Cultural eutrophication is one of the most pervasive courses of water quality impairment in shallow lakes around the world. Extensive nutrient loading may lead to sudden losses of macrophytes in shallow lakes, producing a shift from a clear-water state with abundant macrophytes to a turbid-water state dominated by phytoplankton (Scheffer et al. 1993). This shift can occur over a wide range of total phosphorus concentration, indicating that factors other than nutrients are also responsible for maintaining shallow lake ecosystems in one of the two alternative stable states (Wetzel 2001). In macrophyte-dominated lakes, macrophytes support the clear-water state by suppressing phytoplankton through competition for nutrients and production of allelochemicals and by providing refuge for zooplankton from fish predation (Scheffer et al. 1993). In phytoplankton-rich lakes, zooplanktivorous fish often dominate and stabilize the turbid-water state while preying on the zooplankton, thus reducing the zooplankton pressure on phytoplankton (Gulati 1990).
To restore eutrophic shallow lakes, many restoration projects have been undertaken to induce a shift from the turbid phytoplankton-dominated state to the clear state dominated by macrophytes (Søndergaard et al. 2007; Gulati et al. 2008). Stoking with predatory (piscivorous) fish is one of the biomanipulation methods aimed at reducing the number of zooplanktivorous fish and improving the zooplankton abundance and phytoplankton grazing (Benndorf 1995; Skov & Nilsson 2007). However, restoration attempts are not always successful and largely depend on internal and external factors affecting the resilience of the turbid algae-dominated state of lake ecosystems (e.g. Søndergaard et al. 2007). It is yet unclear how changes in weather conditions may interact with trophic cascade mechanisms and whether this may contribute to switching between alternative stable states in shallow lakes (Scheffer & Van Nes 2007). There is some evidence that even a small increase in temperature may create a shift in turbid shallow lakes to a clear-water phase (Scheffer et al. 2001). Contrary to this view, Jeppesen et al. (2003) presented data showing that probability of a shift to the clear-water state in nutrient-rich lakes decreases with higher temperature. Studies of long-term effects of changing anthropogenic pressure on phytoplankton and macrophyte dynamics can provide useful information on the degree of environmental impairment and extrinsic and intrinsic factors stabilizing the alternative states of lake ecosystems.
The composition of phytoplankton communities changes in relation to increasing lake fertility (Hutchinson 1967). However, the response of phytoplankton to eutrophication is highly dependent on other ecological factors such as light availability. In clear-water lakes, the increased dominance of cyanobacteria along the eutrophication gradient is a well-known phenomenon (e.g. Trifonova 1990). Compared to clear-water lakes, a significantly weaker response of cyanobacteria to increased nutrient loading was observed in brown-water humic lakes (Ptacnik et al. 2008). Light availability is also an important predictor of abundance and distribution of aquatic plants. Low light conditions caused by organically colored waters and sediment resuspension promote the dominance of helophytes (i.e. emergent plants) and floating-leaved plants over submerged ones (Toivonen & Huttunen 1995; Nurminen 2003). Furthermore, due to the low availability of light in humic and turbid lakes, the distribution of submerged aquatic plants is restricted to shallow, littoral habitats (Stewart & Freedman 1989; Nurminen 2003). Although much information is available on the interactions of macrophytes and phytoplankton in clear-water lakes, relatively little is known on how limited availability of light may affect the interaction between macrophytes and phytoplankton along the eutrophication gradient.
