however, at a prolonged flood it may not be pronounced. In the studied river section, the formation of a late summer maximum of phytorunoff (from late August to mid-September) was common; in some years it could exceed the summer peak. In extremely low-water years, a high early-summer maximum of phytorunoff was, apparently, induced by the absence of a second wave of high water, which may be enhanced by favourable weather conditions providing earlier warming of the water. The share of phytorunoff during the open water period made up 97-98% of total annual flow. In general, seasonal dynamics of phytorunoff during high water depended on water discharge, whereas in a summerautumn low-water period – on phytoplankton content. Dynamics of zooplankton biomass runoff also had several peaks (with the major one in the second and third decades of June at spring-summer flood recession) caused by abundant zooplankton inflow from floodplain reservoirs; in the years with extremely high flood, it can reach hundreds of tons per day. Insignificant peaks of zooplankton runoff in mid-, late-summer were specific only for some years. In different years, annual algae runoff was similar (305- 405 tons of chlorophyll), thus indicating stability of autotrophic link development of the Upper Ob ecosystem in the long-term aspect. At the same time, the greatest phytorunoff was noted in the years with low- and medium-water content and the highest summer air temperature. Current annual flow of phytoplankton biomass in the Upper Ob is 2-3 times higher than 50 years ago that is evidence of the increase in productivity of this river ecosystem. As compared to algae, inter-annual fluctuations in zooplankton runoff in the Upper Ob are much more significant, i.e., annual indicators may differ by almost 20 times (0.5-5.8 thousand tons). Most annual zooplankton runoff is formed at flood recession during the June maximum due to its intake from floodplain reservoirs. Interestingly, maximum annual zooplankton runoff tends to appear in the years with minimum annual phytoplankton runoff. Our study demonstrates the dependence of seasonal dynamics of biotic runoff on the features of hydrological river regime. Changes in values of zooplankton and phytoplankton runoff depend on different factors. During spring-summer flooding, phytoplankton runoff is determined by water discharge, and by phytoplankton content – during summer-autumn low water. Maxima of zooplankton runoff during the year, mainly caused by floodplain waters inflow to the river, takes place in early summer when zooplankton runoff is much more influenced by zooplankton content than water volume at flood recession. Despite high biomass, low runoff of zooplankton during summer-autumn low water is a result of low water discharge. As compared to phytoplankton, annual zooplankton runoff varies more significantly (0.5-5.8 thousand tons) because of different formation of June maxima. The presented findings suggest the importance of biotic runoff calculations in assessing the relationship between dynamics of plankton communities from large rivers with hydrological and hydrometeorological factors.