**3. Stressors' effect on eutrophication characteristics parameters**

The stressors' effects can be well highlighted by the mains parameters: temperature, pH, Secchi disc transparency (SD), chlorophyll *a* (CHL), dissolved oxygen (DO), total phosphorus (TP), total nitrogen (TN), and plankton populations [24].

### **3.1. Temperature and aquatic stratification**

or volcanic eruptions. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding greenhouse gas emissions including water vapors (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others which have caused the earth's surface temperature to rise. Atmospheric CO2 concentrations have increased by more than 40% since preindustrial times, from approxi‐ mately 280 parts per million by volume (ppmv) in the eighteenth century to 396 ppmv in 2013.

Freshwater resources are vulnerable to climate change; warming of the climate system increases global average air and influences the hydrological cycle [17]. Climate change associated with the water cycle (**Figure 2**) includes water body and land temperature increase [18], accelerated glaciers melting, decreasing the surface of water and land occupied with snow, increased evaporation and level of lakes water reduction, increased level of coastal marine and ocean and inundation, wetland loss by sea level rise, changes in the seasonal distribution and amount of precipitation, increasing precipitation intensity sometimes as extreme weather—storms, changes in the balance between snow and rain, increasing nutrients' concentration by soil washing and soil erosion [19], increasing acidity in rivers, lakes, seas, and oceans [20]. **Figure 2** shows the conceptual diagram visualizing the main components of

Waters with similar effect filters, however, should respond similar to climate variability. Hydrodynamic patterns are influenced largely by the depth and size of the lake affecting the annual heat budget, temperature stratification during summer and winter, the concentration of oxygen in the hypolimnion, salt solubility, and availability of nutrients. The retention time (a factor depending on morphometry and through-flow) determines if internal or external

**Figure 1** shows the variations of the atmospheric CO2, CH4, N2O per year [7, 16].

**Figure 1.** Atmospheric CO2, CH4, N2O variations per year [7].

climate change and their major effects on freshwaters [21].

*2.2.2. Effect of climate change*

18 Water Stress in Plants

processes dominate.

Usually, the atmospheric temperature influences the temperature of natural water bodies and these two together depend on the geographical location and meteorological conditions (rainfall, humidity, wind velocity, etc.). The water temperature varies with depths, at the surface the temperature is higher than at greater depths, a phenomenon known as aquatic stratification. The water temperature influences the chemical, physical, and biological proc‐ esses. Gas solubility decreases with increase in temperature, and biochemical activity doubles every 10°C of temperature increase. Thus, this kind of heating may cause a thermal impact on receiving water bodies and may influence the whole native community [25, 26].

**Figure 3.** Temperate lakes are thermally stratified in the summer but mix each spring and autumn [28].

Because the lakes and reservoirs are deep enough, they stratify, generating "layers" of water with different physical characteristics. The large differences in density between layers of water determine thermal stratification. The density is influenced by temperature, so at about 3.98 °C the water is most dense (heaviest). The water stratification is seasonal. In the spring time, in temperate climates, immediately after the ice melts, the surface water beings to warm to 0°C. The increasing density of the warming water along with wind cause this surface water to sink and mix with the deeper water, a process called spring turnover. In this period, the water column is cold and has approximately the same temperature (**Figure 3**). During the late spring and summer, the sunlight is absorbed in the water column, heating up with the air average daily temperature increases. In the absence of wind, temperature decreases exponentially with the depth. The lake is now stratified into three layers of water, termed summer stratification. The upper layer, called the epilimnion zone, is in contact with the atmosphere and seasonal climate factors variations, is warm, well-mixed, has a higher pH and higher dissolved oxygen. The thermocline is a plane where the greatest water temperature changes and is very resistant to wind mixing. Hypolimnion starts beneath the metalimnion, extending to the lake bottom, is the coldest layer of a lake in summer, and the warmest layer during winter, usually dark, receives insufficient irradiance (light) for photosynthesis to occur and relatively undisturbed. In deep, temperate lakes, the bottom-most waters of the hypolimnion are typically close to 4°C throughout the year [26, 27].