Sestroretskiy Razliv is the largest water body located in the suburban area of Saint-Petersburg (northwestern Russia). It was established in 1723 to provide for the energy needs of Sestroretsk’s tool manufacturing plant by building a dam on the Sestra River at 5 km from its discharge into the Gulf of Finland. At present, it is an important element of the urban landscape and is under constant anthropogenic pressure. The surrounding areas feature industrial and agricultural enterprises, landfills and highways, and intensive construction has been conducted on the shores of the reservoir since the 1990s. Furthermore, the reservoir is traditionally used as a recreational site. Regular monitoring of water quality parameters of the Sestroretskiy Razliv reservoir was initiated in 1966, when a new water supply plant was constructed and the reservoir has started to be used for drinking water supply. Since 2000, the reservoir is no longer used for water supply purposes. The first comprehensive ecological research on the reservoir was conducted by the Institute of Limnology RAS (St.-Petersburg) in 1980 (Stravinskaya et al. 1984b). The second research on the reservoir, which was carried out by the Institute of Limnology RAS in 2002, showed a drastic deterioration of water quality (Belyakov et al. 2002). Although the sewage treatment plants situated at the tributary of the reservoir, the Chernaya River, were reconstructed in 2012, high values of total phosphorus recorded in the reservoir water indicated that external and internal nutrient loads remained high during the 2010s (Kondratyev et al. 2016). The modern assessment of environmental conditions and biological structure of the Sestroretskiy Razliv reservoir was carried out in 2016 and 2018 by the Institute of Limnology RAS in collaboration with the State Research Institute on Lake and River Fisheries (St.-Petersburg).
The main objective of the present paper was to examine long-term changes in the structure of phytoplankton and macrophyte communities in the Sestroretskiy Razliv reservoir over the period from 1980 to 2018. We investigated whether long-term changes in phytoplankton biomass were related to phosphorus concentration. We were particularly interested how macrophyte and phytoplankton communities interacted under conditions of low light availability caused by relatively high concentration of humic compounds and intensive sediment resuspension, which are characteristic features of the Sestroretskiy Razliv reservoir. In addition, based on the analysis of current environmental conditions and biological structure, restoration measures were proposed for the reservoir.
The Sestroretskiy Razliv reservoir (60°05’00”N, 30°00’00”E) is located in the southwestern part of the Karelian Isthmus in the Lakhta lowland. Sestroretskiy Razliv is the largest water body located within the suburban area of Saint-Petersburg. It has a surface area of 10.3 km2 and water volume of 0.018 km3. Sestroretskiy Razliv is a polymictic shallow reservoir with a mean depth of 2.2 m and a maximum depth of 4.6 m (Stravinskaya et al. 1984a). The shoreline length of the reservoir is about 20 km. The western shore is shallower than the eastern one (Stravinskaya et al. 1984a). The western shore encroaches on the urban area of Sestroretsk. The eastern shore is covered with deciduous and coniferous forests. The territories adjacent to the northern shore are extensive waterlogged lands and bogs. Contemporary bottom deposits in Sestroretskiy Razliv are represented by sand and silt (Sergeeva et al. 1984). Silt is the most common sediment in the reservoir, forming about two thirds of the topmost part of the bottom deposits. It occurs in the form of large patches in the southern, northern and western parts of the reservoir. Sand occurs in the central part and along the western and eastern shores, in the most hydrodynamically active areas of the reservoir. The content of organic matter in the upper layer of sediments varies from 2 to 71%, reaching the highest values (48–71%) in the fine silt fraction.
The two main tributaries to the reservoir are the Sestra River, with a length of 75 km and a catchment area of 399 km2, and the Chernaya River, with a length 35 km and a catchment area of 126 km2. The total catchment area of Sestroretskiy Razliv is 586 km2 (Stravinskaya et al. 1984a).
The first investigation into the reservoir carried out in 1980 showed that the Chernaya River is the main source of contamination of the reservoir (Stravinskaja et al. 1984b). The inflow of the Sestra and Chernaya rivers contributes 96.1% to the water balance of Sestroretskiy Razliv (Veselova & Judin 1984). The reservoir has a rather high water exchange rate (10 yr−1). As the outflow from the reservoir is only moderately regulated, the annual water level fluctuations tend to follow its natural course, but with a lower level in spring and a smaller amplitude. The annual water level fluctuations vary between 0.6 and 0.8 m with the highest water level recorded during wet years.
