INTRODUCTION

Plant pigments are widely studied as indicators of the functioning of plant organisms, communities, and ecosystems (Vinberg, 1960; Swain, 1985; Phillips et al., 2008; Tishchenko et al., 2020). In hydrobiological works, the spatiotemporal distribution of pigments in the pelagic and benthal regions of water bodies is of paramount importance (Struktura …, 2018; Moir et al., 2018). Information on the content of plant pigments in BSs is necessary to restore the production indicators of water bodies and assess their trophic state (Makri et al., 2019; Bernat et al., 2020; Gushulak et al., 2021), as well as to study the impact of climate change on aquatic ecosystems (Rühland et al., 2015; Garnier et al., 2019; Hofmann et al., 2021; Rahaman et al., 2022). The indicator significance of sedimentary pigments for understanding the structural and functional organization of heterogeneous ecosystems is based, first and foremost, on the knowledge of the interaction between water masses and BSs under different hydrodynamic conditions (Alimov, 2000; Cabecinha et al., 2009; Krol et al., 2011; Cardoso-Silva et al., 2022; Wu et al., 2022). Plant pigments are least studied in the BSs of technogenic water bodies, among which small and very small reservoirs that are sources of drinking water predominate (Avakyan et al., 1987; Rumyantsev et al., 2021).

The Uvod Reservoir is a natural monument used to supply water to the population of Ivanovo oblast. The hydrological features of this reservoir are largely due to the influence of water masses irregularly supplied from the large Gorky Reservoir through the Volga–Uvod canal (Zakonnov et al., 2000; Markevich and Elizarova, 2000). Plant pigments in the BSs of the Uvod Reservoir have not previously been studied.

The purpose of this work is to reveal the content and distribution of sedimentary pigments and establish the trophic status of the small Uvod Reservoir with a heterogeneous hydrological regime in the reaches.

MATERIALS AND METHODS

BS samples were taken in 2012 using a GOIN-1 gravity tube from the upper 5-cm layer at stations located in six reaches of the reservoir: Kolbaskinskii (station 20), Ivankovskii (stations 10–15), Uvodskii (stations 18, 19), Tsentralnyi (stations 7–9), Krasotkinskii (stations 16, 17), and Priplotinnyi (station 1–3, 5) (Fig. 1). Pigments were extracted with acetone from samples with natural moisture and determined in the total extract by spectrophotometry using a Lambda 25 spectrophotometer (PerkinElmer, United States). Chl and Ph concentrations were calculated using the Lorenzen’s equations (1967). The ratio of carotenoids and Chl was judged from the ratio of optical densities at the maxima of the absorption spectrum of the corresponding pigments before (E480/E665 index) and after (E480/1.7E665a index) acidification of the extracts. The second indicator takes into account the presence of Ph and therefore reflects the values of the index, which is not affected by Chl destruction products. Calculated index E480/1.7E665a can be more correctly compared with the values known for planktonic algae with an insignificant share of Chl destruction. The type of BS was assessed according to the classification (Butorin et al., 1975), which takes into account particle size distribution, water content, dry volumetric mass, and OM concentration. Organic matter was determined by loss upon ignition (600°C); pigments were determined in the same way as in a number of works (Sigareva et al., 2004, 2013, 2022; Timofeeva et al., 2021).

Fig. 1.
figure 1

Map of standard stations (1–20) in Uvod Reservoir. Reaches: I, Kolbaskinskii; II, Ivankovskii; III, Uvodskii; IV, Tsentralnyi; V, Krasotkinskii; and VI, Priplotinnyi.

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The content of Chl in the plankton of the Uvod Reservoir was calculated from the concentration of Chl + Ph in the average annual sedimentation layer and the average depth. The calculation was based on the regularity revealed in the Volga reservoirs: the average content of Chl in the water column is close to the content of Chl + Ph in the average annual sedimentation layer (Sigareva et al., 2013). The trophic state of the reservoir according to Chl + Ph in BSs was estimated according to (Möller and Scharf, 1986): oligotrophic, <13; mesotrophic, 13–60; eutrophic, 60–120; hypertrophic, >120 µg/g d.s.

