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Differences between lakes and reservoirs

Reservoirs receive through rivers larger inputs of water, as well as soil and pollutant loads than lakes. On the other hand, reservoirs have the potential to flush the pollutants more rapidly than do lakes through water withdrawal (UNEP, 2000).
Unlike lakes, deep reservoirs are distinguished by the presence of a longitudinal gradient in physical, chemical and biological water quality characteristics from the upstream river end to the dam. Thus, reservoirs have three major zones: an upstream riverine zone, a downstream lake-like zone at the dam end, and a transitional zone separating these two zones (Figure 2, Table 1).

Physical functioning of lakes and reservoirs

Development of thermal stratification

Lakes and reservoirs are affected by different meterological (radiation, precipitation and wind) and hydrological processes (inflows, outflows) (Figure 3). Thermal stratification is a major factor influencing the growth and succession of phytoplankton and overall water quality in lakes. It is a solar-radiation-driven process that separates the lake water column into three distinct vertical layers due to the change in water density with temperature (Lampert and Sommer, 2007). Radiation mainly enters the lake at short wavelengths (visible light); it is mostly absorbed near the surface and transformed into heat. Water density is maximum at 4 °C at low turbidity and salinity; it becomes lighte r either by cooling below 4 °C or by warming above 4 °C. Lighter water buoys whereas den ser water sinks. Wind produces turbulence and currents at the surface, that dampen rapidly with depth, creating a transition layer, the metalimnion, between the mixed warm surface waters, the epilimnion, and the colder quiescent deep waters, the hypolimnion. This yields the typical temperature profile of a stratified lake of long residence time with a strong decline in temperature in the metalimion, called thermocline (Figure 4), separating the epilimnion and the hypolimnion, both with rather homogeneous temperatures.
Stratified lakes are separated into three distinct vertical zones (Figure 4):
– the epilimnion, the upper well mixed layer, is marked by little variation in temperatures over depth.
– the metalimnion, a transition layer, between the epilimnion and the hypolimnion, temperatures decrease rapidly with depth. This layer contains the thermocline, a thin but distinct layer in which temperature changes more rapidly with depth than it does in the layers above or below. The location of the thermocline varies with the depth of the outlet used for withdrawal in reservoirs (Casamitjana et al., 2003).
– the hypolimnion, the bottom layer, below the metalimnion, shows little change in temperature with depth.
This type of stratification does not apply to all lakes and reservoirs. Some reservoirs may only have an epilimnion and a metalimnion (Bade, 2005). The location of the thermocline is not governed by solar radiation and wind only, but varies with the depth of the outlet used for withdrawal (Casamitjana et al., 2003) and can also be influenced by inflows.

Stratification and mixing patterns in lakes and reservoirs

Different patterns of seasonal stratification and mixing in lakes are observed throughout the world (Lewis Jr., 1983):
1. Amictic lakes never mix because they are permanently frozen. They exhibit inverse cold water stratification whereby water temperature increases with depth below the ice surface 0 °C. Such lakes are found in Arctic and An tarctic regions and at very high altitudes.
2. Meromictic lakes mix only partially; the deep water layers never intermix either because of high water density caused by dissolved substances or because the lake is protected from wind effects. An example of a meromictic lake is Lake Pavin (Bonhomme et al., 2011).
3. Holomictic lakes mix completely at some time in the year; their temperature and density is uniform from top to bottom at that time. These lakes are classified according to the frequency of mixing:
– Oligomictic lakes do not mix every year. Because such lakes are usually large and have a large heat storage capacity, whether or not they mix completely depends on the local meteorological conditions. An example is Lake Bourget in France (Vinçon-Leite et al., 2014).
– Monomictic lakes mix only once each year, either in summer or in winter:
i. Cold monomictic lakes are found in Polar regions. They are covered by ice throughout much of the year. They thaw, but rarely reach temperatures above 4 °C, and mix in summer.
ii. Warm monomictic lakes mix in winter. These lakes are widely distributed from temperate to tropical climatic regions. One example is Lake Constance, which on average freezes over about once every 33 years because of its large size.
– Dimictic lakes mix twice a year (usually in spring and autumn). During winter they are covered by ice. During summer they are thermally stratified, with temperature-derived density differences separating the warm surface waters (the epilimnion), from the colder bottom waters (the hypolimnion). This is the most common lake type at temperate latitudes.
– Polymictic lakes mix frequently. These are usually shallow lakes that do not develop seasonal thermal stratification. Their stratification can last a few days or weeks. They are found both under tropical and temperate latitudes. An example is Lake Créteil in France (Soulignac et al,. 2014).
This classification is based on mixing regimes in lakes rather than reservoirs that are generally classified as polymictic.