Previous studies of the quality characteristics of water and phytoplankton in the reservoir were conducted in 1980 (Stravinskaja et al. 1984b; Trifonova & Senatskaya 1984) and 2002 (Belyakov et al. 2002). In 1980 and 2002, water quality parameters and phytoplankton were sampled in May, July, August and October at two locations in the central zone and one location near the inflow of the Chernaya River (Fig. 1). In 2016 and 2018, samples for the analysis of water quality and phytoplankton were collected in May, August and October at approximately the same locations as in 1980 and 2002.
Water samples for chemical analysis were collected using a Limnos sampler (2.0 l) and transported to the laboratory in plastic bottles. In the field, pH measurements were made using a Hanna portable pH meter. Water transparency was determined with a Secchi disc. The following chemical analyses were performed in the laboratory using standard methods (Semenov 1977). Inorganic phosphorus (IP) was determined as a sum of dissolved and particulate orthophosphates by a modified molybdenum method of Murthy and Riley (1962), using ascorbic acid as a reducer; total phosphorus (TP) – using the same procedure after oxidation with K2S2O8 in acidic medium. Total nitrogen (TN) was determined by oxidation with K2S2O8 in alkaline medium with
subsequent reduction of NO3− on a Cd-reducer and determination of NO2− with a Griss reagent. Concentration of hydrocarbonates was determined using potentiometric titration with HCl, sulfates – using the turbidimetry method, chlorides – by mercurymetric titration, calcium and magnesium – using the titrimetric method with EDTA. The sum of sodium and potassium was calculated based on the balance of main cations and anions. The content of main ions (S ions) was assessed as a sum of HCO−, 3SO42−, Cl−, Ca2+, Mg2+ and Na+ + K+. Water color was determined using visual titration with a standard Pt-Co-reagent.
Phytoplankton samples for quantitative analysis were collected using a Ruttner water sampler (0.5 l) and preserved with Lugol’s iodine solution in the field. In the laboratory, phytoplankton samples were concentrated by sedimentation up to 50 ml and counted in a Nazhott chamber of 0.05 ml (Guseva 1959) under light microscope AxioLab A1 (CarlZeiss). The biomass (mg l−1) was estimated using geometric approximation as algal biovolume (Trifonova 1990). Phytoplankton identification was based on the modern taxonomic literature (Komárek & Anagnostidis 1999; 2005; Krammer & Lange-Bertalot 1991). Chlorophyll
The previous study of the aquatic vegetation in the reservoir was carried out in 1980 (Katanskaya 1984). In 2016 and 2018, aquatic macrophyte surveys were carried out in the full growing season (July and August). As in the previous study of 1980 (Katanskaya 1984), the taxonomic composition and distribution of macrophyte beds were examined from a boat, applying the phytolittoral mapping method (Katanskaya 1988; Kolada et al. 2012). In the field, the spatial distribution of macrophyte beds of dominant species was plotted on a bathymetric plan. The boundaries of macrophyte beds were determined by GPS coordinates from field studies during the seasonal maximum of vegetation. In order to assess temporal changes in the composition of macrophyte communities, the relative abundance of dominant species and functional vegetation groups (emergent, floating-leaved and submerged) were estimated. The area of macrophyte beds of dominant species was digitized with coordinate data using ArcView GIS 3.2, and then the percentage of these species in the total plant coverage was determined. Plants of each dominant species were collected by the quadrat sampling method to measure the aboveground biomass of macrophytes (Katanskaya 1988). Samples were collected from five randomly distributed plots (0.25 m2 for helophytes and 1 m2 for floating-leaved and submerged plants) by wading in a shallow area of the reservoir. A PVC quadrat frame was placed on the sediment surface and all plant shoots within the frame were removed manually by cutting the stems at the sediment. The collected plants were placed in labelled bags, transferred to the laboratory, air-dried and then weighted. Calculations of the total aboveground macrophyte biomass was based on the calculated areas of macrophyte beds and average biomass values of each dominant species.
In September 2015 and May 2016, the State Research Institute on Lake and River Fisheries carried out a survey on the fish community, the results of which were published earlier elsewhere (Pedchenko et al. 2017).
