179
13 Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship Between Fish Trophic Guilds, Limnology …
found previously in the Itaipu Reservoir (Agostinho et al.
1999), but in lower concentrations. Ammonia nitrogen is
the least found nitrogen form and one of the most frequently
used by primary producers. This is because the ammonium
ion can be used as a nitrogen source during protein synthesis by primary producers. This may happen as ammonia is
one of the main forms of nitrogen excreted by fishes (Wetzel 1993). In the Itaipu Reservoir the average concentration
of this variable was 0.03 mg/L, confirming the results of
Agostinho et al. (1997b), with average values of 0.033 mg/L.
The total TKN is the sum of forms of organic and ammonia nitrogen, which are extremely important for biological
activity. In the Itaipu Reservoir this form of nutrient was the
one which presented the best relationship with the chlorophyll-a concentrations, and positive relationship with water
transparency, as evidenced by multiple regression analysis.
The concentrations of total Kjeldahl nitrogen in the reservoir
had horizontal distribution similar to that found by Agostinho et al. (1999) and Andrade et al. (1988), with the transition
zone having higher concentrations of this nutrient. This was
the variable that best correlated with the chlorophyll-a variable, corroborating the results found by BINI (2003), thus
demonstrating the assimilation of this source of nitrogen by
phytoplankton (Reynolds 1984; Esteves 1988). The relationship between the nitrogen forms of chlorophyll was also observed by Kurmayer and Wanzenbock (1996) and Karjalainen et al. (1998), who found a direct relationship between
the reductions in nitrate and ammonia concentrations and
increase in concentrations of chlorophyll-a in experiments in
ponds with known fish biomasses.
Phosphorus is found in the aquatic environment in the
same way that nitrogen is (organic and inorganic/in particles and dissolved) and together they are the most important nutrients for primary production in aquatic ecosystems.
The lowest average concentration in the lacustrine zone of
Itaipu Reservoir can be explained by sedimentation rates
throughout the watercourse. It is important to stress that the
P total in this zone may have more effect on productivity
(chlorophyll-a) owing to the luminosity factor (Pagioro and
Thomaz 2002). This explains why the phosphorus had no
significant relationship when related to chlorophyll-a and
water transparency in the multiple regression analysis of the
Itaipu Reservoir.
Comparing the total phosphorus concentrations with the
results Agostinho et al. (1999) and Andrade et al. (1988), it
is possible to observe that the reservoir presented a seasonal
longitudinal distribution pattern that is similar in both studies. The concentrations of this nutrient were similar in both
studied zones (Agostinho et al. 1999) and this showed that
the constant reduced concentrations of total phosphorus is an
important implication for the absence of eutrophication in
the Itaipu Reservoir. Tundisi et al. (1993) point out that inhibition of the eutrophication process can be associated with
the retention of this nutrient in cascade systems reservoirs located upstream of the Itaipu Reservoir, and studies conducted by Nogueira et al. (2005) show a decrease of phosphorus
in cascade systems reservoirs. Furthermore, Agostinho et al.
(1999) point out that large sources that carry phosphorus into
the reservoir are limited by the tributaries and the floodplain.
The impact of fish that feed on sediment detritus, making available large concentrations of phosphorus for phytoplankton has been studied by several authors (Lamarra
1975; Drenner et al. 1996; Persson 1997; Starling 1998). In
Paranoá Lake this impact was observed in the stock of tilapia
owing to their omnivorous feeding habits, more than 50 % of
their stomach contents consisted of sediment in the study by
Grando (1989), what through excretion is released into the
water column, increasing the concentration of phosphorus
(Pereira and Ribeiro Filho 2004).
The primary productivity, which can be expressed indirectly by the concentration of chlorophyll, is controlled by
the action of light and nutrient factors (Henry and Simon
1990), considered fundamental to the development of phytoplankton productivity in freshwater ecosystems.
The horizontal distribution of chlorophyll-a concentrations in the Itaipu Reservoir showed significant relationships
with almost all forms of nutrients. The multiple regression
analysis indicates that the nitrate and ammonia nitrogen negatively affect primary productivity, while the total Kjeldahl
nitrogen and total phosphorus positively affected the primary productivity. These results corroborate those by Pagioro
et al. (2005b) in six reservoirs in the state of Paraná, where
they found similar effects of nutrients on chlorophyll-a and
verified the same pattern of distribution.
The phytoplankton community of a reservoir is maintained by continuous input of nutrients (inflows) and by the
nutrients recycled by zooplankton and fishes. A way commonly used to quantify the amount of algae in a specific reservoir is by determining the concentrations of chlorophyll-a
(Straskaba and Tundisi 2000).
The pattern of longitudinal distribution of chlorophyll-a
in the reservoir was constant for the riverine and lacustrine
zones, where the average concentrations found were similar. On the other hand, the transition zone, whose highest
concentrations of chlorophyll were associated with higher
nutrient concentrations, showed a distribution related to the
seasonality of rainfall. Studies by Andrade et al. (1988) corroborate the results of the present study. The highest concentrations of chlorophyll-a in the Itaipu Reservoir, in the
transition zone, are explained by higher concentrations of
total Kjeldahl nitrogen, total phosphorus and nitrate, and, apparently, these correspond to the forms of nutrients that were
influential in increasing concentrations of chlorophyll-a. Pagioro et al. (2005b) commented that the prevalence of high
concentrations of chlorophyll-a in the intermediate zones
(transition) are owing to the ideal relationship between light
13 Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship Between Fish Trophic Guilds, Limnology …
found previously in the Itaipu Reservoir (Agostinho et al.