Epilimnion is cooling down during autumn when decreasing the difference of the density to the hypolimnion. When the temperature of surface and bottom waters and density are approximately the same, winds can mix the entire lake. In winter, the surface water is cooling until it freezes; thus it appears less distinct density stratification because the density difference between 0 and 4°C water is quite small; most of the water column is isothermal at a temperature of 4°C, which is denser than the colder, lighter water just below the ice [26, 29].

Blue-green algae tend to dominate warmer waters while green algae do better under cooler conditions. The toxicity of unionized ammonia is also related to warmer temperatures [30]. The maximum specific growth rate is in the range 5–40°C for members of the Chlorophyta and Bacillariophyta [31]. The optimal temperature for phytoplankton cultures is generally between 20 and 24°C, most commonly cultured species of microalgae tolerate temperatures between 16 and 27°C [32].

### **3.2. pH**

(rainfall, humidity, wind velocity, etc.). The water temperature varies with depths, at the surface the temperature is higher than at greater depths, a phenomenon known as aquatic stratification. The water temperature influences the chemical, physical, and biological proc‐ esses. Gas solubility decreases with increase in temperature, and biochemical activity doubles every 10°C of temperature increase. Thus, this kind of heating may cause a thermal impact on

receiving water bodies and may influence the whole native community [25, 26].

**Figure 3.** Temperate lakes are thermally stratified in the summer but mix each spring and autumn [28].

throughout the year [26, 27].

20 Water Stress in Plants

Because the lakes and reservoirs are deep enough, they stratify, generating "layers" of water with different physical characteristics. The large differences in density between layers of water determine thermal stratification. The density is influenced by temperature, so at about 3.98 °C the water is most dense (heaviest). The water stratification is seasonal. In the spring time, in temperate climates, immediately after the ice melts, the surface water beings to warm to 0°C. The increasing density of the warming water along with wind cause this surface water to sink and mix with the deeper water, a process called spring turnover. In this period, the water column is cold and has approximately the same temperature (**Figure 3**). During the late spring and summer, the sunlight is absorbed in the water column, heating up with the air average daily temperature increases. In the absence of wind, temperature decreases exponentially with the depth. The lake is now stratified into three layers of water, termed summer stratification. The upper layer, called the epilimnion zone, is in contact with the atmosphere and seasonal climate factors variations, is warm, well-mixed, has a higher pH and higher dissolved oxygen. The thermocline is a plane where the greatest water temperature changes and is very resistant to wind mixing. Hypolimnion starts beneath the metalimnion, extending to the lake bottom, is the coldest layer of a lake in summer, and the warmest layer during winter, usually dark, receives insufficient irradiance (light) for photosynthesis to occur and relatively undisturbed. In deep, temperate lakes, the bottom-most waters of the hypolimnion are typically close to 4°C

The pH value is a measure of water acidity or alkalinity and the number expresses the concentration of hydrogen ions indirectly and is expressed to the pH scale (measured on a scale of 0–14). Water pH changes are governed by the amount of free CO2, carbonates, and bicarbonates and are accompanied by the changes in other physicochemical aspects that in turn influence the quality of water. Algal and macrophytes mass increase by the photosyn‐ thesis act, the CO2 increase, in dissolved state, as results of the respiration processes and decomposition of organic matter, reduce the pH [33].

$$\rm{H}\rm{CO}\_{2} + \rm{H}\_{2}\rm{O} \leftrightarrow \rm{H}\_{2}\rm{CO}\_{3} \tag{1}$$

$$\rm H\_2CO\_3 \leftrightarrow HCO\_3^- + H^+ \tag{2}$$

$$\text{HCO}\_3^- \leftrightarrow \text{CO}\_3^{2-} + \text{H}^+ \tag{3}$$

The buffering system CO2/HCO3 <sup>−</sup> /CO3 2− maintains pH around the neutral level. Depending on the current pH level operates these equations in both directions. When pH increases Eqs. (2) and (3) shift to the right.