Prior to statistical analysis, all water quality parameters (except pH), phytoplankton biomass and chlorophyll
The relationships between the phytoplankton biomass and chlorophyll
The measured values of water quality parameters during the years when samples were collected from the Sestroretskiy Razliv reservoir are shown in Figure 2. The data from the whole study period shows that the reservoir has poorly mineralized water, with the content of main ions (S ions) fluctuating between 53 and 76 mg l−1 (Fig. 2a). Despite an increasing trend in average values of S ions from 57 mg l−1 in 1980 to 71 mg l−1 in 2018 in the May–October period (Table 1), no statistically significant differences were found between the sampling years (
Means (and ranges) of the water quality parameters in the Sestroretskiy Razliv reservoir in different years (1980, 2002, 2016 and 2018). The comparison of the means among different years was carried out using ANOVA with the Tukey–Kramer test. The Tukey–Kramer test results are indicated by superscripts.
Parameter | 1980 | 2002 | 2016 | 2018 | ||
---|---|---|---|---|---|---|
Σ ions (mg l−1) | 57 | 67 | 68 | 71 | 1.82 | 0.171 |
(37–84) | (64–71) | (47–98) | (55–85) | |||
HCO3− (mg l−1) | 16ab | 12a | 22ab | 27b | 7.22 | < 0.01 |
(10–23) | (9–15) | (14–32) | (18–41) | |||
Secchi depth (m) | 0.76a | 0.34b | 0.36b | 0.44b | 8.14 | < 0.001 |
(1.00–0.60) | (0.50–0.18) | (0.50–0.15) | (0.75–0.15) | |||
Color (Pt-Co) | 130a | 128a | 178b | 107a | 10.05 | < 0.0001 |
(103–152) | (112–144) | (121–240) | (77–140) | |||
pH | 7.8a | 8.6a | 6.8b | 7.0b | 14.48 | < 0.0001 |
(7.2–8.8) | (7.3–9.8) | (6.7–7.0) | (6.1–8.1) | |||
TP (μg l−1) | 73a | 163b | 96a | 106a | 7.85 | < 0.001 |
(32–158) | (94–233) | (49–150) | (42–170) | |||
PO4-P (μg l−1) | 10a | 31b | 43b | 51b | 25.83 | < 0.0001 |
(4–38) | (19–52) | (13–77) | (26–80) | |||
TN (mg l−1) | 1.18a | 0.72b | 0.62b | 0.70b | 19.10 | < 0.0001 |
(0.82–1.54) | (0.45–0.86) | (0.44–0.92) | (0.39–0.95) |
Letters a and b indicate years with significantly different means.
Bicarbonate alkalinity (HCO3-) varied during the study period from 12 to 35 mg l−1 (Fig. 2b). Although there were significant differences in average values of bicarbonate alkalinity between 2002 and 2018 (
The Secchi depth varied from 0.21 to 0.82 m (Fig. 2c). Significantly higher values of the Secchi depth were recorded in 1980 compared to 2002, 2016 and 2018 (
The color of water ranged from 79 to 228 Pt-Co (Fig. 2d). The maximum values of water color were recorded in late summer (August) in 1980, 2002 and 2018. In 2016, the maximum value of water color was recorded in autumn (October). Comparing different sampling years, a statistically significant increase in the average water color was observed in 2016 (
The pH values of water in the reservoir ranged from 6.1 to 9.5 (Fig. 2e). Higher pH values were recorded in the summer months, when the maximum values of phytoplankton biomass were observed. Significantly higher values of mean pH, fluctuating within a slightly alkaline range (7.8–8.6), were observed in 1980 and 2002 (
Considering May–October average values of the TP concentration, statistically significant differences were observed in 2002 compared to other sampling years (
