Drivers of Phytoplankton Blooms in the Vrutci Reservoir During 2014-2015 and Implications for Water Supply and Management

Dušan Kostić1, Prvoslav Marjanović1, Marko Marjanović1, Ana Blagojević2, Ivana Trbojević2, Dragana Predojević2, Gordana Subakov Simić2, Dragica Vulić1, Vesna Obradović1, Zorana Naunović3

 

1 Institute for the Development of Water Resources “Jaroslav Černi”, Jaroslava Černog 80, 11000 Belgrade, Serbia
2 University in Belgrade Faculty of Biology, Studentski trg 16, 11000 Belgrade, Serbia
3 University in Belgrade Faculty of Civil Engineering, Bulevar kralja Aleksandra 73, 11000 Belgrade, Serbia

 

Abstract

In December 2013 a dense bloom of the cyanobacteria Planktothrix rubescens occurred in the Vrutci Reservoir (Western Serbia) which has been excluded from the water supply system of the City of Uzice to date. As the Vrutci reservoir has been monitored only occasionally with a low spatial resolution, the water quality data are sparse and cannot support a reliable scientific explanation on the pre-conditions that have led to the bloom through a posteriori analysis. During 2014-2015 water and sediment samples from the reservoir and its major tributaries were taken for: physico-chemical, microbiological and phytoplankton investigations. Concurrently, data on external diffuse sources as well as point sources of nutrients within the watershed were collected. During the entire study period P. rubescens dominated the phytoplankton community through prolonged, consecutive blooms. The aim of this study is to search for and discover the drivers of phytoplankton blooms in the Vrutci reservoir and to define the implications for drinking water supply and management, assuming the long term presence of P. rubescens. The study found that: a low diversity of the phytoplankton community, a low TN/TP ratio, significant internal/external nutrient loads, a stable water column and favorable light conditions are the main drivers of cyanobacterial blooms in the Vrutci reservoir. Since the existing WTP technology is not capable of handling high cyanobacterial concentrations, an upgrade of the treatment process is recommended in terms of more efficient removal of cyanobacteria and their byproducts.

Keywords: phytoplankton blooms, eutrophication, water supply, management and operation.

 

Introduction

The problem of eutrophication is one of the most common community concerns as it is strongly related to water quality degradation, which may jeopardize the use of water for many different purposes. Eutrophic lakes face an increased incidence of noxious cyanobacterial blooms and suffer from the depletion of oxygen due to organic matter decomposition (Ansari et al. 2011, Carpenter 2008). In the EU water legislation framework, the eutrophication problem has high priority and is encompassed by several water related EU directives (European Commision 2009). According to the UWWT Directive (91/271/EEC) sensitive areas are defined as:

"Freshwater bodies, estuaries and coastal waters which are eutrophic or which may become eutrophic if protective action is not taken".

When wastewater effluent is to be discharged into the water body, previously declared as a sensitive area, advanced wastewater treatment, based on more efficient nutrient removal is required. Although many EU members, especially those with strong economies, adopted sustainable development strategies which aim to decouple nutrient disposal from population and industry growth and despite the common practice, in many of the EU member states, to apply administrative and technical measures and standards more stringent than those proposed by the EU commission, water pollution and eutrophication remain the second biggest concerns among EU citizens just behind air pollution (EEA 2014, EC 2014, EEA 2015).

The growth of primary producers and seasonal algal succession in lakes and reservoirs is directly dependent on a set of limiting environmental factors such as: water temperature, sunlight, nutrients, and indirectly through: grazing, mortality, food-web competition, water column stability, etc. When nutrients are abundant, growth varies with temperature and available light (Wetzel 1975). Otherwise, production is exclusively controlled by nutrient levels and is relatively insensitive to temperature and light variations. Which factors play a key role in growth limitation, depends mainly on the trophic state of the lake and on the structure of the phytoplankton community. In most freshwater lakes and reservoirs in a temperate climate, primary production is limited by nutrient availability (Bouchard 2005). In most cases observed so far, nitrogen and phosphorus limit phytoplankton growth, and in many lakes primary production is co-limited by both of these nutrients. Many lakes are subjected to seasonal shifts in P and N limitation (Elser et al. 2009, Verduin 1956, Kimmel & Groeger 1984).