The presence of high alkalinity (>100 mg/l) represents considerable buffering capacity and reduces the effects of both photosynthesis and decay in producing large fluctuations in the pH [30].

A minor change in the pH of water determines increasing solubility of phosphorus and other nutrients—making them more available for plants. Increasing accessible nutrients' quantity, determine increasing demand for dissolved oxygen and creates a eutrophic lake where other organisms living in the water become stressed.

The properly pH range for most fish is between 6.0 and 9.0 with a minimum alkalinity of 20 mg/l, with ideal CaCO3 levels between 75 and 200 mg/l, the pH range for most cultured algal species is between 7 and 9, with the optimum range being 8.2−8.7 [20, 32, 33].

In acidic waters' conditions, only some plants and animals survive. Generally, the younger exemplars of most species are more sensitive to environmental changes. **Figure 4** shows the minimum pH level for different species of fish, shellfish, or the insects which they can tolerate the same amount of acid; for example, frogs can tolerate water that is more acidic (i.e., has a lower pH) than trout [20].

**Figure 4.** Recommended minimum pH level to survive for different species of aquatic organisms [20, 33].

### **3.3. Dissolved oxygen**

Dissolved oxygen refers to the level of free, noncompound oxygen present in water. Dissolved oxygen concentration is affected by diffusion and aeration, photosynthesis, respiration, and decomposition. The source of the oxygen in the water is the dissolved oxygen from the air and the primary production (photosynthesis process). Depending on the atmospheric conditions, the oxygen enters the water slowly, diffuses quickly by aeration caused by wind, rapid waterfalls, groundwater, etc. The atmospheric temperature and implicit water temperature influence the water oxygen content, and the dissolved oxygen concentrations decrease as temperature increases.

The consumption of oxygen in the lake is the result of two processes: the oxidative and biochemical decomposition processes and respiration (animals, plants, and microbes consume oxygen). The dissolved organic matter is oxidized and the oxygen is taken up by purely chemical oxidation, photochemical oxidation by UV light [25].

The properly pH range for most fish is between 6.0 and 9.0 with a minimum alkalinity of 20 mg/l, with ideal CaCO3 levels between 75 and 200 mg/l, the pH range for most cultured algal

In acidic waters' conditions, only some plants and animals survive. Generally, the younger exemplars of most species are more sensitive to environmental changes. **Figure 4** shows the minimum pH level for different species of fish, shellfish, or the insects which they can tolerate the same amount of acid; for example, frogs can tolerate water that is more acidic (i.e., has a

species is between 7 and 9, with the optimum range being 8.2−8.7 [20, 32, 33].

**Figure 4.** Recommended minimum pH level to survive for different species of aquatic organisms [20, 33].

Dissolved oxygen refers to the level of free, noncompound oxygen present in water. Dissolved oxygen concentration is affected by diffusion and aeration, photosynthesis, respiration, and decomposition. The source of the oxygen in the water is the dissolved oxygen from the air and the primary production (photosynthesis process). Depending on the atmospheric conditions, the oxygen enters the water slowly, diffuses quickly by aeration caused by wind, rapid waterfalls, groundwater, etc. The atmospheric temperature and implicit water temperature influence the water oxygen content, and the dissolved oxygen concentrations decrease as

lower pH) than trout [20].

22 Water Stress in Plants

**3.3. Dissolved oxygen**

temperature increases.

Daily and seasonal fluctuations in DO may occur in response to algal and bacterial action. The biological activity increases during the spring, summer, and fall when the photosynthetic activity is high. During the summer and winter, most lakes in the temperate climate are stratified. The combination of thermal stratification and biological activity causes patterns in the water chemistry. The major zones of the lake in relating with oxygen concentration and biological activity are the following [35]:





The dissolved concentration varies by season and depth. At turnover (both spring and fall), the O2 in the water is near 100 % saturation (12–13 mg/l at 4°C at sea level pressure).