The May–October average values of the IP concentration were statistically significantly different between the sampling years (
During the whole study period, TN values fluctuated between 0.47 and 1.29 mg N l−1, with maximum TN values recorded in 1980 (Fig. 2h). In 2002, 2016 and 2018, the average TN values were significantly lower compared to those in 1980 (
In 1980, from May to October, phytoplankton biomass in the reservoir varied from 2.8 to 10.6 mg l−1, with the maximum value recorded during the spring peak in May (Fig. 3a). The May–October average value of phytoplankton biomass was 6.8 mg l−1 (Table 2). The chlorophyll
Means (and ranges) of phytoplankton biomass (B), chlorophyll
Parameter | 1980 | 2002 | 2016 | 2018 | ||
---|---|---|---|---|---|---|
B (mg l−1) | 6.8a | 61.0b | 17.6c | 23.1c | 18.02 | < 0.0001 |
(0.3–19.8) | (15.8–99.6) | (3.9–35.6) | (7.0–49.8) | |||
Chl |
16.6a | 84.7b | 43.6ab | 66.6b | 6.87 | < 0.001 |
(0.7–27.2) | (20.5–174.9) | (8.4–122.9) | (27.4–157.6) | |||
% Diatom | 55.8a | 15.7b | 75.6a | 59.7a | 19.30 | < 0.0001 |
(21.2–95.5) | (2.1–58.6) | (20.7–94.0) | (32.5–88.6) | |||
% Cyanobacteria | 14.5aс | 75.9b | 2.4с | 26.2a | 23.94 | < 0.0001 |
(0–46.2) | (26.9–96.4) | (0.3–6.7) | (1.2–62.1) |
Letters a and b and c indicate years with significantly different means.
In 2002, phytoplankton biomass varied from 19.3 to 86.9 mg l−1, with the May–October average of 61 mg l−1 (Fig. 3a, Table 2). The chlorophyll
In 2016, the phytoplankton biomass varied from 6.7 to 31.3 mg l−1, with the May–October average of 17.6 mg l−1 (Fig. 3a, Table 2). The concentration of chlorophyll
In 2018, phytoplankton biomass varied from 8.4 to 47.1 mg l−1 with the May–October average of 23.1 mg l−1 (Fig. 3a, Table 2). The chlorophyll
the total biomass (Fig. 3d). On the other hand, diatoms (mainly taxa of the genus
The measurements made during the whole study period showed statistically significant differences in the May–October average values from different sampling years for the phytoplankton biomass (
The linear correlation analysis showed that the correlation between TP and phytoplankton biomass (
A comparison of the spatial macrophyte distribution in the Sestroretskiy Razliv reservoir between 1980 and 2016 revealed that the largest beds of emergent plants (helophytes) were still located along the western shore and in the northern part of the reservoir near the inflow of the Sestra River (Fig. 6). The outer edge of helophyte beds composed of reeds (
A comparison of the total area covered by aquatic vegetation between 1980 and 2016 showed that it decreased twice, from 157.3 ha to 75.9 ha, respectively (Table 3). The calculations of the reservoir area covered by macrophytes showed its decrease from 14.8% in 1980 to 6.8% in 2016 (Table 3). Considering the functional vegetation groups, the area covered by helophytes and floating-leaved plants decreased by a factor of two. Submerged macrophytes showed the highest reduction in the coverage area by a factor of four. At the same time, the relative contribution of helophytes and floating-leaved plants to the total aquatic vegetation cover remained practically unchanged (Table 3), whereas the relative contribution of submerged plants decreased twice, from 1.6% to 0.8%.