As the amount of nutrients increases as a result of human activities, the growth and reproduction of phytoplankton in lakes is no longer limited by nitrogen and phosphorus. Along with the increase in nutrient levels, lakes will transfer to the next trophic level unless another nutrient or environmental parameter becomes a limiting factor for the growth of the phytoplankton population. If eutrophication occurs, water quality degradation and cyanobacterial blooms might be expected with a high level of certainty. If another nutrient becomes limiting, eutrophication would be postponed, but additional changes could occur. One of the many possible scenarios could be a structural change in the phytoplankton community, which may lead to changes in the zooplankton community, altering other food-web levels as well as the whole lake ecology (Elser et al. 1990).

In recent decades there were many studies dealing with the potential impact of climate change on freshwater resources and vice-versa (Paerl & Paul 2012, Reichwaldt & Ghadouani 2012). Intensified nutrient inputs and increased air and water temperatures, as an outcome of climate change, have a synergistic effect on the eutrophication process. Increased temperatures may result in the increase in primary production even at low nutrient levels. However, increased temperatures at the contact of bottom sediments and overlying waters lead to more intense deoxygenation and increased diffusive nutrient flux from sediments. Furthermore, increased air temperatures promote external nutrient loading by increasing the rate of mineralization in the watershed soils. Consequently, warming can lead to the environmental conditions favored by toxic cyanobacteria even at greater latitudes (Moss et al. 2011, Adrian et al. 2009).

Unlike in natural lakes, the water quality in reservoirs might be altered by operation and management practices. The over extraction of water leads to a drop in water levels and to an increase in the concentration of accumulated nutrients. When water levels fluctuate significantly, sediments become exposed to mineralization and erosion processes; macrophytes cannot survive in the littoral zone enabling an algal dominance (Bakker & Hilt 2015, Moss et al. 2011, Zohary & Ostrovsky 2011). On the other hand, it was shown in previously published research, that controlled water withdrawal may induce strong vertical mixing which harms cyanobacterial populations, reducing their abundance in the vicinity of the water intakes (Lian et al. 2014). There is no doubt that nutrient management which relies on the reduction of external and internal inputs will likely be the most feasible and the most effective measure for the long-term mitigation of eutrophication (Reichwaldt & Ghadouani 2012).

In the case of water supply, eutrophication alters not only the water quality in the impoundment, it also affects the water treatment process and the cost of the operation as well as the final users of the supplied drinking water. Previous research revealed a strong positive correlation between total phosphorus and total organic carbon in temperate lakes. In that sense, nutrient enrichment contributes to organohalide problems in the water supply system. Water supply operators may face increased costs of water treatment as well as taste and odor complaints from their users while they are at risk of being exposed to potentially toxic and carcinogenic organic compounds (Dortch 1997, Walker 1983).

In this study, we analyze the major drivers of phytoplankton growth in the Vrutci Reservoir in Western Serbia during 2014-2015, after a severe algal bloom of the cyanobacterial species Planktothrix rubescens Anagnostidis et Komárek 1988, which took place in December 2013. As the Vrutci reservoir has been monitored only occasionally with a low spatial resolution, the water quality data are sparse and cannot support a reliable scientific explanation on the pre-conditions that have led to the bloom through a posteriori analysis, our focus was on the period after the bloom recorded in December. During 2014-2015 water and sediment samples from the reservoir and its major tributaries were taken for analyses which included: physico-chemical, microbiological and phytoplankton invetsigations. Concurrently, data on external diffusive sources as well as point sources of nutrients within the watershed were collected (JCI, 2015). We also discuss the potential implications for water supply and reservoir management in case of a long-term presence of cyanobacteria. Specifically, we aimed to quantify meteorological conditions, nutrient levels and their relative ratios, which drive seasonal phytoplankton growth.