During summer stratification, in an oligotrophic lake, the oxygen concentration at depth is influenced by physical processes. The absolute concentration of oxygen decreases in the warmer waters (epilimnion) and increases in the cooler waters (metalimnion and hypolimn‐ ion). In a eutrophic lake, the oxygen concentration at depth is influenced by biological processes; the oxidative processes (decomposition and respiration) result in the consumption of oxygen (the dissolved organic matter is oxidized and the oxygen is taken up by purely chemical oxidation, photochemical oxidation by UV light) and the production of oxygen by photosynthesis (primary production):

$$6\text{CO}\_2 + 6\text{H}\_2\text{O} \rightarrow \text{C}\_6\text{H}\_{12}\text{O}\_6 + 6\text{O}\_2\tag{4}$$

Circulation (mixing) and primary production in the epilimnion maintain the oxygen at near 100% saturation. Oxidative consumption reduces the oxygen concentrations in the hypolimn‐ ion and the oxygen concentrations have less than 100% saturation [35].

**Figure 5** shows the variation of temperature and DO with season and depth in lake water. The amount of dissolved oxygen needed varies from creature to creature. Salmon cannot reproduce at a pH levels below 6 mg/l [36]. Coastal fish avoid areas where the DO is below 3.7 mg/l. Bottom feeders, crabs, oysters, and worms need minimal amounts of oxygen (1–6 mg/l), while shallow water fish need higher levels (4–15 mg/l) [37].

**Figure 5.** Seasonal fluctuation of temperature and DO with depth [34].

At low concentration of dissolved oxygen, major changes in the types and amounts of aquatic organisms found living in the water can occur. Species such as fish, mayfly nymphs, stonefly nymphs, caddis fly larvae, pike, trout, and bass that need high concentrations of dissolved oxygen will move out or die. They will be replaced by organisms such as sludge worms, blackfly larvae, and leeches which can tolerate lower dissolved oxygen concentrations. Such phenomena usually occur in late summer, when the temperature is high, the DO low, the rate of photosynthesis is intense, and water transparency is low. A fish which is under stress caused by low oxygen levels in the water is more susceptible to poisoning by insecticides or heavy metals, which can be mobilized under reducing conditions [37, 38].

During winter when water is covered by ice the diffusion of oxygen from the atmosphere in the water cannot be realized and fish, plant, and other organism consumption is greater than the oxygen production by photosynthesis. If the frozen water is covered by snow, the light cannot penetrate and photosynthesis also cannot occur. In this circumstance, the microorgan‐ isms, fish, plants, and decomposition that consume the oxygen will kill [39].

### **3.4. Transparency**

Secchi depth is the most commonly used method to determine the water transparency (clarity). Water transparency determines the depth of the photic zone and consequently affects the lower limit of light penetration which influences the primary productivity of a lake. Changes in water transparency are determined of suspended sediments: inorganic particulates and colloidal organic matter, humic and fulvic acids, phytoplankton (free-floating algae), and zooplankton. Algae are often the dominant influence on the transparency of lake water [40, 41].

For lakes' trophic state evaluation, the Secchi disc transparency parameter will be associated with another like chlorophyll *a*, and phosphorus concentration [12].

### **3.5. Nitrogen compounds**

**Figure 5.** Seasonal fluctuation of temperature and DO with depth [34].

metals, which can be mobilized under reducing conditions [37, 38].

**3.4. Transparency**

24 Water Stress in Plants

isms, fish, plants, and decomposition that consume the oxygen will kill [39].

Algae are often the dominant influence on the transparency of lake water [40, 41].

with another like chlorophyll *a*, and phosphorus concentration [12].