Contribution of different species of helophytes, floating-leaved and submerged plants to the total plant coverage and the area of the Sestroretskiy Razliv reservoir in 1980 and 2016
Ecological groups and species | Area, ha | % of plant cover | % of reservoir area | |||
---|---|---|---|---|---|---|
1980* | 2016 | 1980* | 2016 | 1980* | 2016 | |
Helophytes | ||||||
53.4 | 26.1 | 33.9 | 34.4 | 5.03 | 2.30 | |
42.5 | 16.2 | 27.0 | 21.3 | 4.00 | 1.50 | |
0.3 | 9.5 | 0.2 | 12.5 | 0.03 | 0.90 | |
24.6 | 4.7 | 15.6 | 6.2 | 2.32 | 0.40 | |
(+) | 1.2 | (+) | 1.6 | (+) | 0.10 | |
5.6 | 3.6 | 3.6 | 4.7 | 0.53 | 0.30 | |
Total: | 126.4 | 61.3 | 80.4 | 80.8 | 11.91 | 5.50 |
Floating-leaved plants | ||||||
3.0 | 6.7 | 1.9 | 8.8 | 0.29 | 0.62 | |
0.1 | (+) | 0.1 | (+) | 0.01 | (+) | |
3.4 | 3.2 | 2.2 | 4.2 | 0.32 | 0.30 | |
6.3 | 2.4 | 4.0 | 3.2 | 0.60 | 0.22 | |
0.1 | 0.7 | 0.1 | 0.9 | 0.01 | 0.07 | |
3.6 | 0.1 | 2.3 | 0.1 | 0.34 | 0.01 | |
Mixed beds | 11.9 | 0.9 | 7.6 | 1.2 | 1.13 | 0.08 |
Total: | 28.4 | 14.0 | 18.1 | 18.4 | 2.70 | 1.30 |
Submerged plants | ||||||
2.5 | 0.6 | 1.6 | 0.8 | 0.20 | 0.10 | |
Total: | 157.3 | 75.9 | 100.0 | 100.0 | 14.81 | 6.80 |
* data of Katanskaya (1984); (+) – found sporadically
The total aboveground plant biomass in the reservoir is approximately 684 tons (Table 4). Helophytes, floating-leaved and submerged plants produce 95.7, 4.2 and 0.03% of the total plant biomass, respectively. The comparison of 2016 with 1980 showed that the total plant biomass decreased by a factor of 0.7. The relative contribution of helophytes to the total plant biomass slightly increased from 94% to 96%, while the relative contribution of floating-leaved and submerged plants decreased from 6% to 4% and from 0.06% to 0.03%, respectively (Table 4).
Contribution of ecological groups of aquatic plants to the total aboveground biomass in the Sestroretskiy Razliv reservoir in 1980 and 2016
Ecological group | Aboveground biomass | |||
---|---|---|---|---|
1980* | 2016 | |||
103 kg | % | 103 kg | % | |
Helophytes | 872.6 | 93.95 | 654.9 | 95.72 |
Floating-leaved plants | 55.6 | 5.99 | 29.0 | 4.24 |
Submerged plants | 0.6 | 0.06 | 0.2 | 0.03 |
Total | 928.8 | 100 | 684.2 | 100 |
* data of Katanskaya (1984)
The survey of macrophyte communities in 2016 showed that the relative abundance of helophytes and floating-leaved plants in the reservoir remained fairly constant compared to 1980. On the other hand, there were changes in relative abundance of individual macrophyte species (Table 3). In the functional group of floating-leaved plants, the dominance shifted from
Compared to 1980 and 2016, the significant reduction in the area covered by submerged plants was accompanied by their reduced distribution in the reservoir. In 2016, the distribution of submerged vegetation moved into a shallower zone of the reservoir, compared to 1980. In 1980, submerged plants (mainly
The macrophyte survey conducted in 2018 showed a slight increase in the area covered by mixed beds of low-growing emergent plants (such as
Fish surveys conducted in 2015 and 2016 showed that the fish community in the reservoir was represented by eight taxa belonging to three families: Esocidae (pike), Cyprinidae (bream, bleak, gudgeon, and roach), and Percidae (perch, pikeperch, and ruffe; Pedchenko et al. 2017). The core of the fish community consisted of six taxa (roach, ruffe, bream, perch, pikeperch, and bleak). Roach was the most abundant species in the seine surveys, reaching up to 69% of the total fish abundance. The percid fish (ruffe, perch, and pikeperch) varied in relative abundance from 19% to 28%. Pike was the rarest taxon, its relative abundance in the seine surveys was no more than 0.2%. Cyprinids (roach and bream) reached the highest abundance in the deepest open part of the reservoir. On the other hand, percids (ruffe and perch) were more abundant in reed and bulrush stands. In general, the seine surveys showed that planktivorous species dominated in relation to piscivores in the fish community of the Sestroretskiy Razliv reservoir. The average value of fish standing crop in the reservoir was 98 kg ha−1 (Pedchenko et al. 2017).