Impact of stable stratification on water quality and biodiversity

Thermal stratification is a major factor influencing the growth and succession of phytoplankton and overall water quality in lakes. During stratification period, mixing is dramatically reduced in the hypolimnion, compared to the epilimnion and to a fully mixed lake. Oxygen conditions, nutrient cycling and phytoplankton biomass are affected by this reduced mixing (Huisman et al., 2004; Straile et al., 2003): vertical mixing of oxygen from the lake surface is hindered, which can lead to hypoxia or even anoxia in the hypolimnion, depending on the duration of stratification and on water temperature (Jankowski et al., 2006; Wilhelm and Adrian, 2008).
Thermal stratification favours buoyant species like cyanobacteria over denser species like diatoms (Huisman et al., 2004). Moreover, thermal stratification isolates the epilimnion from the nutrient-richer bottom layers, resulting in phosphorus depletion in the epilimnion. This increases the occurrence of motile algal species such as the dinoflagellate Ceratium hirundinella (Anneville et al., 2002) or the cyanobacterium Planktothrix rubescens (Micheletti et al., 1998). Due to their large body size, these algae are better protected from zooplankton grazing and may build up high standing stocks by the end of the stratification period.
Mixing is an essential process that prevents the build up of high phosphate gradients at the sediment-water interface in the hypolimnion (Marsden, 1989). However, a stratified water column hampers the phosphates released from the sediment in the hypolimnion and prevents their release to the euphotic zone where photosynthesis takes place (DeStasio et al., 1996). The breakdown of thermal stratification after the autumn overturn allows nutrient release into the euphotic zone and may induce phytoplankton blooms.

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Cyanobacteria in freshwater bodies

Cyanobacteria, also known as Cyanophyta, range from unicellular to unspecialized colonial aggregations and form the most widely distributed group of phytoplankton (Reynolds, 2006a). They can be benthic or planktonic, occasionally forming blooms in eutrophic lakes, and are an important component of the picoplankton in both marine and freshwater systems. Their pigmentation includes chlorophyll a, blue and red phycobilins (phycoerythrin, phycocyanin, allophycocyanin, and phycoerythrocyanin) and carotenoids (Barsanti and Gualtieri, 2006).
Cyanobacteria include four main orders, of which three have planktonic representatives (Reynolds, 2006a):
1. Chroococcales: Unicellular or colonial cyanobacteria but never filamentous. Most planktonic genera form mucilaginous colonies (assemblies of cells linked by a viscous exsudate called mucilage), and these are mainly encountered in fresh water. Picophytoplanktonic forms are abundant in the oceans. This order includes Aphanocapsa, Aphanothece, Chroococcus, Cyanodictyon, Gomphosphaeria, Merismopedia, Microcystis, Snowella, Synechococcus, Synechocystis, and Woronichinia.
2. Oscillatoriales: Uniseriate–filamentous cyanobacteria whose cells all undergo division in the same plane. Marine and freshwater genera. This order includes Arthrospira, Limnothrix, Lyngbya, Planktothrix, Pseudanabaena, Spirulina, Trichodesmium and Tychonema.
3. Nostocales: Unbranched–filamentous cyanobacteria whose cells all undergo division in the same plane and certain of which may be facultatively differentiated into heterocysts. Heterocysts are vegetative cells that have been drastically altered (loss of photosystem II, development of a thick, glycolipid cell wall) to provide the necessary anoxic environment for the process of nitrogen fixation. This order includes Anabaena, Anabaenopsis, Aphanizomenon, Cylindrospermopsis, Gloeotrichia and Nodularia.

Ecological and health impacts of toxic cyanobacterial blooms

When they bloom, cyanobacteria have adverse impacts on aquatic ecosystems and human health, with wide-ranging economic and ecological consequences. According to many authors, climate warming is expected to favour the dominance of cyanobacteria on other phytoplankton communities (Paerl and Paul, 2012). An improved understanding of the interactions amongst both the environmental drivers and cyanobacterial physiology is necessary to develop strategies to reduce the risk of more frequent blooms (Brookes and Carey, 2011).

Impacts on ecosystems

The proliferation of cyanobacteria can have numerous consequences. In addition to economic costs for water treatment and losses in tourism, property values, and business (Dodds et al., 2009), cyanobacterial blooms strongly impact their ecosystems. They can deplete oxygen and rise pH. The formation of cyanobacterial blooms in the epilimnion in high light intensities, increases turbidity and decreases light availability (Nõges and Solovjova, 2005) for other primary producers. The increase in pH during intense cyanobacterial blooms may be harmful to certain species of fish (Kann and Smith, 1999). Summer cyanobacterial blooms that lasted for weeks in Chesapeake Bay (USA) increased pH to 10.5; this elevated pH promoted desorption of sedimentary inorganic phosphorus (Gao et al., 2012).