At low concentration of dissolved oxygen, major changes in the types and amounts of aquatic organisms found living in the water can occur. Species such as fish, mayfly nymphs, stonefly nymphs, caddis fly larvae, pike, trout, and bass that need high concentrations of dissolved oxygen will move out or die. They will be replaced by organisms such as sludge worms, blackfly larvae, and leeches which can tolerate lower dissolved oxygen concentrations. Such phenomena usually occur in late summer, when the temperature is high, the DO low, the rate of photosynthesis is intense, and water transparency is low. A fish which is under stress caused by low oxygen levels in the water is more susceptible to poisoning by insecticides or heavy

During winter when water is covered by ice the diffusion of oxygen from the atmosphere in the water cannot be realized and fish, plant, and other organism consumption is greater than the oxygen production by photosynthesis. If the frozen water is covered by snow, the light cannot penetrate and photosynthesis also cannot occur. In this circumstance, the microorgan‐

Secchi depth is the most commonly used method to determine the water transparency (clarity). Water transparency determines the depth of the photic zone and consequently affects the lower limit of light penetration which influences the primary productivity of a lake. Changes in water transparency are determined of suspended sediments: inorganic particulates and colloidal organic matter, humic and fulvic acids, phytoplankton (free-floating algae), and zooplankton.

For lakes' trophic state evaluation, the Secchi disc transparency parameter will be associated

Nitrogen is one macronutrient, which is very important for the primary production in the water body. Nitrogen is a dietary requirement for all organisms, because it is a constituent of all proteins and nucleic acids. Plants consist of approximately 7.5% nitrogen (dry mass).

Depending on water properties, various inorganic nitrogen compounds may be found. Nitrogen is mainly present as N2, NO3 − , HNO2, NO2 − , or HNO3 in aerobic conditions and as N2O, NH3, and NH4 + . Ammonium, nitrate, and nitrite have the most important role in bio‐ chemical processes. Organic nitrogen includes protein, amino acids, urea, and methylamines [1, 3]. The sum of all quantities of nitrogen contained in organic and inorganic compounds is defined as total nitrogen (TN). Total Kjeldahl nitrogen (TKN) represents the sum of the quantities of nitrogen contained in organic compounds and ammonium nitrogen (TKN = org-N + NH4-N [mg/l]). For wastewater, Kjeldahl nitrogen is used as a measure.

The main sources of nitrogen in the water are natural and anthropogenic. The discharge of nitrogen is provided by agriculture from the leachate of fertilizers from the agricultural and natural soil, which mainly contain nitrate, ammonia, urea, and amines and various pesticides, industry (pharmaceutical, alimentary, explosives, dyes, fertilizers, etc.), domestic wastewater (detergents, metabolic human products, etc.), food processing, and atmospheric deposition (**Figure 6**) [1, 42].

Nitrogen is a biological inert gas. The excess of N2 in the water (supersaturations at over 110%) affects several fishes species, causing "gas embolism" or the gas bubble disease.

Ammonia is important as the predominant excretory product of aquatic animals, through the NH3 excreted directly and also by the degradation of the fecal matter, and uneaten feed. Several types of fishes are affected by ammonia at levels higher than 0.1 mg/l. Carps and tilapias can withstand to concentrations of ammonia higher than 3–4 mg/l.

Nitrite (intermediary between ammonium and nitrate) is only present in smaller quantities in natural waters. The NO2 combines with hemoglobin and forms methemoglobin, causing the brown coloration of blood, being toxic to fish [44]. The presence of chloride ions and calcium inhibits the nitrite toxicity [45]. NO2 concentration in hard fresh water pond in fish culture should not exceed 0.1 mg NO2-N/l, and in seawater, 1.0 mg NO2-N/l. Nitrate is the major form of nitrogen used by phytoplankton; no toxic effects to fish have been reported at nitrate level below 100 mg NO3-N/l [47].