In the study period from 1980 to 2018, the simultaneous data on nutrient dynamics and phytoplankton development allowed us to track the long-term changes in the trophic status of the Sestroretskiy Razliv reservoir. According to the phytoplankton biomass level corresponding to the May–October average of 6.8 mg l−1 (Table 2), the reservoir was meso-eutrophic in 1980 (Trifonova & Senatskaya 1984). During the 20 years from the beginning of the 1980s, the annual phosphorus loading doubled (from 2.2 to 4.5 g m−2; Belyakov et al. 2002). Measurements of TP concentration showed an increase in the average TP from 73 μg P l−1 in 1980 to 163 μg P l−1 in 2002 (Table 2), with a value above 100 μg P l−1 considered to be hypertrophic (OECD 1982). Due to the higher phosphorus input, an increase in phytoplankton biomass by a factor of 8 to 9, corresponding to the average value of 61 mg l−1, was recorded in 2002 (Trifonova & Pavlova 2005). In the course of seasonal succession, diatoms were almost completely replaced by cyanobacteria, mainly
Compared with 2002, a 40–35% reduction in TP concentration in 2016 and 2018 was followed by a significant decline in phytoplankton biomass (Tables 1, 2). Despite the decrease in phytoplankton biomass in 2016 and 2018, the Sestroretskiy Razliv reservoir remains a hypertrophic system, as indicated by the average chlorophyll
The composition of macrophyte communities in the Sestroretskiy Razliv reservoir is typical for humic lakes, where the dominance of helophytic and floating-leaved plants over submerged ones is often observed (e.g. Toivonen & Huttunen 1995; Nurminen 2003). When comparing the aquatic vegetation of the reservoir in 1980 and 2016, it is clear that eutrophication had a profound effect on the composition and abundance of macrophytes. The field survey conducted in 1980 showed a very limited development (about 2% of the total area covered by vegetation) of submerged macrophytes, suggesting that the submerged vegetation has been affected by eutrophication since the early 1980s. The further decrease in the total cover of macrophytes, particularly the cover of submerged plants, observed in 2016, suggests that eutrophication of the reservoir has been accompanied by an increasing impact of phytoplankton dominance (Scheffer et al. 1993). The most likely mechanism behind the observed decline in the abundance and species richness of aquatic plants in the Sestroretskiy Razliv reservoir is the shading effect of dense phytoplankton populations on the growth of plants and propagule germination (e.g. Arthaud et al. 2012). Our findings that the decline of submerged macrophytes was more pronounced compared to emergent and floating-leaved plants are in accordance with many studies that report that submerged plants are particularly vulnerable to increasing eutrophication (e.g. Sand-Jensen 1997; Sand-Jensen et al. 2000; Egertson et al. 2004; Papastergiadou et al. 2010). Considering other functional vegetation groups, the effect of increasing eutrophication was also reflected in changes in the composition of emergent plants. In the reported period from 1980 to 2018, the group of helophytes increased in relative abundance and spatial expansion of
Apart from the effect of eutrophication, water level fluctuations apparently had a considerable effect on the composition of macrophyte communities. The analysis of vegetation changes between 2016 and 2018 suggests that the spatial expansion of mixed beds of low-growing emergent plants, typically inhabiting the upper littoral zone and floating-leaved plants, was a likely response to the lowered water level in the summer of 2018. The positive effect of low water levels on the expansion of the area covered by helophytic and floating-leaved plants was previously documented for many lakes (e.g. Chow-Fraser 2005; Hudon et al. 2005; Rusanov 2008; Papastergiadou et al. 2010).