Statistical calculations were performed using MS Excel and Statistica 8.0 software. The variability of indicators was assessed by the coefficient of variation Cv. Regression analysis was used to assess the relationship between indicators. The significance of differences in mean values was assessed by Student’s t‑test (p < 0.05).

Brief description of the Uvod Reservoir. The Uvod Reservoir (57°07′00″ N, 40°51′00″ E) was created in 1937–1939 on the Uvod River, a tributary of the Klyazma River, belonging to the Oka River basin. The reservoir supplies 80% of the drinking water of the large industrial and textile city of Ivanovo (a population of >400 000 people).Footnote 1 The reservoir is elongated from north to south and in shape repeats the flooded channels of the Uvod, Kolbaska, and Krasotka rivers, with steep eroded banks and numerous bays along streams and ravines. In terms of geomorphology, the Uvodskii and Priplotinnyi reaches with an unexpressed littoral and steep banks resemble microcanyons with depths reaching 11–18 m. The Krasotkinskii and Tsentralnyi reaches are characterized by a flat bed, gentle coastal slopes, and a developed littoral. Depending on the water content of the year, the surface area of the reservoir varies from 10.4 to 17.3 km2. Total volume is 0.0685 km3 at the normal water level of 119.6 m of the Baltic system. The length of the reservoir is 19 km, the average width is 0.72 km, the maximum depth is 18.6 m, the average depth is 6 m, and the length of the coastline is 92.6 km. The area of shallow waters with depths of ≤2 m is 2.9 km2. Water transparency along the Secchi disk is ~1.5 m. Thickets of higher aquatic vegetation occupy 10.1% of the water area; hydrophytes dominate (Papchenkov and Markevich, 2003). The Kolbaskinskii reach is the most heavily overgrown. The reservoir is connected with the Volga River by a 78-km-long canal. The Volga waters enter the canal from the Gorky Reservoir downstream of the town of Ples. The hydrological regime is heterogeneous due to periodic fluctuations in the flow of water into the river Uvod and the irregular functioning of the channel (Zakonnov et al., 2000). In the absence of water inflow from the Gorky Reservoir, the flow in the Kolbaskinskii and Krasotkinskii reaches is zero; in the Uvodskii, Tsentralnyi, and Priplotinnyi reaches it is 7.23, 2.38, and 1.96 years–1 respectively. When the Volga water enters through the canal, the flow rate of the Krasotkinskii reach increases to 20.3 and that of the Priplotinnyi increases to 5.5 years–1. The reservoir is characterized by the intensive redeposition (transsedimentation) of alluvium in the channel and littoral parts. Sedimentation rates in the Uvod Reservoir (limits 1.4–4.6, average 2.6 mm/year) are similar to those in large Upper Volga reservoirs (2.1–2.9 mm/year) (Zakonnov, 2007). The area of the bottom is dominated by silt (50%), the share of sands is 30%, macrophyte sediments are 10%, and eroded and transformed soils are 10% (Zakonnov et al., 2000). The reservoir is exposed to anthropogenic heavy metal pollution (Dolotov et al., 2010).

The climate in Ivanovo oblast is temperate continental with cold snowy winters and moderately hot short summers. In the year of observations (2012), almost all seasons were characterized by higher (compared to the norm) temperature and increased precipitation with frequent showers and squalls, as well as an increase in wind activity up to 20 m/s.1

RESULTS

BSs were mainly represented by silts with a predominance of sandy ones (Table 1). The water–physical properties of BSs corresponded to their types and were characterized by low water content and high volumetric mass (Table 1). The exceptions were samples of peaty silt, which is characterized by high water content, low volumetric mass, and a high content of organic matter. The contribution of OM in samples of BSs varied from 1.2 to 47.3% of dry weight.

