67
5 Eutrophication of Lakes
5 Eutrophication of Lakes
5.8.3 Data Analysis
Relationships between environmental parameters and the
density of cyanobacteria were studied using multiple regression analysis. The Akaike’s information criterion (Johnson
and Omland 2004) was used to extract the variables that
significantly increased the amount of explained variation
(∆AIC < 2).
An analysis of variance (ANOVA) indicated significant
differences between the rainy and dry seasons for the reservoirs in the Piranhas River basin for dissolved oxygen,
inorganic dissolved nitrogen, and TP. For the reservoirs in
the Paraíba River basin, only alkalinity was found to have a
significant seasonal difference (Table 5.2).
Differences of limnological factors were observed between the reservoirs in the Piranhas and Paraíba River basins, except for TP and total alkalinity (Table 5.2). In the dry
season, the Piranhas River reservoirs showed higher mean
values for temperature, dissolved oxygen, and dissolved inorganic nitrogen. Higher mean concentrations of TP were
registered in the Paraíba River reservoirs. During the rainy
season, the Paraíba River reservoirs had higher concentrations of dissolved inorganic nitrogen and TP (Table 5.2).
Reservoirs in semiarid regions are considered to be eutrophic when the annual average concentration of TP is higher
than 60 μg l
−1
(Thornton and Rast 1993). During the sampling period, an increase in the concentrations of TP in the
reservoirs was observed, with averages ranging from 39.3 µg
l
−1
(DP = 12.76) in 2006 to 155.89 µg l
−1
(DP = 81.16) in
2009. In 2006, 7.7 % of the reservoirs were categorized as
eutrophic and that percentage increased in each subsequent
year, with 30.8 % in 2007, 42.3 % in 2008, and 57.7 % in
2009 (Fig. 5.10).
We identified 188 taxa in the reservoirs, which were distributed among nine taxonomic groups: Chlorophyceae (61),
Cyanobacteria (49), Bacillariophyceae (34), Euglenophyceae (14), Zygnemaphyceae (19), Dinophyceae (5), Chlamydophyceae (4), Xanthophyceae (1), and Oedogoniophyceae (1). No individual taxon was found to be present in all
the sampled reservoirs. However, Cyclotella meneghiniana,
Cylindrospermopsis raciborskii, Aulacoseira granulate,
Aphanocapsa elachista, and Pseudoanabaena limnetica
were the most frequently identified species. Out of the total
number of identified taxa, 39 occurred only in the dry period
and 30 were found only during the rainy period. Aphanizomenon tropicalis and Coelastrum scabrum were the most
abundant of the 16 taxa that were found only in the Paraíba
River, and Dolichospermum spiroids, Merismopedia tenuissima, and Oocystis lacustris were the most abundant of the
31 taxa found only in the Piranhas River.
The most abundant species of cyanobacteria in the reservoirs were potential toxin producers. These species dominated the phytoplankton community in 16 of the reservoirs,
especially during the dry periods, and represented 73.1 % of
the total individuals and 54.08 % of the total algae during
the rainy season. The occurrence of cyanobacteria blooms in
reservoirs increased significantly between 2006 and 2009. In
2006, only 3 % of the reservoirs were found to have cyanobacteria blooms, while in 2007 that number grew to 20 %, in
2008 it was 45 %, and in 2009 it reached 62 %. Most of the
observed cyanobacteria blooms were found in the Piranhas
reservoirs (Fig. 5.11).
A multiple regression analysis with cyanobacteria density as the dependent variable showed a positive relationship with dissolved oxygen and a negative relationship with
water transparency, pH, and inorganic dissolved nitrogen
(R
2 adjusted = 0.45; p < 0.05). The equation to summarize these
relationships is described by
lnCYN = 13−0.71 lnWT−3.5 pH + 0.29 lnDO−0.19
lnIDN,
where
CYN Cyanobacteria density
WT
Water transparency
DO
Dissolved oxygen
IDN
Inorganic dissolved nitrogen.
Piranhas River
Paraíba River
Variables
Dry
Rainy
Dry
Rainy
Mean
CV (%)
Mean
CV (%)
p
Mean
CV (%)
Mean
CV (%)
p
Water temperature
29.9
6.72
29.57
8.93
0.74
28.2
6.49
28.2
3.96
0.93
Dissolved oxygen
7.95
13.4
7.008
11.5
0.01*
7.82
7.53
8.44
23.3
0.51
Alcalinity
20.4
24.4
17
28.7
0.09
29.2
24.5
18
18
0.01*
Inorganic dissolved
nitrogen
7.49
212
75.07
121
0.01*
2.3
31.4
122
112
0.08
P-ortho
11.80
63.2
33.31
72.1
0.01*
70.2
71
92.6
64.8
0.53
Total-P
168.11
58.11
2.492
135
0.19
1,681.1
58.11
91.02
27.4
0.24
The mean and coefficient of variation (CV) are presented for the two sampling periods as well as the significance level of the differences between
the dry and rainy season for Piranha and Paraíba Rivers ( p)
*p < 0.05
Table 5.2 Limnological measurements in the reservoirs
5 Eutrophication of Lakes
5 Eutrophication of Lakes
5.8.3 Data Analysis
Relationships between environmental parameters and the
density of cyanobacteria were studied using multiple regression analysis. The Akaike’s information criterion (Johnson
and Omland 2004) was used to extract the variables that
significantly increased the amount of explained variation
(∆AIC < 2).
