*2.3.9 Venezuela*

González and Quirós [73], when they considered trends in 16 reservoirs that have different trophic status, found both linear relationship, between total phosphorus (TP) and total nitrogen (TN) with the phytoplankton biomass, and empirical relationship, between total phosphorus concentration and the nitrate/ ammonia quotient which determine the dominance of cyanobacteria. According to the authors' empirical ordination, Venezuelan reservoirs were separated in three groups: group 1 includes reservoirs with low TP (<20 μg/L), while groups 2 and 3 include those reservoirs with moderate to high TP concentrations (>20 μg/L) (**Figure 1**). In group 1, green algae (chlorophyta) are dominant. Group 2 is composed by those reservoirs where nitrate is the dominant inorganic nitrogen compound over ammonia (high nitrate/ammonia quotient), with short water residence time, and the dominant phytoplankton taxa are different from cyanobacteria, while

#### **Figure 1.**

*Relationships between TP and the NO3:NH4 ratio in Venezuelan reservoirs: group 1—TP < 20 μg/L—black circles; group 2—TP > 20 μg/L and non-cyanobacteria dominance—black diamonds; and group 3—TP > 20 μg/L and cyanobacteria dominance—black triangles. The Loma de Níquel reservoir is represented by a white circle, because it represented an intermediate situation between groups. Abbreviation of reservoir names: AFR: Agua Fría, TAG: Taguaza, LAG: Lagartijo, CLA: Clavellinos, TBL: Tierra Blanca, LNI: Loma de Níquel, ECI: El Cigarrón, EPU: El Pueblito, ECU: El Cují, EAN: El Andino, LMA: La Mariposa, LPE: La Pereza, CAM: Camatagua, QSE: Quebrada Seca, PC1: Pao-Cachinche—western wing with uptake point and outlet, and PC2: Pao-Cachinche—eastern wing without outlet, modified from González and Quirós [73].*

group 3 is composed by those reservoirs with low nitrate/ammonia quotient, high residence time of their waters, and the dominant phytoplankton is cyanobacteria.

#### **3. Discussion**

Surveys have shown that more than 40.0% of lakes and reservoirs in the Americas are in eutrophic trophic level [1, 74], and this is a major cause of concern in the developing as well as developed countries.

According to Pratts et al. [75], one of the main problems that affect lakes and reservoirs is the eutrophication. It induces undesirable ecological consequences for the water bodies [1, 3], such as excessive phytoplankton and macrophyte growth. This reduces light penetration and restricts the reoxygenation of water, therefore generating anoxic conditions in the hypolimnetic layers of lakes and reservoirs, as well as high decomposition rates of organic matter that produces a foul smell and makes the water more turbid. Other negative consequences are the proliferation of algal blooms and toxic phytoplankton, fish mortality by suffocation due to drastic oxygen concentration drop during the overturning of waters, proliferation of adequate habitats for vectors of tropical diseases, and loss of biodiversity. These problems are especially important if the water bodies are used for drinking water supply: if these problems are inadequately treated, they may involve serious health risks for human populations.

Regarding the main primary producers in lakes and reservoirs, phytoplankton communities respond quickly to environmental change (as the fertilization of waters with nitrogen and phosphorus) and are indicators of eutrophication [76]. They also show different community dynamics in ecosystems with contrasting trophic states, where high nutrient levels generally favor species belonging to cyanobacteria group.

In most of the studies on lakes and reservoirs in the Americas discussed in the present work, and despite their regional, latitudinal, altitudinal, and climatic differences, eutrophic conditions have led to be dominated by specific species of cyanobacteria. Then, species from the genera *Microcystis*, *Anabaena*, *Planktothrix*, *Oscillatoria*, and *Cylindrospermopsis* seem to be the more widespread dominant organisms under eutrophic conditions in lakes and reservoirs in the American continent. Thus, the cyanobacteria dominance in anthropogenically eutrophic water bodies is an increasing problem that impacts recreation, ecosystem integrity, and human and animal health [77, 78], by deterioration of water quality [79, 80]. The cyanobacterial dominance, on the other side, is under effect of several interacting abiotic (temperature, N/P ratio, other factors) and biotic (intraspecies and interspecific competition in community) factors that usually show different reactions in different environments [81, 82].

According to Reynolds et al. [83] and Bellinger and Sigee [10], cyanobacteria adapts to all types of freshwater environment, including extreme conditions and frequently have the ability to compete with other phytoplankton groups under eutrophic conditions in surface waters. Shapiro [84] and Dokulil and Teubner [81] stated that the ability of "blue-greens" to outcompete other freshwater algae has been attributed to a range of characteristics, including:


**37**

*Eutrophication and Phytoplankton: Some Generalities from Lakes and Reservoirs of the Americas*

• tolerance to low N/P ratios, which is the characteristic for eutrophic lakes—

• depth regulation by buoyancy—avoiding photoinhibition during the early phase of population increase, and allowing algae to obtain inorganic nutrients from the hypolimnion layer when the nutrients decrease in the epilimnion

• tolerance to high pH and low CO2 concentrations, allowing continued growth of "blue-greens" (but not other algae) at the lake surface during the extensive

• symbiotic association with aerobic bacteria—bacterial symbionts at the heterocyst surface provide the local reducing atmosphere required for nitrogen fixation that causes inorganic nutrients accumulation in surface waters, poor in nutrients.