Table 1. Characteristics of BSs at the stations of the Uvod Reservoir in 2012
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Chlorophyll in sediments was degraded in all areas (Table 1); the products of its destruction, Ph, accounted for 73–100% (on average, 89.5 ± 1.4%) of the amount with undestroyed Chl, with little variability (Cv = 6.8%). The content of Chl + Ph varied over a wide range (1–159 µg/g d.s.), with an average of 62.6 ± 10.0 µg/g d.s. (Cv = 62%). The distribution of pigments concentrations over the area of the reservoir bottom is uneven (Table 1). Low concentrations of Chl + Ph were noted in sand deposits in the river section of the Uvodskii reach (station 18), as well as in the shallow Krasotkinskii reach (station 16), in which the flow rate increased most strongly during the operation of the Volga–Uvod canal. High values of Chl + Ph content are associated with clayey and peaty silts. In the deepest Priplotinnyi reach, all the studied stations were characterized by an increased content of sedimentary pigments. The mean concentrations of Chl + Ph in the BSs of the Krasotkinskii (21.5 µg/g d.s.) and Priplotinnyi (78.4 µg/g d.s.) reaches differ most strongly due to differences in water exchange. The concentrations of Chl + Ph per unit area of the bottom and OM are distributed similarly to pigments in the dry mass of the BSs (Table 1). The average content of plant pigments Chl + Ph for the reservoir in sand and silty sand is 16.2 ± 6.1 μg/g d.s., 18.0 ± 7.2 mg/(m2 mm), 0.55 ± 0.22 mg/g OM; in sandy and clayey silts, it is 78.8 ± 10.6, 47.0 ± 3.5, and 0.95 ± 0.06, respectively. In general, sands significantly differ from silts in terms of Chl + Ph content in dry sediment (p < 0.05, t-test 5.1). The content of Chl + Ph in the layer of deposits of natural moisture, taking into account the areas of different types of BSs, reaches 33.6 mg/(m2 mm).

The content of Chl + Ph in sediments is related to the characteristics of the BSs, water content (R2 = 0.86), and dry volumetric mass (R2 = 0.61), by a nonlinear dependence: positive in the first case and negative in the second (Fig. 2). A close relationship is also observed between the concentrations of pigments and OM for the entire dataset (R2 = 0.61). The exclusion of samples of peaty silt enriched with hard-to-mineralize organics leads to an increase in the bond strength (R2 = 0.91).

Fig. 2.
figure 2

Relationships between the content of plant pigments and water content (a) and dry volumetric mass of the BSs (b). On the abscissa axis: (a) water content and (b) dry volumetric mass; along the y axis, Chl + Ph, µg/g d.s.

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The ratio of carotenoids and Chl, index E480/E665, usually used to assess the functional state of plant organisms and communities, varied from 1.34 to 4.79 (Table 1). In sands, this index (2.66 ± 0.75) was characterized by higher values than in silts (1.71 ± 0.06), probably due to the faster Chl degradation in aerated sandy areas (Table 1). Index E480/1.7E665a, which takes into account the effect of Ph on the ratio of carotenoids and Chl, varied from 0.71 to 2.82. Average values of index E480/1.7E665a (1.19 ± 0.11) in BSs is significantly less (p < 0.05, t-test 3.2) than the original index E480/E665 (1.90 ± 0.19), and closer to those in plant communities. For example, in the phytoplankton of the Volga reservoirs, the index E480/E664 (similar to E480/E665) reaches only 0.8–1.3 (Mineeva, 2004).

In production work on phytoplankton (Behrenfeld et al., 2005) and on algal cultures (Kovaleva and Finenko, 2019), the ratios of the concentrations of organic carbon and Chl are studied. An estimation of the values of this indicator in BSs complements the understanding of the causes of its variability in ecosystems. In the Uvod Reservoir, the C/(Chl + Ph) ratio in sandy and clayey silt varied within 400–900; in peaty silt, 2300–2600; and, in clay and coarse sand, 4000–5900. The average for all samples was 1300 ± 400. Generally, C/(Chl + Ph) depends not only on the degree of Chl destruction, but also on the concentration of organic carbon and type of BSs (Fig. 3).