An analysis of variance (ANOVA) indicated significant
differences between the rainy and dry seasons for the reservoirs in the Piranhas River basin for dissolved oxygen,
inorganic dissolved nitrogen, and TP. For the reservoirs in
the Paraíba River basin, only alkalinity was found to have a
significant seasonal difference (Table 5.2).
Differences of limnological factors were observed between the reservoirs in the Piranhas and Paraíba River basins, except for TP and total alkalinity (Table 5.2). In the dry
season, the Piranhas River reservoirs showed higher mean
values for temperature, dissolved oxygen, and dissolved inorganic nitrogen. Higher mean concentrations of TP were
registered in the Paraíba River reservoirs. During the rainy
season, the Paraíba River reservoirs had higher concentrations of dissolved inorganic nitrogen and TP (Table 5.2).
Reservoirs in semiarid regions are considered to be eutrophic when the annual average concentration of TP is higher
than 60 μg l
−1
(Thornton and Rast 1993). During the sampling period, an increase in the concentrations of TP in the
reservoirs was observed, with averages ranging from 39.3 µg
l
−1
(DP = 12.76) in 2006 to 155.89 µg l
−1
(DP = 81.16) in
2009. In 2006, 7.7 % of the reservoirs were categorized as
eutrophic and that percentage increased in each subsequent
year, with 30.8 % in 2007, 42.3 % in 2008, and 57.7 % in
2009 (Fig. 5.10).
We identified 188 taxa in the reservoirs, which were distributed among nine taxonomic groups: Chlorophyceae (61),
Cyanobacteria (49), Bacillariophyceae (34), Euglenophyceae (14), Zygnemaphyceae (19), Dinophyceae (5), Chlamydophyceae (4), Xanthophyceae (1), and Oedogoniophyceae (1). No individual taxon was found to be present in all
the sampled reservoirs. However, Cyclotella meneghiniana,
Cylindrospermopsis raciborskii, Aulacoseira granulate,
Aphanocapsa elachista, and Pseudoanabaena limnetica
were the most frequently identified species. Out of the total
number of identified taxa, 39 occurred only in the dry period
and 30 were found only during the rainy period. Aphanizomenon tropicalis and Coelastrum scabrum were the most
abundant of the 16 taxa that were found only in the Paraíba
River, and Dolichospermum spiroids, Merismopedia tenuissima, and Oocystis lacustris were the most abundant of the
31 taxa found only in the Piranhas River.
The most abundant species of cyanobacteria in the reservoirs were potential toxin producers. These species dominated the phytoplankton community in 16 of the reservoirs,
especially during the dry periods, and represented 73.1 % of
the total individuals and 54.08 % of the total algae during
the rainy season. The occurrence of cyanobacteria blooms in
reservoirs increased significantly between 2006 and 2009. In
2006, only 3 % of the reservoirs were found to have cyanobacteria blooms, while in 2007 that number grew to 20 %, in
2008 it was 45 %, and in 2009 it reached 62 %. Most of the
observed cyanobacteria blooms were found in the Piranhas
reservoirs (Fig. 5.11).
A multiple regression analysis with cyanobacteria density as the dependent variable showed a positive relationship with dissolved oxygen and a negative relationship with
water transparency, pH, and inorganic dissolved nitrogen
(R
2 adjusted = 0.45; p < 0.05). The equation to summarize these
relationships is described by
lnCYN = 13−0.71 lnWT−3.5 pH + 0.29 lnDO−0.19
lnIDN,
where
CYN Cyanobacteria density
WT
Water transparency
DO
Dissolved oxygen
IDN
Inorganic dissolved nitrogen.
Piranhas River
Paraíba River
Variables
Dry
Rainy
Dry
Rainy
Mean
CV (%)
Mean
CV (%)
p
Mean
CV (%)
Mean
CV (%)
p
Water temperature
29.9
6.72
29.57
8.93
0.74
28.2
6.49
28.2
3.96
0.93
Dissolved oxygen
7.95
13.4
7.008
11.5
0.01*
7.82
7.53
8.44
23.3
0.51
Alcalinity
20.4
24.4
17
28.7
0.09
29.2
24.5
18
18
0.01*
Inorganic dissolved
nitrogen
7.49
212
75.07
121
0.01*
2.3
31.4
122
112
0.08
P-ortho
11.80
63.2
33.31
72.1
0.01*
70.2
71
92.6
64.8
0.53
Total-P
168.11
58.11
2.492
135
0.19
1,681.1
58.11
91.02
27.4
0.24
The mean and coefficient of variation (CV) are presented for the two sampling periods as well as the significance level of the differences between
the dry and rainy season for Piranha and Paraíba Rivers ( p)
*p < 0.05
Table 5.2 Limnological measurements in the reservoirs