182
R. A. Ribeiro Filho et al.
voir began in 1985 and persists to this day, and it is of great
importance for understanding the dynamics of fishes in the
reservoir.
Agostinho et al. (1997a) characterized the structure of trophic guilds of fishes of the Paraná River, in which piscivores
were more abundant and diverse. The authors emphasize that
the dynamics of the environment is related to the diversity of
species in trophic categories, as evidenced by the structure of
the diversity of its abiotic component and by the biotic interrelationships governed by the hydrological cycle.
As the Itaipu Reservoir is an artificial damming of the
Paraná River, it is expected that the structure of the fish community should have a strong influence of the hydrological
regime of the river. The study mentioned above found greatest abundance of piscivores and iliophagous, both favored by
the effect of the flood pulse, developing ideal areas that favor
the increase of these stocks (reproduction and feeding). This
effect may explain the large biomass of omnivores and piscivores recorded in this study, in which the riverine and transition zones showed large biomasses of omnivores, and
piscivores were more dominant in the lacustrine zone.
13.4.4 Trophic Relationships (top-down and
bottom-up) in the Itaipu Reservoir:
Direct and Indirect Effects
The theory of trophic cascade in lakes and reservoirs is
based on the fact that each trophic level of the food chain
is inversely and directly related with trophic levels above or
below, following two principles: the loss of energy between
a trophic level and a higher one, and the disturbance in a trophic level brings consequences to the remaining trophic levels. The studies that have tested and confirmed this hypothesis in temperate lakes and reservoirs are numerous (Hrbáček
et al. 1961; Shapiro et al. 1975; Shapiro and Wright 1984;
Carpenter et al. 1985; Persson et al. 1988; Carpenter and
Kitchell 1988; Mcqueen 1990; Power 1990; Brett and Goldman 1996; Brett and Goldman 1997; Hairston and Hairston
1997; Meijer et al. 1999; Jeppesen et al. 2000; Gulati 2001;
Carpenter et al. 2001; Benndorf et al. 2002; Lathrop et al.
2002; Bell et al. 2003; Hansson et al. 2004; Borer et al. 2005;
Mehner et al. 2005), unlike what happens in tropical lakes,
where studies of trophic cascades are scarce (Quirós and
Boveri 1999; Scasso et al. 2001; Hasan et al. 2001; Lazzaro
et al. 2003; Rejas et al. 2005; Jeppesen et al. 2005).
The top-down effects in temperate environments are explained by Mcqueen (1990) and Lazzaro (1997), where relationships between trophic levels suffer negative effect of the
presence of piscivores on the lower levels.
Lazzaro et al. (2003) investigated the effects of trophic
cascades in dams of Pernambuco. The relationships that are
well explained in temperate climate lakes and reservoirs
were not so evident in tropical environments. The authors
suggest that the state of the trophic guilds of fishes plays the
main role in food webs in reservoirs of northeastern Brazil,
where indirect effects between chlorophyll concentration
and the densities of omnivores have a positive relationship,
indicating that omnivores somehow contribute with some
kind of pressure towards the development of chlorophyll-a.
According Jeppesen et al. (2005), this is because the wealth
of fish species is often higher in tropical and subtropical
lakes. South America and Africa have a very rich freshwater
fish fauna and many species of fishes show a partially overlapping niche, increasing the control of prey (Lazzaro 1997).
This effect was also observed by Stein et al. (1995) in lakes
in southern United States.
Fish communities in tropical and subtropical lakes are
often dominated by omnivores, which have great feeding
plasticity, consuming zooplankton, phytoplankton, periphyton, benthic macro invertebrates, and debris. According to
Lazzaro et al. (2003), the piscivores are represented in few
species and in small biomass and a greater biomass of small
carnivores and omnivores generally governs the dynamics of
the trophic chain.
Some species of omnivores are not controlled by the
availability of food resources (zooplankton) and not even by
the pressure of predation as they are bigger in size than their
potential predators (Arcifa et al. 1995). The top-down control by piscivorous fishes is therefore weaker in subtropical/
tropical lakes than in temperate lakes.
Roche and Rocha (2005) discussed the effect of the presence of planktivores in lakes and reservoirs in Brazil, highlighting the effects of predation, food types and feeding selectivity, effects of fries and fingerlings, nutrient availability,
trophic state of the environment and hydrology.
Lazzaro (1987) notes that in most previous studies involving the impact of planktivorous fish the feeding behavior of
the species was not determined and, consequently, the mechanisms that determine the feeding selectivity of the fishes
and the use of resources were not well-understood. The author explains that fishes have great ability to change their
pattern of feeding selectivity, and classifies fishes according
to the way they feed: visual particulate, filtration, pumping,
and filtration drag. In the case of the first two, several factors influence the success of predation, as prey size and light
intensity, and species of young fish that feed by visual mode
particles may switch to filter-feeding by pumping when they
become adults. Lazzaro (1991) demonstrated this effect in
laboratory experiments with a cichlid species, the Geophagus brasiliensis (cará). Roche et al. (2005) tested the feeding selectivity for the same species in experiments in mesocosms in situ in the Lobo-Broa Reservoir (SP). The authors
found that the biomass of Daphnia sp decreased in treated
fish, showing the pressure for food selectivity, confirming
the laboratory studies by Lazzaro (1991).