Table of contents :

1.1 Reservoirs and their ecosystems
1.1.1 Actual and future development of reservoirs
1.1.2 Differences between lakes and reservoirs
1.1.3 Physical functioning of lakes and reservoirs Development of thermal stratification Stratification and mixing patterns in lakes and reservoirs Impact of stable stratification on water quality and biodiversity
1.2 Cyanobacteria in freshwater bodies
1.2.1 Ecological and health impacts of toxic cyanobacterial blooms Impacts on ecosystems Toxin production Microcystins Cylindrospermopsin
1.2.2 Main functional traits and key controlling factors of cyanobacterial blooms Temperature Light Nutrients Vertical migration Grazing Wind mixing and flushing
1.3 Water quality models and phytoplankton dynamics in reservoirs
1.3.1 Lake ecosystem models and modelling procedure
1.3.2 Overview of the most commonly applied hydrodynamic-ecological models CAEDYM DELFT3D-ECOLOGY CE-QUAL-W2 PROTECH PCLAKE IPH-TRIM3D-PCLake MyLake SALMO GLM-AED MELODIA
2.1 Study site
2.1.1 Geology and hydrology of Karaoun Reservoir Reservoir geology Reservoir hydrology Reservoir inflows Reservoir outflows and losses
2.1.2 Current and anticipated uses of Karaoun Reservoir Hydropower production Future water supply to Beirut Irrigation through Canal 900 Future Canal 800 Professional fishing
2.2 Design of a monitoring program
2.2.1 Field measurements Water sampling sites Water sampling method Transparency Phycocyanin profile measurements Water temperature, pH and conductivity measurements Dissolved oxygen
2.2.2 Laboratory analyses Phytoplankton microscopic identification and counting Chlorophyll-a quantification Nutrient analysis Cylindrospermopsin analysis
2.2.3 Measurements used to validate the model
2.3 Model description
2.3.1 DYRESM description
2.3.2 CAEDYM description Growth rate Temperature Light Cyanobacteria vertical migration Respiration, Mortality & Excretion
2.3.3 DYRESM-CAEDYM input data
2.4 Evaluation methods
2.4.1 Phytoplankton biodiversity
2.4.2 Trophic state
2.4.3 DYRESM-CAEDYM model performance
3.1 Introduction
3.2 Trophic state and algal succession in Karaoun Reservoir before 2012
3.2.1 Nutrient concentrations and trophic state
3.2.2 Algal succession and biodiversity
3.3 Trophic state and algal succession at Karaoun Reservoir in 2012 and 2013
3.3.1 Hydrological conditions
3.3.2 Physico-chemical parameters Transparency Dissolved oxygen Specific conductivity Water temperature and thermal stratification Nitrate and ammonium Total phosphorus and orthophosphate
3.3.3 Chlorophyll-a and phycocyanin fluorescence
3.3.4 Phytoplankton composition and biovolumes
3.3.5 Phytoplankton groups seasonal succession
3.3.6 Zooplankton community
3.3.7 Trophic level and diversity index
3.4 Environmental drivers of the succession of phytoplankton groups in Karaoun Reservoir
3.4.1 Settling of diatoms after establishment of thermal stratification in early spring
3.4.2 Disappearance of green algae after nutrient limitation and temperature elevation in late spring
3.4.3 Cyanobacteria dominance at high temperature and low nutrient concentrations between late spring and early autumn
3.4.4 Dominance of dinoflagellate at low irradiance and water temperature in autumn
3.5 Comparison with other Mediterranean lakes and reservoirs
3.5.1 Morphological and hydrological characteristics
3.5.2 Eutrophication level and integrated water management
3.5.3 Phytoplankton diversity
3.5.4 Toxic cyanobacterial succession
3.6 Conclusion
4.1 Introduction
4.2 Results
4.2.1 Physico-chemical conditions
4.2.2 Replacement of Aphanizomenon ovalisporum by Microcystis aeruginosa at high temperature
4.2.3 Cylindrospermopsin detection
4.2.4 Comparison between A. ovalisporum and CYN distribution in the water column
4.2.5 Absence of correlation between Cylindrospermopsin concentration and A. ovalisporum biovolumes
4.3 Discussion
4.3.1 Aphanizomenon ovalisporum blooms in Karaoun Reservoir
4.3.2 Competition between Microcystis aeruginosa and Aphanizomenon ovalisporum
4.3.3 Relation between cylindrospermopsin concentrations and A. ovalisporum
4.3.4 Disappearance of CYN from water column by degradation or sedimentation
4.4 Conclusion
5.1 Introduction
5.2 Description of input data to DYRESM-CAEDYM
5.3 DYRESM-CAEDYM configuration
5.4 Thermal model calibration and verification
5.5 Biological model calibration and validation
5.6 Succession of Aphanizomenon ovalisporum and Microcystis aeruginosa according to DYRESMCAEDYM
5.7 Model performance
5.8 Model limitations
5.9 Conclusion
General conclusion


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