Fig. 3.
figure 3

Correlations of C/(Chl + Ph) with the content of pigments and organic carbon in the Uvod Reservoir. The C/(Chl + Ph) ratio is along the y axis; along the abscissa, (a) Chl + Ph, mg/g d.s., for all samples; (b) OM, mg/g d.s., for all samples; (c) Chl + Ph, mg/g d.s., without peaty silt; and (d) OM, mg/g d.s., without peaty silt.

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The restoration of the Chl concentration of phytoplankton in the Uvod Reservoir, based on the content of Chl + Ph (33.6 mg/m2 mm) in the surface layer of the BSs, the depth of the reservoir 6 m, and the average annual sedimentation rate of 2.6 mm/year, showed that the average long-term Chl content in water reaches 14.6 µg/L.

DISCUSSION

The productive properties of aquatic ecosystems depend on a complex of biotic and abiotic factors, including the geomorphology of the water body, conditions in the watershed, and climate dynamics (Alimov, 2000; Struktura …, 2018; Rahaman et al., 2022). Hypotheses have been put forward that, with a decrease in the size of a reservoir, carbon accumulation increases (Winslow et al., 2015) and the length of the food chain decreases (Post et al., 2000). The study of the content and spatial distribution of sedimentary pigments in the Uvod Reservoir supplemented the ideas about the productivity of small water bodies (with an area of ≤20 km2) with a heterogeneous hydrological regime due to unique features. Thus, the geomorphology of the reservoir is characterized by the presence of areas of the “canyon” type (with a poorly developed littoral, steep banks, and deep channel) and, the hydrodynamics, by the contrast flow (from zero to 20.3 years–1) due to the irregular operation of the Volga–Uvod canal; the structure of the bottom sediment complex is due to redeposition of alluvium.

The geomorphological parameters of the Uvod Reservoir contribute to the overgrowth of the reservoir with hydrophytes, in contrast to the large Volga reservoirs, which are overgrown mainly with helophytes and hygrohelophytes (Papchenkov and Markevich, 2003; Structura …, 2018). It can be expected that the pigment indicators of hydrophytes (Sigareva and Timofeeva, 2023), which are involved in the formation of BBs, contribute to a slight decrease in the content of Chl + Ph, an increase in the relative content of carotenoids, and an increase in the C/(Chl + Ph) ratio in sediments.

The production indicators of plankton and benthos are ambiguously related to hydrodynamics. For example, at a high flow velocity in the upper reaches of reservoirs, a decrease in the concentration of pigments in phytoplankton and BSs is usually noted (Mineeva, 2004; Sigareva et al., 2013; Timofeeva et al., 2021). However, in the Uvod Reservoir, the phytoplankton biomass increases (from 1.18 to 2.51 g/m3) when the operating channel provides an increase in turbulence and flow (Zakonnov et al., 2000; Markevich and Elizarova, 2000). The average concentration of sedimentary pigments exceeds that in the large Cheboksary and Gorky reservoirs with water exchange rates of 20.9 and 6.1 years–1, respectively (Sigareva et al., 2013; Timofeeva et al., 2021), which indicates the absence of a clear relationship between the average content of Chl + Ph in BSs and water exchange. At the same time, in each of the considered reservoirs, an increase in the concentration of sedimentary pigments was noted from the most flowing upper sections to the near dams.

The results suggest that the features of the Uvod Reservoir contribute to a “leveling” of the values of the production indicators of the benthal. A sign of leveling out can be considered a decrease in the gradient of pigment concentrations between sandy and silty sediments: in the Uvod Reservoir, the content of Chl + Ph in silts is 4.7 times higher than in sands, as opposed to the reservoirs of the Upper Volga, where it reaches 24 (Sigareva and Timofeeva, 2001). Hydrodynamic activity, the functioning of planktonic and benthic communities, climate change, and eutrophication can be factors in smoothing out of the heterogeneity of the spatial distribution of sedimentary pigments (Fisher et al., 1980; Krol et al., 2011; Struktura …, 2018; Hofman et al., 2021).

