Assessment of the relationship between chlorophyll content and biogenic elements in large lowland resevoirs

In the second half of the 20th century, with increased rates of eutrophication, universal quantitative relationships between the content of biogenic elements and chlorophyll (Chl) were obtained, which serves as a marker of algae biomass, as well as an indicator of the trophic status of water bodies. Most of these studies were performed on lakes, however, the patterns and ratios found for lakes were not always observed in reservoirs, young ecosystems with high development dynamics. Long-term studies of phytoplankton productivity are carried out in the reservoirs of the Upper Volga. These studies have shown that a direct relationship of Chl with total nitrogen (Ntot) and phosphorus (Ptot) is rarely found and is at low correlation coefficients. A more successful attempt was to indirectly assess this relationship using the ratios of Chl/Ptot and Chl/Ntot, which was named “phytoplankton response” (Vinberg, 1987) corresponding to the term “efficiency” (Kalf, Knoechel, 1978). Our first data on Chl/Ptot and Chl/Ntot in the Upper Volga reservoirs refer to 1980-1990 (Mineeva and Razgulin, 1995; Mineeva, 1993; Mineeva, 2004). At present, new data have been obtained, which have made it possible to follow long-term trends in the development of phytoplankton in the reservoirs of the Upper Volga in connection with the availability of biogenic elements under variations in the trophic status of the reservoirs. Samples were collected in August 2015-2018 at 26 stations in three large reservoirs of the Upper Volga (56°51'-58°22'N, 35°55'-38°25'E): Ivankovo, Uglich and Rybinsk reservoirs. Chlorophyll content was determined by the fluorescence method (Gol’d et al., 1986) in integral water samples taken from the surface to the bottom. Data on biogenic elements obtained at the same time by Dr Irina Stepanova are given in our joint publication (Mineeva et al., 2021). The standard software packages for a personal computer were used for statistical processing (calculation of mean values and standard error of the mean (M±mM), correlation and variation coefficients, regression equations and graphing). In this research, we revealed that nutrient content in the Upper Volga reservoirs varied within similar limits, the average Ntot/Ptot ratio was the same, and the average values of Ptot and Ntot decreased in Rybinsk reservoir. Chl content was typical of the summer phytoplankton maximum in the Upper Volga reservoirs (See Table 1). Ptot content did not change much in Ivankovo and Uglich reservoirs, but in Rybinsk reservoir it increased in 2015 and decreased in 2016. The more stable content of Ntot decreased a little in the cool 2017. Chl concentrations of 20-30 μg/L obtained in Rybinsk reservoir in 2015 and 2018, in Uglich reservoir in 2017 and in Ivankovo reservoir in 2015 correspond to the eutrophic category; concentrations of 35-52 μg/L (Rybinsk reservoir in 2016, Uglich reservoir in 2015, 2016 and 2018, Ivankovo reservoir in 2016-2018) - to hypertrophic category, and only in 2017 in Rybinsk reservoir did they decrease to a moderately eutrophic level of 13 μg/L (See Fig. 1). It was found out that under these conditions Chl content per unit of biogenic elements varied in a wide range: Chl/Ptot from 0,03 to 2,13, Chl/Ntot from 2,55 to 123, with maximum values in the highly eutrophic Shosha reach in Ivankovo reservoir (August 2018). Chl/Ntot slightly differed in Ivankovo and Uglich reservoirs, but increased in Rybinsk reservoir; Chl/Ptot decreased in the range of Ivankovo, Uglich and Rybinsk reservoirs (See Table 1). Most of both ratios belong to narrower limits: 0.10-0.50 for Chl/Ptot (67% of all values) and 10–40 for Chl/Ntot (51%) (See Table 2). The average Chl/Ptot ratio for the entire data set is 0.43±0.03, and Chl/Ntot is 29.8±1.9 with variation coefficients of 77.0 and 63.9%. The interannual changes (See Fig. 1) show that all the maximum ratios were obtained in 2016, while the minimum in Ivankovo reservoir was in 2015, in Rybinsk reservoir in 2015 and 2017 and in Uglich reservoir in 2017 and 2018. Our results demonstrate that both ratios correlate with each other (See Fig. 2), as well as with the Chl content (See Fig. 3). The correlation of both ratios with Chl weakens in Ivankovo reservoir, probably due to the abundant development of macrophytes. We can observe the same in all reservoirs in the cool 2017 with a decrease in ChlCyan (the abundance of cyanoprokaryotes) and an increase in the proportion of ChlBac (the abundance of diatoms). Both ratios are insignificantly dependent on hydrological factors as well as on Ptot and Ntot content, but the situation can change in individual reservoirs and in different years of observation (See Table 3). As it was demonstrated, both ratios increase with the growth of trophy estimated by Chl. They do not significantly differ in mesotrophic and moderately eutrophic waters, but become much higher in eutrophic and highly eutrophic waters, indicating a more efficient consumption of nutrients in the latter. At the same time, Ptot and Ntot change little along the Chl gradient (See Table 4). Chl/Ptot decreases with the increase in Ptot and grows slightly with the increase in Ntot; Chl/Ntot does not change over the entire range of Ptot and Ntot; but both ratios vary along the gradient of Ntot/Ptot (See Table 5) that serves as an indicator of the biogenic limitation of phytoplankton and a factor regulating the development of algae. A retrospective analysis shows significant interannual fluctuations of both ratios in Rybinsk reservoir and a tendency to increase them in Uglich and Ivankovo reservoirs (See Fig. 1). Thus, Chl/Ptot and Chl/Ntot ratios are useful for analyzing the relationship between the development of phytoplankton and nutrient supply that is necessary in order to assess the efficiency of their use and their availability to algae cells. The results obtained allow us to conclude that phytoplankton in the Upper Volga reservoirs is less sensitive to the presence of nitrogen and more dependent on phosphorus.

