Ecological Potential Assesment of the Uvac Reservoirs Based on Phytoplankton Communities

Marija Pećić1, Ivana Trbojević1, Dušan Kostić2, Marko Marjanović2, Dragana Predojević1, Gordana Subakov Simić1

 

1 University of Belgrade, Faculty of Biology, Institute of Botany and Botanical Garden ,,Jevremovac", Takovska 43, 11000 Belgrade, Serbia.

2 The Jaroslav Černi Institute for the Development of Water Resources, Jaroslava Černog 80, 11226 Pinosava, Belgrade, Serbia.

 

Abstract

In the canyon of the Uvac River in west Serbia, three reservoirs have been built with an important role in the hydroelectric power supply system: Sjenica, Zlatar and Radoinja. Considering that research conducted in previous years recorded massive cyanobacterial blooms in some of those reservoirs whose ecological potential was estimated as moderate, there was a need for a characterization of the phytoplankton communities of these significantly modified water bodies as well as a reassessment of their ecological potential. The results of this research showed that all three of the investigated reservoirs are characterized by stable phytoplankton communities resistant to environmental stress and dominated prevalently by typical planktonic forms. In total, 64 cyanobacterial and algal taxa from 8 divisions were identified. It was also found that the phytoplankton parameters indicate that the investigated reservoirs have good ecological potential, even maximal, which was confirmed by the results of the chemical parameters, with the exception of the Sjenica Reservoir whose potential was assessed as moderate, because of the oxygen concentration.

Keywords: phytoplankton, ecological potential, Uvac, Sjenica Reservoir, Zlatar Reservoir, Radoinja Reservoir.

Introduction

 

Since the Uvac River is characterized by a significant amount of water and high slope (3-10%) and flow velocity (Mandić et al., 1996), the dams were built in order to form three reservoirs in this river's canyon - Sjenica, Zlatar and Radoinja reservoirs. All three reservoirs have a significant role in the hydroelectric power supply system in Serbia, and Sjenica and Zlatar reservoirs are part of a Special Nature Reserve, the "Uvac River Gorge". The first in the string of reservoirs and the most recently built Sjenica Reservoir is 25 km long with a maximum depth of 108 m, the second one, Zlatar Reservoir is up to 70 m deep with a surface area of 7.25 km2 and the third, the smallest one, Radoinja Reservoir is 30 m deep and 11 km long (Stanković, 2005). The Sjenica and Zlatar reservoirs represent typical dimictic water bodies with summer thermal stratification of the water column, while the Radoinja Reservoir is a compensation basin fed by deep hypolimnetic water from the upstream reservoir that has a remarkable influence on the seasonal distribution of physical and chemical water properties (Mićković et al., 2015).

Previous studies of the Sjenica Reservoir water quality based on phytoplankton communities had recorded relatively low algal diversity (21 taxa only) with the diatom dominance and massive blooms of the cyanobacteria, Plankthotrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek (Vasiljević et al., 1995). This natural phenomenon can lead to many environmental issues such as the reduction in biodiversity, leading to disruption of the trophic chain, a negative impact on the image of the lake, and potential toxic risks to both animal and human health (Jacquet et al., 2005), thus environmental awareness should be increased. Also, cyanobacterial blooms decrease the ecological potential of artificial and significantly modified water bodies, where ecological status/potential is an expression of the quality of the structure and proper functioning of the aquatic ecosystems according to the Water Framework Directive (2000/60/EC), which corresponds to the meaning of the terms "ecological health" and "ecological integrity" of those ecosystems (Kelly et al., 2008). Achieving and maintening good ecological status/potential of all aquatic ecosystems, especially those intended for water supply and protected natural resources (Trifunov et al., 2007), is the main objective of the mentioned Directive (Kelly et al., 2008; Delgado et al., 2012; Moravcová et al., 2013). In Serbia, the assessment of the ecological status/potential of surface waters is regulated by the Rulebook on the Parameters of Ecological and Chemical Status of Surface Waters and the Parameters of the Chemical and Quantitative Status of Groundwater ("The Official Gazette of the Republic of Serbia", No. 74/2011), with the prior assignment of the examined ecosystem to a certain type of aquatic ecosystem according to the Rulebook on the Determination of Water Bodies of Surface and Groundwater ("The Official Gazette of the Republic of Serbia", No. 96/2010).