 

Materials and Methods

Study site

The Vrutci Reservoir (43°50′34″N, 19°41′36″E) is a temperate mid-altitude, multiple use reservoir on the Djetinja River in Western Serbia. The catchment of the Djetinja River belongs to the Western Morava river basin, which is a part of the Sava and Danube river basins. The dam was built in 1984 to provide water storage for the purposes of water supply, flood protection, sediment retention and low-flow management during periods of drought. The operational level of the reservoir is designed at 621.3 m a.s.l. at which time the reservoir stores 40.2 million cubic meters of water, whilst the surface area covers 1.92 square kilometers. The average retention time is estimated at 250 days. The Vrutci Reservoir has a complex elongated shape, 7 kilometers in length with a maximum depth of approximately 50 meters in the vicinity of the dam, Figure 1. The catchment has a total area of 160 square kilometers and is sparsely populated with almost no industry. According to the Strategic Environmental Impact Assessment of the city of Uzice General Spatial plan, the watershed of the Vrutci reservoir is determined as an area with a small to negligible pollution status (Urban Development PU of the city of Uzice 2012).

In December 2013, the reservoir experienced a bloom of the potentially toxic cyanobacterial taxa Planktothrix rubescens, following which state authorities placed a ban on the use of drinking water for a period of 42 days before an alternative water source could be put into operation. During 2014-2015 the reservoir and major tributaries were sampled once a month with water samples taken for analyses which included: physico-chemical, microbiological and phytoplankton analysis. Placeholders of different colors mark sampling locations, Figure 1.

 

Fig01
Figure 1: The multiple use Vrutci Reservoir on the Djetinja River in Western Serbia. In December 2013 the reservoir experienced a bloom of the potentially toxic cyanobacterial species P. rubescens. Depicted above are the sampling locations of water from the reservoir, it’s perennial and seasonal tributaries.

 

Sampling procedure and laboratory analyses

All sampling procedures and laboratory analyses were conducted in accordance to the standards and accredited procedures and methods of the Jaroslav Cerni Institute Water Quality Laboratory. All analytical procedures are in compliance with ISO standards in case they are prescribed; otherwise they are in accordance with internationally applied standardized methods for water quality and sediment analysis: Standard Methods for Water and Wastewater Examination (APHA 2005).

For water sampling, Van Dorn (2.2L) vertical beta bottle was used. Sediment sampling we conducted using a Van Veen grab sampler as well as a Mondsee sediment corer for undisturbed sediment samples. Water samples for qualitative and quantitative phytoplankton analyses were collected with a phytoplankton net and Ruttner bottle respectively. Prior to water sample collection the water column was investigated with the aid of a multiparameter probe (YSI 6600V2-2). The water discharge of the reservoir's major tributaries was measured once a month with an ultrasonic flow metering device (Nivus).

Undisturbed reservoir sediment samples were taken during the summer of 2014. Surface sediment layers were analyzed by means of sequential extraction to determine the different forms of phosphorus present. The white circles in Figure 2 mark the positions where sediment samples were collected.

 

Fig02
Figure 2: White circles mark the locations where undisturbed sediment samples were taken with the aid of a Mondsee sediment corer in the summer of 2014. In total 24 samples were taken and analyzed by means of sequential extraction for different forms of phosphorus.

 

Results and Discussion

Major stressors and nutrient sources: External and internal nutrient loadings

According to the 2011 Census (JCI 2014), there were 2817 residents living within the Vrutci catchment. They are mainly involved in small-scale farming around their rural households. According to the 2012 Cattle Census (JCI 2014) 9708 livestock units (2671 standard animal units) were registered within the Vrutci watershed. Wastewater from households and farms is collected in a decentralized manner. It is usually disposed of in septic tanks or by direct release into surrounding streams without any treatment. In the upper catchment of the Djetinja River, in the area of Kaludjerske Bare, there are lodging capacities with 430 beds in total, without operational wastewater treatment. Industrial facilities are not numerous within the catchment area, except for a few small sawmills and a few small butcheries.

About 70 percent of the total catchment area is covered in forests. A mountainous and hilly terrain with steep slopes makes the catchment area prone to erosion during sudden snowmelt and intensive precipitation events (JCI 2014). The construction of previously designed antierosional dams has not been completed thus far and therefore significant amounts of eroded material enter the Vrutci Reservoir during snowmelt or precipitation runoff.