Pigment composition of phytoplankton and its long-term dynamics in reservoirs of the Upper Volga

According to field observations during 1995–2005 and 2012–2016, the pigment composition of phytoplankton in the Upper Volga reservoirs is given. With an average Chl a content of 13,1±1,4 to 52,5±2,7 μg/L in the Ivankovo reservoir and from 4,1±0,9 to 27,5±2,5 μg/L in the Uglich reservoir, an increase in Chl a was noted after an abnormally hot 2010. The content of Chl a in the Ivankovo reservoir retaining eutrophic status does not go beyond the limits of long-term fluctuations. In the Uglich reservoir, an increase in the absolute and relative amount of Chl a was revealed, indicating the transition of the reservoir to the eutrophic category.

Regularities and Factors of Formation of Winter Hydro/chemical Regime of the Uglich Reservoir

The article presents the results of the study of the winter hydro/chemical regime of the Uglich reservoir which is the second stage of the Volga-Kama cascade of reservoirs. In winter period the main influence on the formation of the hydrochemical regime of the Uglich reservoir is exerted by water coming from the Ivankovsk reservoir which located upstream. It was found that water in the reservoir is hydrocarbonate, mineralization varies from 243 to 351 mg per liter, calcium and magnesium concentrations reach 60.9 and 17.5 mg per liter. Concentrations of mineral phosphorus in the reservoir are 0.043–0.076 mg per liter, total gross phosphorus in the upper relief of the Uglich HPP reach 0.138 mg per liter, nitrate concentrations are 0.14–0.84 mg per liter.The main indicators of hydro/chemical regime of the Uglich reservoir have spatio-temporal variability. Its change from year to year, as well as from target to target. Global climate changes have not yet had a significant impact on the winter hydro/chemical regime of the reservoir.

The Volga River Reservoirs Soil Complex Spatial-temporal Transformation. Communication 3. Assessment of the Morphological/metric Characteristics Variations as a Result of the Bottom Sediments Accumulations in the uglich Reservoir.

Comparison of assessment methods of the reservoirs’ hydro/morpho/metric characteristics changes due to accumulation of bottom sediments and abrasive/erosion processes have been done. The results of the Uglich reservoir bottom sediments monitoring have demonstrated that the morphological/metric characteristics practically have not changed over the 70-yeat period of the exploitation, the Kalyazin lake/river area being the only exclusion. The areas with the transformed soils left as they were, while the areas with sand drift decreased, the areas with clay sediments decreased, and wetlands appeared. Spits, sand-bars, sprinkles and banks that divide a part of water area have been formed at open shallow sites and river mouths. The reservoir hydro/ morphological /metric characteristics have been assessed by the results of monitoring soil surveys in situ, that is, by direct measurements of the bottom sediments thickness and by the bottom topography digital model that had been carried out independently in the Institute of Inland Waters Biology and State Oceanographic Institute. Over the whole period of the reservoir exploitation 28.6 million m3 of bottom sediments have been accumulated, while the water volume loss was 22 million m3. The discrepancy was about 0.6 %. Both methods have given good results and are entitled to further use in researches. Deposits distribution has been mapped. Sedimentation volumes and rates have been calculated, suspended matter balance has been produced, and situation forecasts of the main ground types and bottom sediments formation ways have been made by the 100the anniversary of the reservoir exploitation.