The aim of this study was to assess the ecological potential of the Uvac reservoirs and estimate diversity, abundance and cyanobacterial biomass in the phytoplankton communities of these reservoirs.

 

Materials and Methods

The chemical analyses of the collected water samples (composite) was estimated by standard methods (APHA, 1995) in the laboratory of the Jaroslav Černi Institute for the Development of Water Resources in Belgrade, and the estimation of chlorophyll a and oxygen concentrations and pH values were conducted by YSI 6600 V2 sonde, in situ. Water transparency was measured by Secchi disc (Secchi's depth).

The natural phytoplankton communities were collected from the all of the three reservoirs with a plankton net (net frame 25 cm, mesh size 22-23 µm) in August. Concentrated phytoplankton samples were placed into 100 ml plastic flacons and fixed with Lugol's iodine solution. Detailed qualitative analysis of phytoplankton population structures was made using a Carl Zeiss AxioImager M.1 microscope equipped with a digital camera AxioCam MRc5 and AxioVision 4.8 software. Particular taxa were identified using standard taxonomic literature (Starmach, 1974, 1983, 1985; Ettl, 1978; Huber-Pestalozzi et al., 1983; Popovský and Pfiester, 1990; Komárek and Anagnostidis, 1998, 2005; John et al., 2002; Hofman et al., 2013; Komárek, 2013).

The phytoplankton for quantitative analysis was sampled using a Van Dorn bottle and thereafter poured into 1 l plastic laboratory bottles. Each of them was fixed with Lugol's iodine solution. Utermöhl's method (Utermöhl, 1958) was applied for quantitative analyses of phytoplankton using the Leica DMIL inverted-microscope. Hydro-Bios plankton chambers were used for counting individuals. The results of this method were expressed as a number of individuals per millilitre and a number of cells per millilitre. Biovolume and biomass of phytoplankton representatives were estimated from the approximate geometric volume of least 25 observed individuals of each taxon (Hillebrand et al., 1999) and expressed in millimetre cube per millilitre (mm3/ml) and milligram per litre (mg/l), respectively.

In order to estimate the trophic state of the investigated water bodies, Carlson's Trophic State Index was calculated using the values of the three independent variables: Secchi's depth (TSI(SD)), the chlorophyll a content of a water column (TSI(CHL)), and total phosphorus content (TSI(TP)) (Carlson, 1977; Carlson and Simpson, 1996). The obtained values are used for characterizing a lake's trophic state or overall water body health (Carlson and Simpson, 1996).

The Shannon diversity index is calculated according to the standard formula (Shannon, 1948) and obtained values were used for water quality assessment (Wilhm and Dorris, 1968; Wilhm, 1970).

 

Results and Discussion

Results of physical and chemical parameters for different profiles (epilimnion, metalimnion and hypolimnion) in all of the three investigated reservoirs are presented in Table 1. The total phosphorus (TP) content in average for the whole water column were 0.089 mg/l in the Sjenica, 0.078 mg/l in the Zlatar and 0.087 mg/l in the Radoinja reservoirs, while the measured chlorophyll a concentration recorded in the Sjenica, Zlatar and Radoinja reservoirs were 4.6 µg/l, 1.4 µg/l and 1.8 µg/l, respectively. The estimated water transparency varied from 1.5 m in the Sjenica Reservoir to 4.5 m in the Zlatar and Radoinja reservoirs during the investigation.

Carlson's trophic index calculated using the average values of appropriate parameters of the three water layers (epilimnion, metalimnion and hypolimnion) of the stratified water is shown in Figure 1.

 

fig01
Figure 1: Carlson's trophic index of the investigated reservoirs based on TSI (TP) - total phosphorus content, TSI (SD) - Secchi’s depth and TSI (CHL) - chlorophyll a content.

 

tab01

 

The obtained values for Carlson's trophic index based on total phosphorus content (TSI (TP)) indicated that all three reservoirs could be defined as eutrophic. Calculation of the same index based on the chlorophyll a content (TSI (CHL)) and Secchi's depth (TSI (SD)) defined both Radoinja and Zlatar reservoirs as oligotrophic reservoirs, whereas the same parameters characterized the Sjenica Reservoir as mesotrophic according to TSI (CHL), and eutrophic according the TSI (SD) what is similar to the findings of Čađo et al. (2003). Concentrations of total phosphorus in all three investigated reservoirs are quite uniform considering the fact that this nutrient does not limit primary production, but the ratio TN:TP (Sjenica 10.5, Zlatar 9 and Radoinja 10.3) indicates that nitrogen is probably the main nutrient limiting primary production (LAKEWATCH, 2000).