The formation of water budgets, dissolved and particulate matter, reveals that the four major tributaries: the Djetinja River, Jovac stream, Rocnjak and Jasik form a dominant contribution of the external loading into the Vrutci Reservoir. Around 80% of water, dissolved matter, suspended matter, dissolved phosphorus and total phosphorus is introduced into the reservoir via the Djetinja River. In terms of TOC and ammonia, the contribution of the Djetinja River is even higher, 90 and 85 percent respectively. The catchment of the Rocnjak stream, which is another tributary, has a higher than average population density thus in terms of total nitrogen and nitrates, the Rocnjak stream is a significant contributor to the external load of nitrogenous compounds, Figure 3.

The bar diagram in Figure 4, depicts the results of the sequential extraction analysis of the sampled bottom sediments. Different forms of phosphorus in the thin surface layer are represented with different colors. The major portions of phosphorus are bonded to aluminum (Al) and calcium (Ca). Soluble or readily bio-available phosphorus (represented in blue in the bottom of the bar) is a relatively small portion of the total phosphorus which does not exceed 3-5% of TP (Petković et al. 2016). Under anoxic conditions and low pH values iron-bound (Fe-P) phosphorus (red color) could also be released from the bottom sediments into the overlying waters and might become available to primary producers. Whether the released phosphorus is going to reach the photic zone or not, and become available to primary producers depends primarily on the lake morphometry and vertical transport.

If the bioavailable phosphorus content is displayed as a unit mass per unit area, the potential internal load might be estimated. On average, bottom sediments have the total capacity to release around 200 kilograms of phosphorus per hectare per annum, Figure 5. Anoxic conditions and water column disturbances in the vicinity of the bottom can promote phosphorus flux from sediments to the water phase. If it is assumed that sediment layers cover somewhere between 100 and 150 hectares of the lake bed, the total potential of the sediment to reaches between 2 and 3 tons of phosphorus released per year into the water column.

 

Fig03
Figure 3: Relative contributions of four major tributaries: Djetinja, Rocnjak, Jovac and Jasik to the external loads in terms of: water, dissolved and particulate matter. As expected, the Djetinja River, as the biggest tributary contributes the most to the external loading of the Vrutci Reservoir.

 

Fig04
Figure 4: Results of sequential extraction analysis of bottom sediments. A thin layer (2.5 centimeters) of surface sediments was analyzed for different forms of phosphorus. The major portions of phosphorus are bonded to aluminum and calcium. Bioavailable or water soluble phosphorus is a small portion of TP which is mainly below 3% of TP.

 

Fig05
Figure 5: Bioavailable phosphorus stored in sediments of the Vrutci Reservoir displayed as mass per unit area. In the worst case scenario, assuming that all bioavailable phosphorus stored in sediments enters the water column, 2 to 3 tons of phosphorus could be released from sediments annually.

 

Phytoplankton of the Vrutci Reservoir

The phytoplankton of the Vrutci Reservoir is poorly diversified with a strong dominance of the cyanobacterium P. rubescens. One hundred and seven species divided into seven functional groups were determined (JCI 2015). Phytoplankton biomass has varied over time and with the respect to the different water column strata. Most of the observed, depth-averaged, values were below 2 mg/L in all three layers of water. In August 2015, the phytoplankton biomass reached its maximum value of 15.5 mg/L in the thermocline, followed by a sharp decline in the next three months, Figure 6.

 

Fig06
Figure 6: Time series of phytoplankton biomass in: epilimnion, thermocline and hypolimnion of the Vrutci Reservoir. Phytoplankton biomass reached its maximum value in the August 2015.

 

Environmental factors that promote phytoplankton growth: water column stability and TN:TP ratio

Unlike diatoms, the most species of cyanobacteria prefer a stable water column (Reynolds et al. 1987, Lindenschmidt & Chorus 1989, Wagner and Adrian 2009). P. rubescens is a cold water stenotherm species which is spread over the entire water column during the winter mixing of the water body. During summer stratification P. rubescens usually resides in the metalimnion, where it adjusts vertical position searching for optimal light and temperature conditions using buoyancy regulation (Reynolds 1984, Walsby et al. 2001, Legnani et al. 2005). Such behavior was observed in the Vrutci Reservoir as well. During the stable summer stratification, P. rubescens resided in a thin and compact layer in the metalimnion, whilst during the winter and early spring it was spread all over the water column.