Nitrate (NO3-N) and ammonia (NH4-N) concentrations are highly variable during the lake seasonal cycles. For deep stratified lakes, nitrate is higher during mixing events and usually decreases in late summer and fall. NH4-N is generated by heterotrophic bacteria as the primary nitrogenous end product of the organic matter decomposition, and is readily assimilated by plants in the trophogenic zone [35]. NH4-N concentrations are usually low in oxygenated waters of oligo- to mesotrophic deep lakes because it is nitrified. At lower dissolved oxygen values, nitrification of ammonia ceases and higher amounts of NH4-N from the sediments are released [48].

**Figure 6.** Distribution on depth of the ammonia, nitrate, nitrite, redox potential, and temperature in a temperate lake [43].

#### **3.6. Phosphorus compounds**

Phosphorus is one very important macronutrient, which limits the primary production in freshwater. Though phosphorus is the 11th most abundant mineral in the earth's crust, it does not exist in a gaseous state. Phosphorus (P) is an essential nutrient for life, playing a role in the deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine diphosphate (ADP), and adenosine triphosphate (ATP).

**Figure 7.** Natural phosphorus cycling between the source, terrestrial, and aquatic ecosystems [6].

The phosphorus in the natural water body is provided by anthropogenic (industrial and agricultural sources) and natural sources. The phosphorus increase is caused by domestic wastewater (detergents and soaps, pesticides, food wastes, and human metabolic waste) [49, 50], food processing industries (meat, vegetable, and cheese processing) [51], distillery, synthetic and natural (cow dung, pig dung, and poultry manure) fertilizers used in agroeco‐ system [52], agricultural runoff and domestic sewage, phosphate mines [53], and it is very slow, being largely insoluble from mineral matter of rocks (**Figure 7**) [4].

The quantities of phosphorus entering the surface drainage vary with the amount of phos‐ phorus in catchment soils, topography, vegetative cover, quantity and duration of runoff flow, land use, and pollution.

The total phosphorus in aquatic systems, occurs in three forms: inorganic phosphorus (orthophosphate and polyphosphate), particulate organic phosphorus, and dissolved organic phosphorus (soluble and insoluble). The dissolved phase includes inorganic phosphorus, organic phosphorus excreted by organisms, and macromolecular colloidal phosphorus.

Particulate matter includes living and dead plankton, precipitates of phosphorus, phosphorus adsorbed to particulates, and amorphous phosphorus [54]. Total phosphorus concentrations greater than 30μg/l cause algal blooms in lakes and reservoirs.

Aquatic plants require inorganic phosphate, orthophosphate ions (PO4 3−) for nutrition [55]. This form of phosphate is transferred to consumers and decomposes as organic phosphate soluble and insoluble [35].

The deposition of phosphorus into lake sediments occurs by mechanisms such as:


### **3.7. Total nitrogen to total phosphorus ratio, TN/TP (redfield ratio)**

**Figure 6.** Distribution on depth of the ammonia, nitrate, nitrite, redox potential, and temperature in a temperate lake

Phosphorus is one very important macronutrient, which limits the primary production in freshwater. Though phosphorus is the 11th most abundant mineral in the earth's crust, it does not exist in a gaseous state. Phosphorus (P) is an essential nutrient for life, playing a role in the deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine diphosphate (ADP), and

**Figure 7.** Natural phosphorus cycling between the source, terrestrial, and aquatic ecosystems [6].

[43].

26 Water Stress in Plants

**3.6. Phosphorus compounds**

adenosine triphosphate (ATP).

Nitrogen and phosphorus are two nutrients, which are necessary for microorganism's growth. The nitrogen can be present in three species: nitrate, nitrite, and ammonia, all species are highly soluble in aquatic environment. If nitrogen is in low concentration, the microorganisms can use the nitrogen from atmosphere; also the nitrogen cannot be limited in aquatic systems. The phosphorus is most important nutrient after nitrogen, its concentration controls the plants growth, and it can be easily uptake by precipitation. The total nitrogen to total phosphorus ratio parameter indicates the stage of plant growth [57, 58].