The amount of Ph, which, as a rule, depends on light conditions unfavorable for photosynthesis, in the BSs of the Uvod Reservoir is characterized by high values (89.5 ± 1.4% in total with Chl) with little variability, like in the Volga reservoirs, in which the photosynthetic zone is less deep than the depth of the reservoir (Sigareva et al., 2004; Structure …, 2018; Timofeeva et al., 2021). For comparison, in BSs of large shallow lakes, in which the euphotic zone covers the entire water column, the contribution of Ph (to the total with undestroyed Chl) to BSs reaches 60% (Sigareva et al., 2022). The same indicator in the phytoplankton of the Uvod Reservoir (29.8 ± 1.1%) and the cascade of the Volga reservoirs (33.5 ± 0.8%) is estimated as relatively low similar values (Markevich and Elizarova, 2000; Mineeva, 2004).

Other indicators of the physiological state of plant communities and the transformation of the pigment fund (E480/E665 and E480/1.7E665a) change according to the content of Ph. E480/E665 values (from 1.34 to 4.79, average 1.90 ± 0.19) in the sediments of the Uvod Reservoir less than in the BSs of other Volga reservoirs, including the Gorky Reservoir (Timofeeva et al., 2021). For integral samples of phytoplankton from the Uvod Reservoir, E480/E664 (similarly to E480/E665) is from 0.89 to 1.53 (Markevich and Elizarova, 2000). In this case, the average value of E480/E664 was 1.16 in June 1993 and 1.11 in July 1995, which is close to the phytoplankton of the mesotrophic waters of the Volga reservoirs (1.20 ± 0.01) (Mineeva, 2004), as well as for macrophyte leaves (Sigareva and Timofeeva, 2023). The preservation of plant pigments when their synthesis is impossible is usually facilitated by anoxic conditions and an aphotic environment (Cardoso-Silva et al., 2022).

The C/(Chl + Ph) ratio in aquatic ecosystems varies significantly depending on the functional activity of plant organisms and their communities. In phytoplankton, this ratio is ~100 (Behrenfeld et al., 2005); in leaves of macrophytes, it is 76; and, in stems, 408 (Sigareva and Timofeeva, 2023). In BSs, the C/(Chl + Ph) ratio increases due to the degradation of pigments and reaches 1300 in the Uvod Reservoir. In general, the ratio depends on the type of BSs and increases in the order sandy and clayey silt, peaty silt, clay, and coarse sand.

According to (Möller and Scharf, 1986), the range of Chl + Ph concentrations in the sediments of the Uvod Reservoir covers all trophic categories, from oligotrophic to hypertrophic. The concentrations of the eutrophic and mesotrophic types were most frequently recorded, and the concentrations of the oligotrophic and hypertrophic types were much less common (Table 1). The average concentration of Chl + Ph for the reservoir, taking into account the areas of soils of different types (58.5 ± 6.7 μg/g d.s.), belongs to the mesotrophic category. This result corresponds to the mesotrophic status according to the pigment characteristics of phytoplankton obtained 20 years ago. According to the results of 1993 and 1995 (Markevich and Elizarova, 2000), in the Uvod Reservoir, the phytoplankton Chl concentration, the contribution of Ф, and the E480/E664 corresponded to typically mesotrophic waters of the Volga reservoirs, where Chl is 5.4 ± 0.1 μg/L, Ph is 33.5 ± 0.8%, and E480/E664 is 1.20 ± 0.01 (Mineeva, 2004). However, the average Chl + Ph concentration in sediments in 2012 corresponds to the upper limit of mesotrophy, while the Chl concentration and phytoplankton biomass correspond to the average level of mesotrophy. Judging by the long-term dynamics of plant pigments in water and BSs, an increase in the productivity of the Volga reservoirs was observed in 2012 (Struktura …, 2018); therefore, it can be assumed that the concentrations of Chl of phytoplankton in the Uvod Reservoir in 1993 and 1995 (5.1 and 5.7 µg/L) were lower than in the period of our studies. An approximate calculation confirmed the reality of this assumption: the restored Chl concentration of planktonic algae in 2012, based on the content of Chl + Ph in sediments, is estimated at 14.6 μg/L, which characterizes the trophic state of the Uvod Reservoir as typically eutrophic (Vinberg, 1960).