The vertical profiles of water temperature and chlorophyll fluorescence observed in July 2015 are given in the Figure 7a. The maximum chlorophyll fluorescence is at 6.5 m depth and is aligned with the highest abundance of cyanobacteria and with the inflection point of the temperature profile. Water samples taken from that particular depth are cloudy or turbid and have an opaque red tint, Figure 7b. In these water samples, the phytoplankton community is almost fully represented by P. rubescens cells, Figure 7c.

 

Fig07

a) Vertical profiles of temperature and Chl fluorescence-July 2015

b)Turbid and reddish color water sample taken from the 6.5 depth-peak in Chl fluoresc.

c) Microscopic view on Planktothrix rubescens cells and filaments

Figure 7: During the summer stratification, Cyanobacteria are the major portion and the most abundant representative of phytoplankton community. P. rubescens resides in the thin and compact layer in metalimnion at 6.5 meters depth (a). Water sample taken from that depth is turbid and reddish (b) because of the high abundance of P. rubescens cells (c).

 

Many lakes exhibit seasonal shifts in N and P limitations during the seasonal algal succession. According to the seasonal changes of the TN:TP ratio in different water column strata, the Vrutci Reservoir showed a combination of a N and N&P limited environment, when phytoplankton biomass had its peak in the thermocline during the summer (August) 2015, Figure 8 and Figure 6. Growth of primary producers in epilimnion was limited by phosphorus availability during the May, while growth in the metalimnion was limited by phosphorus during the October 2014, and at the beginning of summer 2015.

 

Fig08
Figure 8: Sesonal changes in TN:TP ratio in the Vrutci Reservoir. Many lakes exhibit seasonal shifts in N and P limitations for growth of primary producers.  Phytoplankton biomass reached its peak in the metalimnion in the summer 2015, when the Vrutci reservoir was sort of combined N and N&P limited environment.

 

Implications for Water Supply

A high algal content in drinking water sources may alter water supply in many different ways. Algal presence increases raw water turbidity and if present, cyanobacteria and their byproducts may induce health risks. Phytoplankton die-off and biomass decomposition consumes dissolved oxygen resulting the changes in pH and redox potential on the water and sediment interface.

After an intense cyanobacterial bloom in December 2013 and a 42 day long ban on the use of drinking water, the Vrutci Reservoir has remained out of operation to date. The city of Užice and the surrounding area is supplied with drinking water from a karstic spring, namely Sušička Vrela, as an alternative water source.

The Vrutci Reservoir has water intakes at three different elevations: 587.5 m, 600 m and 612 m a.s.l. As shown previously, during the period of summer stratification, phytoplankton resides in a thin sub-layer of the metalimnion predominantly. This fact should be considered during the selection of an appropriate water intake for the purpose of water supply, as it is quite simple to avoid the intrusion of cyanobacteria and their byproducts, at a level exceeding that which is required, by choosing a water intake which is outside of the zone of high cyanobacterial abundance. The diagram in Figure 9 depicts the relative positions of the water intakes and lake layers. Data about the water temperature is available for the period stretching between December 2013 - December 2015. If it is assumed that cyanobacterial presence in high concentrations is related to the presence of a metalimnion, the deepest water intake was outside the zone of cyanobacterial presence for the majority of the given timespan and as such was at a low risk of cyanobacterial intrusion.

 

Fig09
Figure 9: The Vrutci Reservoir has three water supply intakes at three different elevations: 587.5, 600 and 612 m a.s.l. The diagram above shows the positions of the water intakes in relation to the lake layers. For the majority of the study duration the deepest water intake was outside of the zone of high cyanobacterial abundance which was usually observed in the metalimnion during summer stratification.

 

Final Remarks

Climate change and the enrichment in nutrient content act mutually towards the propagation of eutrophication symptoms in freshwater ecosystems. In many lakes primary production is controlled by nutrient availability. However increased temperatures as a result of climate change, may lead to an increase in primary production even at low nutrient levels.

