Long-Term Dynamics of Chlorophyll in Plankton of Different Sites in a Large Plain Reservoir

On the basis of field observations in 2009–2019, the seasonal and long-term dynamics of chlorophyll at six standard stations of the Rybinsk Reservoir is considered. The fluorescence method is used to determine chlorophyll. The average chlorophyll content at stations varies from the minimum 3.5–10.8 to the maximum 16.9–40.5 μg/L in different years and from 12.0 at station 5 (Sredny Dvor) up to 21.6 μg/L at station 2 (Mologa) on average for the entire period. The seasonal dynamics of chlorophyll is characterized by spring; summer; and, in some years, autumn maxima. With the same type of seasonal dynamics, the change in chlorophyll at the stations is to varying degrees coupled in the years of observation and is most often correlated in neighboring areas. Significant differences in the amount of chlorophyll at the stations are observed during surveys carried out within one day and are determined by the complex hydrological structure of the reservoir; active dynamic processes; and, in the spring, by the thermal regime. The presence of stable large circulation zones smoothes out the spatial differences of chlorophyll, the average seasonal concentrations of which do not differ significantly at all six stations in years with an average water level, and at four stations of the Main Reach in extremely high-water years. A close correlation in long-term dynamics is revealed for the average chlorophyll concentrations for the growing season at six stations, as well as at each station and the reservoir as a whole. The results of the work confirm the reliability of the data obtained for assessing the ecological state of the Rybinsk Reservoir.

THE EFFECT OF CLIMATE CHANGE ON THE THERMAL REGIME IN THE RYBINSK RESERVOIR

According to the data of the Rybinsk Hydrometeorological Observatory (HMO) and archival data of the Laboratory of Hydrology, Papanin Institute for Biology of Inland Waters, Russian Academy of Sciences, the seasonal variability of the main climate parameters, air temperature (Rybinsk HMO) and water temperature in the Rybinsk Reservoir have been studied at the modern stage of global warming (2001–2019). Over the period of 1976–2019, the rate of an increase in the average annual air temperature was 0.50°C /10 years in the littoral zone of the Rybinsk Reservoir. Changes in the timing of the onset and end of the climatic seasons of the year and an increase in their duration have been determined. It is found that during the modern period the average surface air temperature was higher than the climatic norm in all months of the year (1960–1990). During the growing season (conventionally May–October), its maximum increase was recorded in July, 1.5°C, May and September, 1.2°C. It is shown during the modern period of intensive global warming the average decadal water temperature in the spring, summer, and autumn seasons increased compared to the norm. The maximum positive anomalies were recorded in the second-third decade of May, 2.8–2.3°C and July, 2.0°C. According to observations at the standard stations, significant differences were recorded in water mass heating: in July in anomalously warm summer of 2010 the average water temperature was 27°C in the surface layer and 18.5°C in the near bottom layer; in the cold summer of 2017, the temperature was 18.5 and 16.0°C, respectively. The monitoring data on the water temperature in the reservoir indicate an increase in the number of years with anomalous thermal conditions as a result of climate change. The earlier temperature stratification of water masses (the end of May) and decrease in the difference in the temperature between the surface and near-bottom water layers have been observed.

The results of calibrating satellite altimetry measurements from the space geodetic system “GEO-IK-2” in the water area of the Rybinsk reservoir

The authors present the results of satellite altimetry measurements calibration from the space geodetic system “GEO-IK-2” in the water area of the Rybinsk reservoir, obtained through two independent methods. In the first case, the altimetry measurements were calibrated by comparing the sea surface heights with respect to the reference ellipsoid, which were calculated from radio altimeter measurements, and from the processing of ground GNSS measurements performed at the sub-satellite point at the time of passing the GEO-IK-2 spacecraft. For this, in the summer of 2020, special geodetic measurements were carried out in the water area of the Rybinsk reservoir. The GNSS receiver was mounted on the Sea Buoy, which was towed by boat to the specified sub-satellite points, the coordinates of which were predetermined from the forecast of satellite orbits. At the same time, base stations for GNSS observations were installed on the coast. The largest difference in the sea surface height does not exceed 13,9 cm. The second method is cross-calibration by radio altimeter measurements from other altimetry spacecraft. In this case, the tracks of the first and second satellites within a relatively short time interval cross one common point, at which the sea surface height is determined. For this, the Jason-3 spacecraft was selected, the track of which was crossed by the GEO-IK-2 vehicle track on September 11, 2020 over the water area of the Rybinsk Reservoir. The difference in sea surface heights was 9,6 cm.

SPECIFIC FEATURES OF FEEDING EUROPEAN VENDACE COREGONUS ALBULA (L.) IN RYBINSK RESERVOIR

The article describes Coregonus albula (L.) inhabiting the open pelagial of the Main reach and the river reaches of the Rybinsk Reservoir together with other fish species that form the feeding colonies of planktophagous fish. The feeding spectra of vendace for the summer-autumn period are determined. Mass species of cladocerans are present in the diet of fish feeding in the productive biotopes of the main reach of the reservoir, in the zones of stable high concentrations of feed invertebrates in the summer, during the maximum development of zooplankton. In this period of the year there dominate Bosmina zooplankton (the relative significance ratio in nutrition (IR) varies from 67.2 to 98.5%), Daphnia species (IR 98.1%) and Bythotrephes species (22.1%). The diet of vendace is more diverse in the estuaries of large tributaries of the river reaches. There they feed on the larvae and adults of amphibiotic insects Chironomidae, Simuliidae and Syrphidae, whose larvae live on the bottom, while adult insects keep a ground-air lifestyle. An invasive amphipod species Gmelinoides fasciatus presents in the diet of vendace in the near-dam zone of the reservoir in autumn. The appearance of benthic organisms in the nutrition of vendace in the autumn period coupled with a significant decrease of plankton invertebrates in the diet may be explained, on one hand, by a seasonal decrease in the biomass of zooplankton in the reservoir’s pelagial and search for new forage places, and, on the other hand, a food competition from the another plankotophage, the Ponto-Сaspian kilka. Despite the fact that vendace is morphologically adapted to feeding on small invertebrates in the water column, it demonstrates a certain degree of plasticity in the selection of food organisms in the Rybinsk reservoir, and its feeding spectrum changes depending on the season and productivity of the reservoir

WATER BALANCE AND ECOLOGICAL CONDITIONS OF THE RYBINSK RESERVOIR IN THE EXTREMELY HIGH-WATER YEAR OF 2017

This study analyzes the inflow, runoff and water level of the Rybinsk Reservoir during the extremely highwater year of 2017. It was the second-highest on record high-water year in the reservoir basin during the whole period of the reservoir operation. The total annual inflow to the reservoir significantly exceeded its long-term average and amounted to 52,148 km3. Such an increase in the inflow is due to rising air temperatures in winter time under the increasing influence of global warming and high rainfall. To characterize ecological conditions in the reservoir we use data on its water balance, mean ten-day period water temperature in May-October provided by Yaroslavl Center for Hydrometeorology and Environmental Monitoring and materials from six hydrological and hydrobiological expeditions conducted by researchers from the Institute for Biology of Inland Waters RAS (IBIW RAS) in May-October, 2017. During the expeditions to the reservoir, integrated hydrological, hydrochemical and hydrobiological investigations were conducted at standard stations. These studies have been carried out by IBIW researchers since 1962 to monitor the dynamics of ecological conditions of the waterbody.