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 (N_tot) and phosphorus (P_tot) is rarely found and is at low correlation coefficients. A more successful attempt was to indirectly assess this relationship using the ratios of Chl/P_tot and Chl/N_tot, which was named “phytoplankton response” (Vinberg, 1987) corresponding to the term “efficiency” (Kalf, Knoechel, 1978). Our first data on Chl/P_tot and Chl/N_tot 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±m_M), 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 N_tot/P_tot ratio was the same, and the average values of P_tot and N_tot decreased in Rybinsk reservoir. Chl content was typical of the summer phytoplankton maximum in the Upper Volga reservoirs (See Table 1). P_tot 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 N , decreased a little in the cool 2017. Chl concentrations of20-30 |rg/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/P_tot from 0,03 to 2,13, Chl/N_tot from 2,55 to 123, with maximum values in the highly eutrophic Shosha reach in Ivankovo reservoir (August 2018). Chl/N_tot slightly differed in Ivankovo and Uglich reservoirs, but increased in Rybinsk reservoir; Chl/P_tot 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/P_tot (67% of all values) and 10-40 for Chl/N_tot (51%) (See Table 2). The average Chl/P_tot ratio for the entire data set is 0.43±0.03, and Chl/N_tot 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 Chl_Cyan (the abundance of cyanoprokaryotes) and an increase in the proportion of Chl_Bac (the abundance of diatoms). Both ratios are insignificantly dependent on hydrological factors as well as on P_tot and N_tot 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, P_tot and N_tot change little along the Chl gradient (See Table 4). Chl/P_tot decreases with the increase in P_tot and grows slightly with the increase in N_tot; Chl/N_tot does not change over the entire range of P_tot and N_tot; but both ratios vary along the gradient of N_tot/P_tot (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/P_tot and Chl/N_tot 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. The paper contains 3 Figures, 5 Tables and 31 References. The Author declares no conflict of interest.

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