1999), but in lower concentrations. Ammonia nitrogen is
the least found nitrogen form and one of the most frequently
used by primary producers. This is because the ammonium
ion can be used as a nitrogen source during protein synthesis by primary producers. This may happen as ammonia is
one of the main forms of nitrogen excreted by fishes (Wetzel 1993). In the Itaipu Reservoir the average concentration
of this variable was 0.03 mg/L, confirming the results of
Agostinho et al. (1997b), with average values of 0.033 mg/L.
The total TKN is the sum of forms of organic and ammonia nitrogen, which are extremely important for biological
activity. In the Itaipu Reservoir this form of nutrient was the
one which presented the best relationship with the chlorophyll-a concentrations, and positive relationship with water
transparency, as evidenced by multiple regression analysis.
The concentrations of total Kjeldahl nitrogen in the reservoir
had horizontal distribution similar to that found by Agostinho et al. (1999) and Andrade et al. (1988), with the transition
zone having higher concentrations of this nutrient. This was
the variable that best correlated with the chlorophyll-a variable, corroborating the results found by BINI (2003), thus
demonstrating the assimilation of this source of nitrogen by
phytoplankton (Reynolds 1984; Esteves 1988). The relationship between the nitrogen forms of chlorophyll was also observed by Kurmayer and Wanzenbock (1996) and Karjalainen et al. (1998), who found a direct relationship between
the reductions in nitrate and ammonia concentrations and
increase in concentrations of chlorophyll-a in experiments in
ponds with known fish biomasses.
Phosphorus is found in the aquatic environment in the
same way that nitrogen is (organic and inorganic/in particles and dissolved) and together they are the most important nutrients for primary production in aquatic ecosystems.
The lowest average concentration in the lacustrine zone of
Itaipu Reservoir can be explained by sedimentation rates
throughout the watercourse. It is important to stress that the
P total in this zone may have more effect on productivity
(chlorophyll-a) owing to the luminosity factor (Pagioro and
Thomaz 2002). This explains why the phosphorus had no
significant relationship when related to chlorophyll-a and
water transparency in the multiple regression analysis of the
Itaipu Reservoir.
Comparing the total phosphorus concentrations with the
results Agostinho et al. (1999) and Andrade et al. (1988), it
is possible to observe that the reservoir presented a seasonal
longitudinal distribution pattern that is similar in both studies. The concentrations of this nutrient were similar in both
studied zones (Agostinho et al. 1999) and this showed that
the constant reduced concentrations of total phosphorus is an
important implication for the absence of eutrophication in
the Itaipu Reservoir. Tundisi et al. (1993) point out that inhibition of the eutrophication process can be associated with
the retention of this nutrient in cascade systems reservoirs located upstream of the Itaipu Reservoir, and studies conducted by Nogueira et al. (2005) show a decrease of phosphorus
in cascade systems reservoirs. Furthermore, Agostinho et al.
(1999) point out that large sources that carry phosphorus into
the reservoir are limited by the tributaries and the floodplain.
The impact of fish that feed on sediment detritus, making available large concentrations of phosphorus for phytoplankton has been studied by several authors (Lamarra
1975; Drenner et al. 1996; Persson 1997; Starling 1998). In
Paranoá Lake this impact was observed in the stock of tilapia
owing to their omnivorous feeding habits, more than 50 % of
their stomach contents consisted of sediment in the study by
Grando (1989), what through excretion is released into the
water column, increasing the concentration of phosphorus
(Pereira and Ribeiro Filho 2004).
The primary productivity, which can be expressed indirectly by the concentration of chlorophyll, is controlled by
the action of light and nutrient factors (Henry and Simon
1990), considered fundamental to the development of phytoplankton productivity in freshwater ecosystems.
The horizontal distribution of chlorophyll-a concentrations in the Itaipu Reservoir showed significant relationships
with almost all forms of nutrients. The multiple regression
analysis indicates that the nitrate and ammonia nitrogen negatively affect primary productivity, while the total Kjeldahl
nitrogen and total phosphorus positively affected the primary productivity. These results corroborate those by Pagioro
et al. (2005b) in six reservoirs in the state of Paraná, where
they found similar effects of nutrients on chlorophyll-a and
verified the same pattern of distribution.
The phytoplankton community of a reservoir is maintained by continuous input of nutrients (inflows) and by the
nutrients recycled by zooplankton and fishes. A way commonly used to quantify the amount of algae in a specific reservoir is by determining the concentrations of chlorophyll-a
(Straskaba and Tundisi 2000).
The pattern of longitudinal distribution of chlorophyll-a
in the reservoir was constant for the riverine and lacustrine
zones, where the average concentrations found were similar. On the other hand, the transition zone, whose highest
concentrations of chlorophyll were associated with higher
nutrient concentrations, showed a distribution related to the
seasonality of rainfall. Studies by Andrade et al. (1988) corroborate the results of the present study. The highest concentrations of chlorophyll-a in the Itaipu Reservoir, in the
transition zone, are explained by higher concentrations of
total Kjeldahl nitrogen, total phosphorus and nitrate, and, apparently, these correspond to the forms of nutrients that were
influential in increasing concentrations of chlorophyll-a. Pagioro et al. (2005b) commented that the prevalence of high
concentrations of chlorophyll-a in the intermediate zones
(transition) are owing to the ideal relationship between light