The eutrophication phenomenon is in a direct relationship with water quality degradation and with the increased incidence of noxious cyanobacterial blooms. For that reason, among EU citizens, water pollution and eutrophication are considered as the second most common concern just behind air pollution. When water bodies previously declared as sensitive areas, according to the UWWTD, serve as assimilators of effluents, advanced wastewater treatment is required.

Existing water reservoirs need to meet multiple water demands meaning that water quality considerations must be taken into account in reservoir management as there could be severe implications for: water supply, bathing, fisheries, irrigation and wildlife conservation due to impaired water quality. In terms of water reservoir management, water quality is not only a synonym for water chemistry but it also usually acts as a constraint for multiple water uses, or it might be considered as a reservoir use when the water quality is a purpose for itself (when water is provided to assimilate effluents) (Dortch 1997).

Eutrophication affects water supply in a top-down manner. As it affects the quality of the impounded water, and it influences the water treatment process as well as end-users. A water body which suffers from eutrophication contains high levels of particulates and dissolved organic matter, a large proportion of cyanobacteria in the phytoplankton community, anoxic zones from the metalimnion towards the bottom of the hypolimnion, a high pH in the surface layer and a relatively low pH in the anoxic zones. Water quality degradation affects almost all of the stages of the water treatment process. The presence of dissolved organics hinders floc formation and disinfection whilst the presence of particulate matter affects filtration by clogging filters and by reducing the time between consecutive back washings. Whilst the end-users are at risk of being exposed to drinking water of an inappropriate quality, they may face higher fees due to the higher costs of water treatment (Walker 1983).

The phytoplankton community of the Vrutci Reservoir is poorly diversified with a high share of cyanobacteria in the overall phytoplankton abundance. During the entire study period P. rubescens dominated the phytoplankton community through prolonged, consecutive blooms and its presence was recorded in almost all water samples. During the period of strong thermal stratification P. rubescens resided in a thin layer of the metalimnion with optimal thermal and light conditions. The highest concentrations were recorded in August in the metalimnion and they coincide with the lowest TN/TP ratio. During that part of the season, the water column was anoxic in the two zones, at the top and at the bottom of the hypolimnion. Deoxygenation triggers the denitrification process where nitrogen gas escapes the anoxic zones. Part of it is captured by nitrogen fixators while the rest is released to the atmosphere. At the same time, phosphorus is released from bottom sediments into the overlying water column and becomes potentially available to primary producers.

Proposed resilience and remediation strategies generally rely on: modified operation, restrictions in nutrient loadings and on the upgrade of the water treatment process. As cyanobacteria tends to settle in a compact and thin layer in the metalimnion, selection of a proper multi-level intake is proposed as a low-cost and simple measure to avoid the intrusion of cyanobacteria into the water supply system. A restriction in nutrient loads should be achieved via improved sanitation on the upper catchment, proper storage of manure and by erosion reduction techniques. The water treatment process should be upgraded towards improved: flocculation, sedimentation, filtration, disinfection and cyanobacterial byproducts removal.

Further efforts are foreseen in the monitoring of water quantities and water quality of the Vrutci Reservoir. Establishing a programme of continuous monitoring will facilitate water, nutrient and carbon balancing calculations. The contribution of torrential tributaries to the nutrient and organic matter loads is unknown so far and should be subject to further investigation. Further research efforts should encompass zooplankton and the role of fish species in nutrient and carbon cycling as well. Additional data on erosion processes in the watershed and groundwater interactions should give a better insight into the sources of allochthonous nutrients and carbon, contributing to a better understanding of the metabolism of the reservoir in it's entirety.

 

Acknowledgements

The authors would like to thank the Ministry of Education, Science and Technological Development of the Republic of Serbia for their support during the research which was carried out under the "Monitoring and Modeling of Rivers and Reservoirs (MORE) - Physical, Chemical, Biological and Morphodynamic Parameters" project -TR37009. Part of the research was conducted within the programme of exploratory works for the purposes of the project "Data and documentation collection for the purposes of the rehabilitation of the Vrutci Reservoir and the completion of the project "Reconstruction of the DWTP on Cerovića hill". The authors would also like to thank the Ministry of agriculture and environmental protection, The Republic Directorate for water and the PUC "Vodovod Užice" for financing this project.

 

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