180
R. A. Ribeiro Filho et al.
and phosphorus, most found in these regions, increasing primary productivity. The authors emphasize that this pattern in
the distribution of chlorophyll-a is not always found and this
is because other factors (such as residence time, reservoir
size, morphometry) influence the metabolism of primary
productivity.
It is important to stress that the impact of diffuse pollution (agricultural) and punctual pollution may be increasing
nutrient levels in the Itaipu Reservoir, as it can be seen in
satellite images of the region where the Itaipu Reservoir is
located (Stivari et al. 2005). Another aspect to be analyzed
with regard to the transition zone is that, besides a steeper
topography, a large swine farming activity is practiced in the
São Francisco Verdadeiro e Falso River micro basins, producing pollution owing to an increase in nutrients caused by
lack of waste treatment of these farms (Benassi, S.F., personal communication). Most of these properties are located
near the banks of tributaries of the reservoir, explaining the
increase of nutrient concentrations, and consequent higher
occurrence of cyanobacteria in this region.
13.4.2 Trophic State Index (TSI)
The Trophic State Index is intended to classify water bodies according to nutrient enrichment and is obtained through
statistical analysis of linear regression models (Toledo Jr
et al. 1983). Water transparency many times does not show
values that represent the trophic status of the environment,
especially for the oligotrophic limit value. For this reason,
we started to use the weighted average of the indices calculated by assigning a lower weight ratio to water transparency
(Secchi disk reading), without having to remove it from the
calculation of the TSI average.
The results obtained show that the transition zone has a
higher trophic status than the other zones, what can be related to the higher nutrient input (diffuse pollution), as discussed above, or by the ratio of optimal light with the nutrient concentrations. The mean values of TSI indicate a slight
improvement of the trophic state of water, according to spatial and temporal patterns. The values found for lacustrine
zones indicate that these regions are oligotrophic. On the
other hand, the riverine and transition zones showed higher
values, reaching concentrations that indicate a mesotrophic
level with some eutrophic values in the transition zone. Nevertheless, the average value for the entire reservoir, disconsidering the zone divisions, indicated that its waters can be
classified as mesotrophic, corroborating studies by Andrade
et al. (1988). These changes in trophic status among different regions were also found by Zanata (1999) and Minillo
(2005), in which precipitation and diffuse pollution were the
factors that best explained the changes of the trophic status.
Another important factor to consider is the cascade system upstream of the Itaipu Reservoir, present in the Tietê,
Grande, and Paranapanema rivers, which supplies waters
to the Paraná River, forming the reservoir. This factor may
have a great influence, as the concentrations of nutrients and
particulate matter undergo a lowering effect through the successive reservoirs in cascade systems (Nogueira et al. 2005;
Andrade et al. 1988).
13.4.3 Analysis of Longitudinal Distribution
of Biological Communities
The responses of the communities in reservoirs under manipulated conditions or not, are incomplete, as they are altered
or destroyed before complete interactions. The instability
of the formation of a new environment makes communities
unstable owing to anthropogenic activities and damming
(Agostinho and Gomes 2005). The process of spatial and
temporal succession of communities in reservoirs depends
on the speed of filling, withdrawal of vegetation or not, on
the establishment of an extensive coastal zone which increases the substrate and the processes of colonization of the
basin (Tundisi 1986a).
According to Tundisi (1990) the seasonal cycle of phytoplankton in reservoirs is due in large part to hydrodynamic changes. The pulses produce sudden changes in the
system, with rapid discharges in the surface, producing impacts owing to the input of material in suspension, nutrient
availability and solar radiation (which causes changes in
the specific composition of the phytoplankton), as well as
the frequency of turbulence caused by the action of winds
(which affects the spatial distribution, horizontal and vertical
of phytoplankton in reservoirs). The authors emphasize that
the pattern of horizontal distribution determines the distribution characteristics of phytoplankton, resulting, in many
cases, in algal blooms owing to the accumulation produced
by the horizontal surface currents.
The relationship between the composition and distribution of phytoplankton and the physicochemical regimes are
complex and the interaction can be understood only with
long-term studies (Tundisi 1990). The variation in density,
according to the main groups, shows a distribution pattern
among the zones of the reservoir, in which we observed the
dominance of cyanobacteria (76 %) and that the remaining groups of algae were less abundant. These results corroborate those by Pinto Coelho et al. (2005), who obtained
cyanobacteria abundances of values of 11,000 ind./mL, and
Andrade et al. (1988), in the Itaipu Reservoir, with the highest densities of cyanobacteria Mycrocystis kutzing. Only in
the transition zone cyanobacteria blooms were recorded, a
fact that can be explained by higher nutrient availability in
the rainy season and by the pH values (Shapiro and Wright
R. A. Ribeiro Filho et al.
and phosphorus, most found in these regions, increasing primary productivity. The authors emphasize that this pattern in
the distribution of chlorophyll-a is not always found and this
is because other factors (such as residence time, reservoir
size, morphometry) influence the metabolism of primary
productivity.