The ratio of nitrogen:phosphorus 10:1 is ideal for aquatic plant growth, the ratio higher than 10:1 indicates phosphorus limited systems; and nitrate accumulates in abundance in water and the ratio less than 10:1, nitrogen limited systems; nitrate will be used soon as input in the water body [30, 38, 59].

## **3.8. Chlorophyll** *a* **(CHL)**

Chlorophyll *a* is used as a trophic state indicator. It indicates the ratio between planktonic primary production and algal biomass. The algal biomass generates the main problems resulting from eutrophication. It is easier to measure the value of CHL, the algal biomass. Chlorophyll *a* presents a great variability of the cellular chlorophyll content (0.1–9.7% of fresh algal weight), which is influenced by algal types. Seasonally, a great variability in individual cases can be expected due to composition of species, nutrient availability, and light conditions [3, 61].

### **3.9. Plankton populations**

The general effect of eutrophication on the trophic chain consists of excessive growth phyto‐ plankton and macrophytes vegetation, shift to bloom-forming algal species, which might be toxic or inedible, green or brown coloration of the water, increase in the biomass of benthic and epiphytic algae, change in the species composition of macrophytes vegetation, increase of consumer species biomass, increase of fish killing incidence, reduction in species diversity especially macrophytes, frequent occurrence of low dissolved oxygen events (particularly overnight), large pH changes [6, 14].

The major consequence of eutrophication concerns is oxygen availability. By daylight, photosynthesis phytoplankton produces oxygen and biomass and at night, organisms (animal and plants) and microorganisms by respiration and microorganisms by aerobic decomposing (oxidation) of the dead biomass, consume the oxygen. When the all oxygen will be consumed the oxygen from the sulfate will be used by the anoxic bacteria, will release sulfur which will capture the free oxygen still present in the upper layers and in the deep layer there will be accumulated hydrogen sulfite, which has the smell of rotten eggs.

The changes in the water will lead to important changes in the plankton population. Macro‐ algae, phytoplankton (diatoms, dinoflagellates, chlorophytes), and cyanobacteria (blue/green algae) will experience excessive growth; some of these organisms can release toxins in the water and be toxic themselves. Gelatinous aggregation that floating on the water surface can be produced by blue-green algae and diatoms. In 1982 and 1983, large amounts of gelatinous aggregations were observed on the Aegean Sea [62].

Macroalgal proliferations, the massive developments in spring and summer, also called green tides, are repeatedly observed in the marine environment. The species implicated are fre‐ quently from the genus Ulva, Monostroma, Enteromorpha, Elodea Myrisphyllus in fresh waters, Chaetomorpha, and Cladophora. The increasing amount of these type of macroalgae determine the decreasing amount of much more interesting species for biodiversity (autoch‐ thonous long-living) such as Fucus. Accumulation of large amounts of these species on beaches can induce numerous nuisances including odor, making it impossible to be used [63].

Most sensitive to oxygen availability, the zooplankton (fish and shellfish, animals with and without limited active locomotion, etc.) may die in oxygen limitation or in water with excessive alkalinity (intense photosynthesis), or toxicity from dangerous metabolic produces (cyano‐ toxins) or cells themselves of cyanobacteria and other microorganisms.

Humans or animals may be exposed to toxins through the consumption of contaminated drinking water, direct contact with fresh water or the inhalation of aerosols. Toxic compounds can be found free in the water or are cells bound. The normal processes used in treating water for drinking purposes are not efficient so as to remove the free toxins from the water.

Toxins induce damage in animals and humans by acting at the molecular level and conse‐ quently affecting cells, tissues, and organs. The main toxin groups include hepatotoxins, neurotoxins, and dermatotoxins, which produce the cyclic peptides, alkaloids, and lipopoly‐ saccharides. The nervous, digestive, respiratory, and cutaneous system may be affected [63].