R. A. Ribeiro Filho et al.
voir began in 1985 and persists to this day, and it is of great
importance for understanding the dynamics of fishes in the
reservoir.
Agostinho et al. (1997a) characterized the structure of trophic guilds of fishes of the Paraná River, in which piscivores
were more abundant and diverse. The authors emphasize that
the dynamics of the environment is related to the diversity of
species in trophic categories, as evidenced by the structure of
the diversity of its abiotic component and by the biotic interrelationships governed by the hydrological cycle.
As the Itaipu Reservoir is an artificial damming of the
Paraná River, it is expected that the structure of the fish community should have a strong influence of the hydrological
regime of the river. The study mentioned above found greatest abundance of piscivores and iliophagous, both favored by
the effect of the flood pulse, developing ideal areas that favor
the increase of these stocks (reproduction and feeding). This
effect may explain the large biomass of omnivores and piscivores recorded in this study, in which the riverine and transition zones showed large biomasses of omnivores, and
piscivores were more dominant in the lacustrine zone.
13.4.4 Trophic Relationships (top-down and
bottom-up) in the Itaipu Reservoir:
Direct and Indirect Effects
The theory of trophic cascade in lakes and reservoirs is
based on the fact that each trophic level of the food chain
is inversely and directly related with trophic levels above or
below, following two principles: the loss of energy between
a trophic level and a higher one, and the disturbance in a trophic level brings consequences to the remaining trophic levels. The studies that have tested and confirmed this hypothesis in temperate lakes and reservoirs are numerous (Hrbáček
et al. 1961; Shapiro et al. 1975; Shapiro and Wright 1984;
Carpenter et al. 1985; Persson et al. 1988; Carpenter and
Kitchell 1988; Mcqueen 1990; Power 1990; Brett and Goldman 1996; Brett and Goldman 1997; Hairston and Hairston
1997; Meijer et al. 1999; Jeppesen et al. 2000; Gulati 2001;
Carpenter et al. 2001; Benndorf et al. 2002; Lathrop et al.
2002; Bell et al. 2003; Hansson et al. 2004; Borer et al. 2005;
Mehner et al. 2005), unlike what happens in tropical lakes,
where studies of trophic cascades are scarce (Quirós and
Boveri 1999; Scasso et al. 2001; Hasan et al. 2001; Lazzaro
et al. 2003; Rejas et al. 2005; Jeppesen et al. 2005).
The top-down effects in temperate environments are explained by Mcqueen (1990) and Lazzaro (1997), where relationships between trophic levels suffer negative effect of the
presence of piscivores on the lower levels.
Lazzaro et al. (2003) investigated the effects of trophic
cascades in dams of Pernambuco. The relationships that are
well explained in temperate climate lakes and reservoirs
were not so evident in tropical environments. The authors
suggest that the state of the trophic guilds of fishes plays the
main role in food webs in reservoirs of northeastern Brazil,
where indirect effects between chlorophyll concentration
and the densities of omnivores have a positive relationship,
indicating that omnivores somehow contribute with some
kind of pressure towards the development of chlorophyll-a.
According Jeppesen et al. (2005), this is because the wealth
of fish species is often higher in tropical and subtropical
lakes. South America and Africa have a very rich freshwater
fish fauna and many species of fishes show a partially overlapping niche, increasing the control of prey (Lazzaro 1997).
This effect was also observed by Stein et al. (1995) in lakes
in southern United States.
Fish communities in tropical and subtropical lakes are
often dominated by omnivores, which have great feeding
plasticity, consuming zooplankton, phytoplankton, periphyton, benthic macro invertebrates, and debris. According to
Lazzaro et al. (2003), the piscivores are represented in few
species and in small biomass and a greater biomass of small
carnivores and omnivores generally governs the dynamics of
the trophic chain.
Some species of omnivores are not controlled by the
availability of food resources (zooplankton) and not even by
the pressure of predation as they are bigger in size than their
potential predators (Arcifa et al. 1995). The top-down control by piscivorous fishes is therefore weaker in subtropical/
tropical lakes than in temperate lakes.
Roche and Rocha (2005) discussed the effect of the presence of planktivores in lakes and reservoirs in Brazil, highlighting the effects of predation, food types and feeding selectivity, effects of fries and fingerlings, nutrient availability,
trophic state of the environment and hydrology.
Lazzaro (1987) notes that in most previous studies involving the impact of planktivorous fish the feeding behavior of
the species was not determined and, consequently, the mechanisms that determine the feeding selectivity of the fishes
and the use of resources were not well-understood. The author explains that fishes have great ability to change their
pattern of feeding selectivity, and classifies fishes according
to the way they feed: visual particulate, filtration, pumping,
and filtration drag. In the case of the first two, several factors influence the success of predation, as prey size and light
intensity, and species of young fish that feed by visual mode
particles may switch to filter-feeding by pumping when they
become adults. Lazzaro (1991) demonstrated this effect in
laboratory experiments with a cichlid species, the Geophagus brasiliensis (cará). Roche et al. (2005) tested the feeding selectivity for the same species in experiments in mesocosms in situ in the Lobo-Broa Reservoir (SP). The authors
found that the biomass of Daphnia sp decreased in treated
fish, showing the pressure for food selectivity, confirming
the laboratory studies by Lazzaro (1991).