A total of 64 algal species classified in 8 divisions (Cyanobacteria (13), Dinophyta (7), Chrysophyta (5), Xanthophyta (1), Cryptophyta (3), Bacillariophyta (9), Euglenophyta (3) and Chlorophyta (23)) were detected by qualitative analysis of the samples (Table 2). Most of the recorded species were members of Chlorophyta. Although earlier research noted the massive occurrence of the Planktothrix rubescens blooms caused by eutrophication of the Uvac reservoirs, particularly in the Sjenica Reservoir (Blaženčić et al. 1990; Vasiljević et al., 1995), the presence of this potentially toxic cyanobacterium was not recorded in our study. Also, most of the identified taxa are known as typical planktonic forms (Reynolds, 2006).

 

tab02

 

The phytoplankton community in the Zlatar Reservoir has been characterized by the highest diversity - 41 recorded taxa, in the Sjenica Reservoir 40 taxa were recorded, while the lowest diversity was notified in the Radoinja reservoir (27 taxa). As shown in Figure 2, the highest contribution to the total diversity in the Sjenica Reservoir was made by green algae, which is in accordance with the findings of Čađo et al. (2015) for the summer period. There was less diversity of green algae in the Zlatar Reservoir and the least in the Radoinja Reservoir, but in these two reservoirs there was an increased presence of Cyanobacteria and Dinophyta, respectively (Figure 2).

 

fig02
Figure 2: Number and share of detected taxa per divisions in the investigated reservoirs.

 

Comparative analysis of the phytoplankton communities of the three investigated reservoirs showed that great similarities in the total diversity exist among the present algal divisions recorded in the reservoirs, except for the green algae and cyanobacteria (Figure 2). Speaking in terms of total species diversity, the greatest extent of similarity was found between the Sjenica and Zlatar reservoirs. In terms of the Shannon diversity index, the Radoinja and Zlatar reservoirs have a high similarity, while the Sjenica Reservoir has the lowest phytoplankton diversity (Figure 3), especially in metalimnion, where very low water quality is detected according to Wilhm and Dorris (1968) and Wilhm (1970). The average values of the diversity index show that the water in all the reservoirs is of moderate quality (Wilhm and Dorris, 1968; Wilhm, 1970).

 

fig03
Figure 3: Shannon diversity index for different profiles in the water columns of the investigated reservoirs.

 

Comparative phytoplankton biomass in reservoirs is presented in Figure 4. Comparing the obtained values of the algal biomass of the investigated reservoirs, revealed that the highest one occurred in metalimnion and epilimnion of the Sjenica Reservoir, whereas the lowest primary production, according to biomass, was recorded in the Zlatar Reservoir. Phytoplankton biomass in the Sjenica Reservoir was dominated by a species of the Dinophyta division (Ceratium hirundinella) (Figure 5) in epilimnion, while metalimnetic and hypolimnetic communities were characterized by explicit diatom dominance in the biomass, primarily caused by very numerous populations of a large diatom Ulnaria delicatissima var. angustisima (average length 250 – 300 µm) (Figure 6). Ulnaria delicatissima var. angustisima abundance was the highest at 8 m depth (276 individuals (cell)/ml), in metalimnion, and it is interesting to observe a drastic decrease in oxygen in this water layer (Table 1), which is known as a metalimnetic oxygen minimum and can be described as a negative heterograde oxygen curve (Effler et al., 1998). This kind of oxygen distribution is characteristic in deep reservoirs, but is still atypical and driven by complex interactions of various processes in lake/reservoirs, such as phytoplankton respiration and/or decay, non-migrating dense zooplankton populations and interflow of substances with high oxygen demands - colored dissolved organic matter (CDOM) (Effler et al., 1998; Zhang et al., 2015). Many algae, including diatoms, are capable of heterotrophic metabolism in light restricted or substrate rich environments (Tuchman et al., 2006), and according to our results, this may also be the case with Ulnaria delicatissima var. angustisima. Still, it should be noted that the metalimnetic oxygen minimum in the Sjenica Reservoir could also be caused by advanced decomposition processes. However, the fact is that oxygen concentrations in lakes/reservoirs are a clear indicator of ecosystem health and water quality, and thus understanding of oxygen distribution in lakes and developing strategies to maintain minimum concentrations of oxygen should be one of the primary tasks in lake management (Zhang et al., 2015), implicating that the Sjenica Reservoir water quality should be monitored more frequently.