It is important to stress that the impact of diffuse pollution (agricultural) and punctual pollution may be increasing
nutrient levels in the Itaipu Reservoir, as it can be seen in
satellite images of the region where the Itaipu Reservoir is
located (Stivari et al. 2005). Another aspect to be analyzed
with regard to the transition zone is that, besides a steeper
topography, a large swine farming activity is practiced in the
São Francisco Verdadeiro e Falso River micro basins, producing pollution owing to an increase in nutrients caused by
lack of waste treatment of these farms (Benassi, S.F., personal communication). Most of these properties are located
near the banks of tributaries of the reservoir, explaining the
increase of nutrient concentrations, and consequent higher
occurrence of cyanobacteria in this region.
13.4.2 Trophic State Index (TSI)
The Trophic State Index is intended to classify water bodies according to nutrient enrichment and is obtained through
statistical analysis of linear regression models (Toledo Jr
et al. 1983). Water transparency many times does not show
values that represent the trophic status of the environment,
especially for the oligotrophic limit value. For this reason,
we started to use the weighted average of the indices calculated by assigning a lower weight ratio to water transparency
(Secchi disk reading), without having to remove it from the
calculation of the TSI average.
The results obtained show that the transition zone has a
higher trophic status than the other zones, what can be related to the higher nutrient input (diffuse pollution), as discussed above, or by the ratio of optimal light with the nutrient concentrations. The mean values of TSI indicate a slight
improvement of the trophic state of water, according to spatial and temporal patterns. The values found for lacustrine
zones indicate that these regions are oligotrophic. On the
other hand, the riverine and transition zones showed higher
values, reaching concentrations that indicate a mesotrophic
level with some eutrophic values in the transition zone. Nevertheless, the average value for the entire reservoir, disconsidering the zone divisions, indicated that its waters can be
classified as mesotrophic, corroborating studies by Andrade
et al. (1988). These changes in trophic status among different regions were also found by Zanata (1999) and Minillo
(2005), in which precipitation and diffuse pollution were the
factors that best explained the changes of the trophic status.
Another important factor to consider is the cascade system upstream of the Itaipu Reservoir, present in the Tietê,
Grande, and Paranapanema rivers, which supplies waters
to the Paraná River, forming the reservoir. This factor may
have a great influence, as the concentrations of nutrients and
particulate matter undergo a lowering effect through the successive reservoirs in cascade systems (Nogueira et al. 2005;
Andrade et al. 1988).
13.4.3 Analysis of Longitudinal Distribution
of Biological Communities
The responses of the communities in reservoirs under manipulated conditions or not, are incomplete, as they are altered
or destroyed before complete interactions. The instability
of the formation of a new environment makes communities
unstable owing to anthropogenic activities and damming
(Agostinho and Gomes 2005). The process of spatial and
temporal succession of communities in reservoirs depends
on the speed of filling, withdrawal of vegetation or not, on
the establishment of an extensive coastal zone which increases the substrate and the processes of colonization of the
basin (Tundisi 1986a).
According to Tundisi (1990) the seasonal cycle of phytoplankton in reservoirs is due in large part to hydrodynamic changes. The pulses produce sudden changes in the
system, with rapid discharges in the surface, producing impacts owing to the input of material in suspension, nutrient
availability and solar radiation (which causes changes in
the specific composition of the phytoplankton), as well as
the frequency of turbulence caused by the action of winds
(which affects the spatial distribution, horizontal and vertical
of phytoplankton in reservoirs). The authors emphasize that
the pattern of horizontal distribution determines the distribution characteristics of phytoplankton, resulting, in many
cases, in algal blooms owing to the accumulation produced
by the horizontal surface currents.
The relationship between the composition and distribution of phytoplankton and the physicochemical regimes are
complex and the interaction can be understood only with
long-term studies (Tundisi 1990). The variation in density,
according to the main groups, shows a distribution pattern
among the zones of the reservoir, in which we observed the
dominance of cyanobacteria (76 %) and that the remaining groups of algae were less abundant. These results corroborate those by Pinto Coelho et al. (2005), who obtained
cyanobacteria abundances of values of 11,000 ind./mL, and
Andrade et al. (1988), in the Itaipu Reservoir, with the highest densities of cyanobacteria Mycrocystis kutzing. Only in
the transition zone cyanobacteria blooms were recorded, a
fact that can be explained by higher nutrient availability in
the rainy season and by the pH values (Shapiro and Wright