The dominance of cyanobacteria in eutrophic water bodies can also be explained according to the main meaningful functional groups proposed by Reynolds [85, 86] and Reynolds et al. [87], based on nutrient availability and stability of the water column. Thus, cyanobacteria can dominate in the phytoplanktonic community in an array of warm, mixed, and fertilized (mainly phosphorus) water bodies, with

In most of the eutrophic lakes and reservoirs in the Americas, as were in various

researches on freshwater systems in the temperate regions, there are important increases in level of nutrients, mainly in level of phosphorus, which leads to excessive phytoplankton biomass firstly in a predominance of cyanobacteria. Likewise, in most cases, the dominant cyanobacterial species belong to the genera *Microcystis*, *Anabaena*, *Planktothrix*, *Oscillatoria*, and *Cylindrospermopsis*, which have toxic strains that can cause potential health problems, particularly if the water bodies are

Authors would like to thank the Inter American Network of Academic of

We also thank Dr. Diego Rodríguez and an anonymous reviewer for improving

Sciences (IANAS) for all its help in gathering the information.

The authors declare no conflict of interest.

• resistance to zooplankton grazing by both mechanical and chemical

allowing continued growth when N becomes limited;

low transparency values and limitation in C and N [87, 88].

*DOI: http://dx.doi.org/10.5772/intechopen.89010*

layer from mid- to late-summer;

interference;

**4. Conclusions**

used for drinking water supply.

**Acknowledgements**

the language translation.

**Conflict of interest**

bloom formation; and

*Eutrophication and Phytoplankton: Some Generalities from Lakes and Reservoirs of the Americas DOI: http://dx.doi.org/10.5772/intechopen.89010*


The dominance of cyanobacteria in eutrophic water bodies can also be explained according to the main meaningful functional groups proposed by Reynolds [85, 86] and Reynolds et al. [87], based on nutrient availability and stability of the water column. Thus, cyanobacteria can dominate in the phytoplanktonic community in an array of warm, mixed, and fertilized (mainly phosphorus) water bodies, with low transparency values and limitation in C and N [87, 88].

### **4. Conclusions**

*Microalgae - From Physiology to Application*

in the developing as well as developed countries.

**3. Discussion**

group 3 is composed by those reservoirs with low nitrate/ammonia quotient, high residence time of their waters, and the dominant phytoplankton is cyanobacteria.

Surveys have shown that more than 40.0% of lakes and reservoirs in the Americas are in eutrophic trophic level [1, 74], and this is a major cause of concern

According to Pratts et al. [75], one of the main problems that affect lakes and reservoirs is the eutrophication. It induces undesirable ecological consequences for the water bodies [1, 3], such as excessive phytoplankton and macrophyte growth. This reduces light penetration and restricts the reoxygenation of water, therefore generating anoxic conditions in the hypolimnetic layers of lakes and reservoirs, as well as high decomposition rates of organic matter that produces a foul smell and makes the water more turbid. Other negative consequences are the proliferation of algal blooms and toxic phytoplankton, fish mortality by suffocation due to drastic oxygen concentration drop during the overturning of waters, proliferation of adequate habitats for vectors of tropical diseases, and loss of biodiversity. These problems are especially important if the water bodies are used for drinking water supply: if these problems are inadequately treated, they may involve serious health risks for human populations. Regarding the main primary producers in lakes and reservoirs, phytoplankton communities respond quickly to environmental change (as the fertilization of waters with nitrogen and phosphorus) and are indicators of eutrophication [76]. They also show different community dynamics in ecosystems with contrasting trophic states, where high nutrient levels generally favor species belonging to cyanobacteria group. In most of the studies on lakes and reservoirs in the Americas discussed in the present work, and despite their regional, latitudinal, altitudinal, and climatic differences, eutrophic conditions have led to be dominated by specific species of cyanobacteria. Then, species from the genera *Microcystis*, *Anabaena*, *Planktothrix*, *Oscillatoria*, and *Cylindrospermopsis* seem to be the more widespread dominant organisms under eutrophic conditions in lakes and reservoirs in the American continent. Thus, the cyanobacteria dominance in anthropogenically eutrophic water bodies is an increasing problem that impacts recreation, ecosystem integrity, and human and animal health [77, 78], by deterioration of water quality [79, 80]. The cyanobacterial dominance, on the other side, is under effect of several interacting abiotic (temperature, N/P ratio, other factors) and biotic (intraspecies and interspecific competition in community) factors that usually show different reactions in

According to Reynolds et al. [83] and Bellinger and Sigee [10], cyanobacteria adapts to all types of freshwater environment, including extreme conditions and frequently have the ability to compete with other phytoplankton groups under eutrophic conditions in surface waters. Shapiro [84] and Dokulil and Teubner [81] stated that the ability of "blue-greens" to outcompete other freshwater algae has

• optimum growth at high temperatures not preferred by other phytoplankton

• high survival ability in the water column compared to other species under low

**36**

different environments [81, 82].

groups, as in diatoms;

been attributed to a range of characteristics, including:

light tolerance caused by extensive algal bloom;

• increasing trend in temperature due to climate change;

In most of the eutrophic lakes and reservoirs in the Americas, as were in various researches on freshwater systems in the temperate regions, there are important increases in level of nutrients, mainly in level of phosphorus, which leads to excessive phytoplankton biomass firstly in a predominance of cyanobacteria. Likewise, in most cases, the dominant cyanobacterial species belong to the genera *Microcystis*, *Anabaena*, *Planktothrix*, *Oscillatoria*, and *Cylindrospermopsis*, which have toxic strains that can cause potential health problems, particularly if the water bodies are used for drinking water supply.