The ecological concept underlying the restoration practice implies that shallow lakes can be in two alternative stable states: the clear-water state with abundant submerged macrophytes and the turbid-water state with small abundance of submerged plants and extensive development of phytoplankton (Scheffer et al. 1993). One of the main conclusions drawn from practical applications is that reducing the external nutrient loading alone cannot lead to successful restoration of eutrophic lakes. High internal phosphorus loads from sediments and changes in the biological structure of eutrophic lakes delay the recovery to the clear-water state (Søndergaard et al. 2007). Among the important feedback mechanisms delaying the lake’s recovery, small abundance of submerged vegetation and high abundance of planktivorous fish should be mentioned. Macrophytes play an important structuring role in shallow lakes, stabilizing the clear-water state via allelopathy, competition with phytoplankton for nutrients and refuge for zooplankton (Scheffer et al. 1993). On the other hand, the high biomass of planktivorous fish reduces the top-down control of zooplankton over phytoplankton, thus affecting the macrophyte-dominated state (Gulati 1990). In the Sestroretskiy Razliv reservoir, the average standing crop of fish (with the dominance of planktivorous species over piscivorous) was 98 kg ha−1, which is close to the lowest value of fish production variability in eutrophic lakes (100–400 kg ha−1; Gulati et al. 2008). According to the alternative stable state concept (Scheffer et al. 1993), the Sestroretskiy Razliv reservoir is a phytoplankton-dominated system. Negative effects associated with small abundance of macrophytes and dominance of planktivorous fish are the most likely structural mechanisms stabilizing the phytoplankton-dominated state in the reservoir. The dark color and high turbidity of water due to wave-induced sediment resuspension are the additional factors limiting the growth of submerged plants and preventing the regime from shifting from phytoplankton to macrophyte dominance. Our results suggest that despite the limited availability of light in the Sestroretskiy Razliv reservoir, submerged macrophytes were still negatively impacted by phytoplankton, especially during warm years when underwater light was stronger and summer cyanobacterial blooms occurred. Therefore, the reduction of nutrient loading as a restoration measure will reduce the phytoplankton biomass and thus remains the most effective method for restoring the submerged vegetation in the reservoir.
The comparison of data on the abundance and composition of macrophyte and phytoplankton communities in 2016 and 2018 showed that the phytoplankton-dominated state remained stable in the reservoir, despite considerable fluctuations in water turbidity and temperature. As far as the phytoplankton development is concerned, the comparison of 2016 with 2018 showed that a slight increase in phytoplankton biomass occurred in 2018 at a higher summer temperature (Fig. 3a,b, Table 2). In the colder year of 2016, the area covered by submerged plants did not change in response to lower phytoplankton biomass and remained similar to the area observed in 2018 with greater phytoplankton development. This indicates that in 2016, high water turbidity due to wave-induced sediment resuspension limited the growth of submerged vegetation despite the lower level of algae-induced turbidity. Based on data collected from Dutch shallow lakes, Scheffer et al. (2001) showed that a small increase in temperature could lead to a shift toward a clear-water phase due to increased zooplankton grazing activity. Therefore, our findings seem to contradict this view and suggest that other mechanisms, such as higher temperature-dependent algal growth, reduced feeding of zooplankton on cyanobacteria and higher predation pressure of fish on zooplankton were responsible for the observed increase in phytoplankton biomass at increasing temperature (Jeppesen et al. 2003).
Based on the above-mentioned characteristic features of the Sestroretskiy Razliv reservoir, the management approach to the restoration of the reservoir should include:
Further reduction in nutrient loads from both the catchment area and the sediment to reduce nutrient concentrations in water of the reservoir. To reduce the internal nutrient input, harvesting and removal of helophyte vegetation at sites with high density of macrophytes should be implemented.
Biomanipulation by introducing native predatory (piscivorous) species such as pike and pikeperch fry should be applied to minimize populations of planktivorous fish (Benndorf 1995; Skov & Nilsson 2007). This will produce the top-down trophic cascade effects on fish and lead to the re-establishment of submerged macrophytes.
The comparison of data from 1980 to 2002 showed an increase in the total phosphorus concentration and phytoplankton biomass indicating a change in the trophic status of the Sestroretskiy Razliv reservoir from meso-eutrophic to hypertrophic. The increase in phytoplankton biomass coincided with a shift in the community composition from diatoms to cyanobacteria with the dominance of