68
M. Callisto et al.
The proposed model to describe the cyanobacteria density
in the reservoirs was found to have residuals with a normal
distribution (W = 0.96; p = 0.73) by the analysis assumptions
(Fig. 5.12).
The availability of phosphorus has been regarded as one
of the most important factors for determining phytoplankton
biomass and water quality in lakes and temperate wetlands
(Dilon and Rigler 1974) of tropical and subtropical regions
(Attayde and Bozelli 1999; Huzart et al. 2006). However,
reservoirs have different characteristics than natural lakes.
For example, they often have unique morphology and hydrodynamics, as well as high loading rates of nutrients from
the sediments and the drainage basin (Wetzel 1990). Thus,
the driving factors of limnological dynamics are not likely to
be similar in lakes and reservoirs, and therefore, the system
responses to nutrient enrichment are likely to be different
(Tundisi et al. 1990).
Our models demonstrated that cyanobacteria blooms,
which are a regular problem in Brazilian semiarid reservoirs
(Vasconcelos et al. 2011), did not have a direct relationship
with phosphorus concentrations. This result suggests that
phosphorus concentrations were not a good indicator of the
trophic level of these reservoirs. The low transparency of the
water was owing to high concentrations of suspend solids,
which favors the growth of cyanobacteria while inhibiting
other classes of phytoplankton.
The high suspended solids concentrations may have been
influenced by an increase of aquaculture activities in the
Brazilian semiarid reservoirs. The practice of farming fish
in net cages is an important factor related to the elevated
concentrations of nutrients and Chla, as well as a reduction of water transparency. The impact of fish farming on
reservoirs is readily observable from the visible release of
dissolved or suspended materials and metabolites from food
remains (Starling et al. 2002; Lazzaro et al. 2003; Guo et al.
2009; Borges et al. 2010). These substances are associated
with a high temperature, which favors an increase in the
density of phytoplankton, and particularly cyanobacteria
(Padisák 1997; Chorus and Bartram 1999; Reynolds et al.
1997), which may explain the high densities of cyanobacteria observed in the studied reservoirs.
Additional factors should be considered to help explain
eutrophication in semiarid waters. Throughout the study
period, the concentrations of TP and densities of cyanobacteria were clearly increasing. This can be explained by the
increasing level of anthropogenic activities in the area and
by the climatic features of the region. The long time necessary for water renewal is an important natural event that
can maximize this problem. In addition to the low rainfall,
other factors also influenced the eutrophication, including
nutrient loads from tributaries during the rainy periods; furthermore, because renewal requires large inputs of relatively
clean water, which did not occur during this period, the trophic levels of the reservoirs will likely be maintained or even
worsen over time.
Fig. 5.10 Evolution of increasing phosphorus concentrations from
2006 to 2009 in the Paraíba River Basin
Fig. 5.11 Occurrence of cyanobacteria blooms in the Paraíba River
Basins and the evolution of cyanobacteria densities from 2006 to 2010
Fig. 5.12 Dispersion of the predicted values as a function of the residual pattern
M. Callisto et al.
The proposed model to describe the cyanobacteria density
in the reservoirs was found to have residuals with a normal
distribution (W = 0.96; p = 0.73) by the analysis assumptions
(Fig. 5.12).
The availability of phosphorus has been regarded as one
of the most important factors for determining phytoplankton
biomass and water quality in lakes and temperate wetlands
(Dilon and Rigler 1974) of tropical and subtropical regions
(Attayde and Bozelli 1999; Huzart et al. 2006). However,
reservoirs have different characteristics than natural lakes.
For example, they often have unique morphology and hydrodynamics, as well as high loading rates of nutrients from
the sediments and the drainage basin (Wetzel 1990). Thus,
the driving factors of limnological dynamics are not likely to
be similar in lakes and reservoirs, and therefore, the system
responses to nutrient enrichment are likely to be different
(Tundisi et al. 1990).
Our models demonstrated that cyanobacteria blooms,
which are a regular problem in Brazilian semiarid reservoirs
(Vasconcelos et al. 2011), did not have a direct relationship
with phosphorus concentrations. This result suggests that
phosphorus concentrations were not a good indicator of the
trophic level of these reservoirs. The low transparency of the
water was owing to high concentrations of suspend solids,
which favors the growth of cyanobacteria while inhibiting
other classes of phytoplankton.
The high suspended solids concentrations may have been
influenced by an increase of aquaculture activities in the
Brazilian semiarid reservoirs. The practice of farming fish
in net cages is an important factor related to the elevated
concentrations of nutrients and Chla, as well as a reduction of water transparency. The impact of fish farming on
reservoirs is readily observable from the visible release of
dissolved or suspended materials and metabolites from food
remains (Starling et al. 2002; Lazzaro et al. 2003; Guo et al.
2009; Borges et al. 2010). These substances are associated
with a high temperature, which favors an increase in the
density of phytoplankton, and particularly cyanobacteria
(Padisák 1997; Chorus and Bartram 1999; Reynolds et al.
1997), which may explain the high densities of cyanobacteria observed in the studied reservoirs.
Additional factors should be considered to help explain
eutrophication in semiarid waters. Throughout the study
period, the concentrations of TP and densities of cyanobacteria were clearly increasing. This can be explained by the
increasing level of anthropogenic activities in the area and
by the climatic features of the region. The long time necessary for water renewal is an important natural event that
can maximize this problem. In addition to the low rainfall,
other factors also influenced the eutrophication, including
nutrient loads from tributaries during the rainy periods; furthermore, because renewal requires large inputs of relatively
clean water, which did not occur during this period, the trophic levels of the reservoirs will likely be maintained or even
worsen over time.
Fig. 5.10 Evolution of increasing phosphorus concentrations from
2006 to 2009 in the Paraíba River Basin
Fig. 5.11 Occurrence of cyanobacteria blooms in the Paraíba River
Basins and the evolution of cyanobacteria densities from 2006 to 2010
Fig. 5.12 Dispersion of the predicted values as a function of the residual pattern