The Uvod Reservoir significantly differs in pigment characteristics from the large Gorky Reservoir, the waters of which enter through the Volga–Uvod Canal. A number of indicators (silt area and pigment content in certain types of BSs and in BSs as a whole) indicate that siltation and eutrophication in the Uvod Reservoir are more pronounced than in the Gorky Reservoir. Thus, the mesotrophic status of the Uvod Reservoir is characterized by higher concentrations of sedimentary pigments than in the Gorky Reservoir: in the former, the average concentration of Chl + Ph, calculated taking into account the areas of different types of BSs, is 2.3 times higher than in the latter (Timofeeva et al., 2021). Both values are mesotrophic, although, in the Uvod Reservoir, it is the final phase of mesotrophy and, in the Gorky Reservoir, typical mesotrophy. Differences in concentrations according to the trophic trait were also revealed for certain types of BSs. Thus, in the Uvod Reservoir, silts are characterized by eutrophic values, while in the Gorky Reservoir they are mesotrophic. At the same time, in the Uvod Reservoir, silts occupy a larger part of the area (50%) than in the Gorky Reservoir (32%).

It is interesting to compare the reservoir with a small natural reservoir, Lake Myastro, similar to the Uvod Reservoir in area (13.1 km2) and average depth (5.4 m). In this lake, the concentration of sedimentary pigments is low (6.4 ± 6.2 μg/g d.s.) (Smolskaya and Zhukova, 2020), almost an order of magnitude less than in the reservoir. However, according to the literature data, Chl concentrations in the phytoplankton of the compared water bodies are similar. In Lake Myastro in 1991–1998, the Chl content was 4.3 ± 1.9; in 2012 it was 5.0 ± 5.2 µg/L (Zhukova et al., 2016), which is comparable with the Uvod Reservoir in 1993 and 1995 (Solov’eva, 1996; Markevich and Elizarova, 2000). The similarity of reservoirs in terms of Chl concentration in phytoplankton is not traceable if we take into account the calculated concentration of Chl (14.6 µg/L) in phytoplankton in the Uvod Reservoir in 2012. The reason for differences in water bodies in the content of plant pigments could be the long-term dynamics of abiotic and biotic processes, which change more significantly in the reservoir than in the lake.

CONCLUSIONS

The first data on plant pigments in the BSs of the Uvod Reservoir, which is unique in terms of hydrodynamics, support the hypothesis of more intensive carbon accumulation in small water bodies. The spatial distribution of plant pigments in BSs depends on the features of the hydrological regime, the geomorphology of the reservoir, and the irregular operation of the Volga–Uvod canal, and it is consistent with the nature of the BSs, like in other natural and technogenic water bodies. Indicators of the state of the pigment fund (the contribution of Ph, the ratio of carotenoids and Chl, and the ratio of organic carbon to the sum of Chl + Ph) correspond to a strong degree of destruction, which is usually observed when the irradiation is insufficient for photosynthesis in the hypolimnion of deep sections of water bodies, as well as during aeration due to intense hydrodynamic activity. The trophic state of the Uvod Reservoir is characterized by sedimentary pigments as the final stage of mesotrophy; by the calculated Chl concentration of phytoplankton, it is typically eutrophic, which indicates eutrophication due to the primary production of plankton OM. Productivity indicators (concentration of sedimentary pigments and contribution of silts in the bottom sediment complex) reflect a higher rate of eutrophication and siltation in the Uvod Reservoir when compared with the Gorky Reservoir, which is connected through the Volga–Uvod canal. The pigment characteristics of BSs can be used to obtain integral information about the production properties of aquatic ecosystems, including the reconstruction of the average concentration of Chl in phytoplankton, to monitor and develop ways to manage water resources.