The symptoms observed on mice using acute doses of hepatotoxins are liver injury and death from liver hemorrhage and cardiac failure within a few hours of exposure. Chronic exposure induces liver injury and promotes the growth of tumors, and cancer. The species of microor‐ ganisms that cause the toxic effect are Microcystis, Schizotrix, Plectonema Phormidium, Lyngbia, *Cylindrospermopsis raciborskii*, Anabaena, *Planktothrix agardhii*, Aphanizomenon, Oscillatoria, and Spirula.

Neurotoxins affect the mice and aquatic birds by causing death in a few minutes through respiration arrest. Anabaena, Oscillatoria, Aphanizomenon, Lyngbia are the species respon‐ sible for neurotoxins production. Dermatotoxins induce irritant and allergic response in tissues by contact. Lyngbia, Schizothrix, Oscillatoria are most important species which produce the dermatotoxins.

In marine water, over 40 algal species produce the toxins the most important microalgae; Dinophysis, Alexandrium, Gymnodinium, Prorocentrum, Pseudonitzschia (diatoms) are frequently observed and represent a risk for seafood consumers.

The effects include [63]:

**3.8. Chlorophyll** *a* **(CHL)**

28 Water Stress in Plants

**3.9. Plankton populations**

overnight), large pH changes [6, 14].

[3, 61].

Chlorophyll *a* is used as a trophic state indicator. It indicates the ratio between planktonic primary production and algal biomass. The algal biomass generates the main problems resulting from eutrophication. It is easier to measure the value of CHL, the algal biomass. Chlorophyll *a* presents a great variability of the cellular chlorophyll content (0.1–9.7% of fresh algal weight), which is influenced by algal types. Seasonally, a great variability in individual cases can be expected due to composition of species, nutrient availability, and light conditions

The general effect of eutrophication on the trophic chain consists of excessive growth phyto‐ plankton and macrophytes vegetation, shift to bloom-forming algal species, which might be toxic or inedible, green or brown coloration of the water, increase in the biomass of benthic and epiphytic algae, change in the species composition of macrophytes vegetation, increase of consumer species biomass, increase of fish killing incidence, reduction in species diversity especially macrophytes, frequent occurrence of low dissolved oxygen events (particularly

The major consequence of eutrophication concerns is oxygen availability. By daylight, photosynthesis phytoplankton produces oxygen and biomass and at night, organisms (animal and plants) and microorganisms by respiration and microorganisms by aerobic decomposing (oxidation) of the dead biomass, consume the oxygen. When the all oxygen will be consumed the oxygen from the sulfate will be used by the anoxic bacteria, will release sulfur which will capture the free oxygen still present in the upper layers and in the deep layer there will be

The changes in the water will lead to important changes in the plankton population. Macro‐ algae, phytoplankton (diatoms, dinoflagellates, chlorophytes), and cyanobacteria (blue/green algae) will experience excessive growth; some of these organisms can release toxins in the water and be toxic themselves. Gelatinous aggregation that floating on the water surface can be produced by blue-green algae and diatoms. In 1982 and 1983, large amounts of gelatinous

Macroalgal proliferations, the massive developments in spring and summer, also called green tides, are repeatedly observed in the marine environment. The species implicated are fre‐ quently from the genus Ulva, Monostroma, Enteromorpha, Elodea Myrisphyllus in fresh waters, Chaetomorpha, and Cladophora. The increasing amount of these type of macroalgae determine the decreasing amount of much more interesting species for biodiversity (autoch‐ thonous long-living) such as Fucus. Accumulation of large amounts of these species on beaches

can induce numerous nuisances including odor, making it impossible to be used [63].

toxins) or cells themselves of cyanobacteria and other microorganisms.

Most sensitive to oxygen availability, the zooplankton (fish and shellfish, animals with and without limited active locomotion, etc.) may die in oxygen limitation or in water with excessive alkalinity (intense photosynthesis), or toxicity from dangerous metabolic produces (cyano‐

accumulated hydrogen sulfite, which has the smell of rotten eggs.

aggregations were observed on the Aegean Sea [62].