 

fig04
Figure 4: Phytoplankton biomass for different water column profiles in the investigated reservoirs.

 

fig05
Figure 5: Ceratium hirundinella and Aphanocapsa planctonica.

 

fig06
Figure 6: Ulnaria delicatissima var. angustisima.

 

Ceratium hirundinella accounted for the majority of phytoplankton biomass in the epilimnion and metalimnion of Lake Zlatar, while hypolimnion has been dominated by diatoms. In the Radoinja Reservoir, diatoms dominated all three water layers.

Physical and chemical parameters in all of the three investigated reservoirs are quite similar, except oxygen concentration (Table 1) which is responsible for the decrease in the Sjenica Reservoir's ecological potential. Based on physical and chemical parameters, the ecological potential of the Zlatar and Radoinja reservoirs is good, while the Sjenica Reservoir has a moderate ecological potential based on the Serbian legislation ("The Official Gazette of the Republic of Serbia", No. 74/2011). When phytoplankton parameters are considered, those relevant for assessing the ecological potential of the three investigated reservoirs in accordance with the aforementioned Serbian legislation are: percent of Cyanobacteria in the total phytoplankton biomass, total phytoplankton abundance and chlorophyll a concentration (Predojević, 2017; "The Official Gazette of the Republic of Serbia", No. 74/2011). Based on these phytoplankton parameters, all of the three investigated reservoirs have good ecological potential, even maximal, bearing in mind that values of appropriate parameters are far below limiting values between good and moderate class of ecological potential ("The Official Gazette of the Republic of Serbia", No. 74/2011). Total abundance of phytoplankton was 258 cell/ml in the Radoinja, 315 cell/ml in the Zlatar and 227 cell/ml in the Sjenica reservoirs, while chlorophyll a concentration was 4.6 µg/l, 1.4 µg/l and 1.8 µg/l in the Sjenica, Zlatar and Radoinja reservoirs, respectively. The percentage of Cyanobacteria in the total phytoplankton biomass does not exceed 1% (from 0.03 to 0.3%). Bearing in mind that the worst parameter defines ecological status/potential (Water Framework Directive, 2000/60/EC) only the Sjenica reservoir is characterized as an ecosystem with moderate, while the other two have good ecological potential, although phytoplankton parameters alone characterize all three reservoirs as ecosystems with good (even maximal) ecological potential.

Denić et al. (2015) have conducted a detailed investigation of the assessment of the ecological potential of the Sjenica Reservoir at all three lake sites that were based on both biological (phytoplankton, phytobenthos and macroinvertebrates) and physico-chemical quality elements. This research showed that the ecological potential of the Sjenica Reservoir could be assessed as moderate (III class), poor (IV class) at the dam site and as bad (V class) at the entrance to the reservoir, therefore it appears as if the health of this reservoir has improved.

According to the World Health Organization (Chorus and Bartram, 1999), if the number of cyanobacteria present in the water column does not exceed 2000 cells per litre, it is not considered to present a risk of a harmful cyanobacterial bloom and consequential occurrence of cyanotoxins in the water column, during monitoring of reservoirs used for water supply. However, weekly sampling and continuous monitoring of raw water are recommended.

 

Conclusions

The analyses of the summer phytoplankton communities in three investigated water reservoirs showed that all three had a stable phytoplankton structure that further pointed to water ecosystems which are resistant to rapid and sudden changes in environmental conditions. Those findings are confirmed by the relatively high diversity of cyanobacteria and algae with 64 recorded taxa, in total. The assessment of ecological potential based on the phytoplankton community indicated good, even maximal ecological potential of the investigated reservoirs, although chemical parameters in the case of the Sjenica Reservoir determine it as an ecosystem with moderate ecological potential, while in the case of the other two reservoirs chemical parameters indicate good potential. Afterall, the ecological integrity of the investigated reservoirs seems to have improved compared with the previous research, which is additionally confirmed by the fact that there was an absence of Planktothrix rubescens in our results whose massive occurrence in past years affected the stability of those ecosystems.

 

Acknowledgements

Support for this research was provided by the Jaroslav Černi Institute for the Development of Water Resources under the auspices of the Ministry of Science and Technological Development, Republic of Serbia, Project No. OI176020.

 

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