63
5 Eutrophication of Lakes
In lakes located in temperate regions, phosphorus is the
main limiting resource of primary productivity and the concentration of phosphorus in the water is correlated with algae
biomass (Dillon and Rigler 1974; OECD 1982). On the other
hand, Thornton and Rast (1989, 1993) showed that reservoirs
from semiarid regions respond differently to eutrophication
than do lakes in temperate regions, from which the classical concepts of eutrophication were developed. Thus, the
primary functions that influence the limnological dynamics
may not be similar between these two regions, and, therefore, system responses to nutrient enrichment may be different (Thornton 1990; Tundisi et al. 1990).
Because the TSI was developed in temperate lakes, its application to tropical reservoirs has been widely questioned
(Bouvy et al. 2000; Huzsar et al. 2000; Costa et al. 2006;
Panosso et al. 2007). Thornton and Rast (1993) proposed
that concentrations of TP above 60 μg l
−1
and Chlaabove
12 μg l
−1
are indicative of a eutrophic reservoir in semiarid
regions. This designation is set at the point where the algal
biomass would be limited more by light than by the concentration of phosphorus.
A typical phenomenon in eutrophic lakes is the occurrence
of blooms of cyanobacteria, which are common planktonic
organisms living in different aquatic ecosystems. Although
they are primarily related to eutrophication (Watson et al.
1997), their distribution is not limited to high-productivity
ecosystems (Reynolds 1997). Cyanobacteria blooms are
globally distributed and are regulated by many environmental variables linked by geographical and ecological aspects
(Ferrão-Filho et al. 2009).
Cyanobacteria blooms affect water quality by changing the pH, transparency, and biodiversity, and by producing odors and/or toxins, such as cyanotoxins (Blahová
et al. 2008). Toxic blooms of cyanobacteria are common in
man-made lakes in the semiarid regions of Brazil and they
frequently cause human health threats (Molica et al. 2005;
Costa et al. 2006; Vasconcelos et al. 2011).
Although cyanobacteria blooms in reservoirs in semiarid
Brazil are mainly related to the eutrophic conditions of these
systems, other factors may also contribute to the success of
cyanobacteria. These factors include environmental constancy, annual rain deficit and lack of water renewal, high
temperatures, high pH, low N/P ratio, low ammonium concentrations, and the absence of efficient predators (Bouvy
et al. 1999, 2000; Huszar et al. 2000, 2006; Barbosa et al.
2010; Dantas et al. 2010).
et al. 2006; Leunda et al. 2009; Puntí et al. 2009). Several
previous studies (e.g., Feio et al. 2006; Aroviita et al. 2010)
have shown that these changes are known to affect ecological assessments based on reference conditions that represent
the systems only for a given season. However, the seasonal
variability of temperature and precipitation observed during
the 2 years of sampling in this study was not reflected in the
benthic communities of the reservoirs. Seasonal variability
in the communities was unpredictable and similar to the interannual variability. Other authors studying both subtropical systems (China) and temperate systems (Canada) have
observed that rainfall and flood pulses did not influence the
distribution of Chironomids in reservoirs because they are
well adapted to fluctuations in the water level (Zhang et al.
2010; Furey et al. 2006).
The ANOSIM global R values for the three reservoirs
showed wide variability within the sampling periods (ANOSIM Serra Azul: Global R = 0.054, p = 0.001; ANOSIM Ibirité: R = 0.166, p = 0.001; and ANOSIM Vargem das Flores:
R = 0.113, p = 0.001). According to the TSI, 29 sites were
classified as oligotrophic in the Serra Azul reservoir, and
only one site was classified as mesotrophic. The Vargem das
Flores reservoir had 26 sites classified as oligotrophic, with
1 site as mesotrophic and 3 sites as eutrophic. Over the 2
years of samples in the Ibirité reservoir, 28 sites were identified as eutrophic and 2 sites were mesotrophic (Fig. 5.6).
The environmental variables were found to have higher
values in the eutrophic reservoir than in the oligotrophic reservoir. For example, the median electrical conductivity was
280.50 and 26.36 mS cm
−1
for the eutrophic and oligotrophic
reservoirs, respectively. The exception was for the S, which
had the highest average values in the oligotrophic reservoir
(see Table 5.1). The decrease in diversity of the benthic communities in the Ibirité reservoir can likely be explained by an
increase in the trophic status owing to the land use and occupation of the areas surrounding this reservoir. The spatial
distribution of some of the environmental variables showed
that one arm of the reservoir had high concentrations of nutrients (TN and high levels of TN/TP; Fig. 5.7).
5.7.6 Semiarid Lentic Ecosystems
Hydrological fluctuations can cause significant changes to
lakes and reservoirs in arid and semiarid regions, where
both seasonal and annual variability can cause changes in
the shape and size of the ecosystems (Sánches-Carrillo et al.
2007). The reservoirs in these regions can suffer significant
intraannual variations of water volume, surface area, water
residence time, and depth, which may affect their physical,
chemical, and biological features.
5 Eutrophication of Lakes
In lakes located in temperate regions, phosphorus is the
main limiting resource of primary productivity and the concentration of phosphorus in the water is correlated with algae
biomass (Dillon and Rigler 1974; OECD 1982). On the other
hand, Thornton and Rast (1989, 1993) showed that reservoirs
from semiarid regions respond differently to eutrophication
than do lakes in temperate regions, from which the classical concepts of eutrophication were developed. Thus, the
primary functions that influence the limnological dynamics
may not be similar between these two regions, and, therefore, system responses to nutrient enrichment may be different (Thornton 1990; Tundisi et al. 1990).
Because the TSI was developed in temperate lakes, its application to tropical reservoirs has been widely questioned
(Bouvy et al. 2000; Huzsar et al. 2000; Costa et al. 2006;
Panosso et al. 2007). Thornton and Rast (1993) proposed
that concentrations of TP above 60 μg l
−1
and Chlaabove
12 μg l
−1
are indicative of a eutrophic reservoir in semiarid
regions. This designation is set at the point where the algal
biomass would be limited more by light than by the concentration of phosphorus.
A typical phenomenon in eutrophic lakes is the occurrence
of blooms of cyanobacteria, which are common planktonic
organisms living in different aquatic ecosystems. Although
they are primarily related to eutrophication (Watson et al.
1997), their distribution is not limited to high-productivity
ecosystems (Reynolds 1997). Cyanobacteria blooms are
globally distributed and are regulated by many environmental variables linked by geographical and ecological aspects
(Ferrão-Filho et al. 2009).
Cyanobacteria blooms affect water quality by changing the pH, transparency, and biodiversity, and by producing odors and/or toxins, such as cyanotoxins (Blahová
et al. 2008). Toxic blooms of cyanobacteria are common in
man-made lakes in the semiarid regions of Brazil and they
frequently cause human health threats (Molica et al. 2005;
Costa et al. 2006; Vasconcelos et al. 2011).
Although cyanobacteria blooms in reservoirs in semiarid
Brazil are mainly related to the eutrophic conditions of these
systems, other factors may also contribute to the success of
cyanobacteria. These factors include environmental constancy, annual rain deficit and lack of water renewal, high
temperatures, high pH, low N/P ratio, low ammonium concentrations, and the absence of efficient predators (Bouvy
et al. 1999, 2000; Huszar et al. 2000, 2006; Barbosa et al.
2010; Dantas et al. 2010).
et al. 2006; Leunda et al. 2009; Puntí et al. 2009). Several
previous studies (e.g., Feio et al. 2006; Aroviita et al. 2010)
have shown that these changes are known to affect ecological assessments based on reference conditions that represent
the systems only for a given season. However, the seasonal
variability of temperature and precipitation observed during
the 2 years of sampling in this study was not reflected in the
benthic communities of the reservoirs. Seasonal variability
in the communities was unpredictable and similar to the interannual variability. Other authors studying both subtropical systems (China) and temperate systems (Canada) have
observed that rainfall and flood pulses did not influence the
distribution of Chironomids in reservoirs because they are
well adapted to fluctuations in the water level (Zhang et al.
2010; Furey et al. 2006).
The ANOSIM global R values for the three reservoirs
showed wide variability within the sampling periods (ANOSIM Serra Azul: Global R = 0.054, p = 0.001; ANOSIM Ibirité: R = 0.166, p = 0.001; and ANOSIM Vargem das Flores:
R = 0.113, p = 0.001). According to the TSI, 29 sites were
classified as oligotrophic in the Serra Azul reservoir, and
only one site was classified as mesotrophic. The Vargem das
Flores reservoir had 26 sites classified as oligotrophic, with
1 site as mesotrophic and 3 sites as eutrophic. Over the 2
years of samples in the Ibirité reservoir, 28 sites were identified as eutrophic and 2 sites were mesotrophic (Fig. 5.6).
The environmental variables were found to have higher
values in the eutrophic reservoir than in the oligotrophic reservoir. For example, the median electrical conductivity was
280.50 and 26.36 mS cm
−1
for the eutrophic and oligotrophic
reservoirs, respectively. The exception was for the S, which
had the highest average values in the oligotrophic reservoir
(see Table 5.1). The decrease in diversity of the benthic communities in the Ibirité reservoir can likely be explained by an
increase in the trophic status owing to the land use and occupation of the areas surrounding this reservoir. The spatial
distribution of some of the environmental variables showed
that one arm of the reservoir had high concentrations of nutrients (TN and high levels of TN/TP; Fig. 5.7).
5.7.6 Semiarid Lentic Ecosystems
Hydrological fluctuations can cause significant changes to
lakes and reservoirs in arid and semiarid regions, where
both seasonal and annual variability can cause changes in
the shape and size of the ecosystems (Sánches-Carrillo et al.
2007). The reservoirs in these regions can suffer significant
intraannual variations of water volume, surface area, water
residence time, and depth, which may affect their physical,
chemical, and biological features.