Procesos políticos, confianza y dilemas morales

Variation of Chl-a concentration in 2010, when the reservoir was aerated, is presented in Figure 30. The Chl-a concentrations at the surface at Site A increased from 14.43 µg/L in June to 43.84 µg/L by the end of July, 2010. After September, the Chl-a

concentrations decreased as a general trend toward fall months (Figure 30). Similar variation in Chl-a concentrations were observed at all four sampling depths at Site A during the entire period.

Analysis of vertical distribution of Chl-a at Site A shows in the beginning of 2010, the Chl-a concentrations in deeper layers were similar to the Chl-a concentrations at the surface. These results suggest that mixing due to aeration dispersed phytoplankton into deeper layers of the reservoir. Higher Chl-a concentrations for about two weeks in the mid- summer at Secchi depth were observed at Site A, which is located close to diffusers and in the deepest part in the impoundment (Figure 30). Althought, observed differensces, Chl-a

concentrations on the bottom were relatively higher and increased in summer monts indicating that aeration was still able to disperce phytoplankton cells deeper in the reservoir.

Figure 30.Chl-a concentrations at Site A (2010) with aeration in the entire period.

0.00 20.00 40.00 60.00 80.00 100.00

22-May 21-Jun 21-Jul 20-Aug 19-Sep 19-Oct 18-Nov

Ch l a ( µg /L ) date (day-month) At Surface At Secchi depth At 2 × Secchi depth 1.50m from the bottom

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Higher Chl-a concentrations were also observed at the surface and the Secchi depth at Site B (Table A10), but no vertical Chl-a differences were observed at Sites C and D (Tables 21 and 26, respectively.). Similarly, relatively higher Chl-a concentrations below the surface has been observed in many deep lakes. Such accumulation of phytoplankton has been related to adaptation of phytoplankton to avoid higher light intensity on the surface (Camacho et al., 2003). Hence occurrence of such accumulation, of Chl-a below the surface suggest that the artificial mixing in the HMD is gentle and not strong.

For the whole period ANOVA results show no significant differences in Chl-a

concentrations between depths at Site A (p=0.68, Table E54.). Similarly, no differences between sampling depths were found at Sites B (p=0.78), C (p=0.61), and D (p=0.47) (Tables E55, E56, and E57, respectively). Since the Chl-a fluctuatelargelyover time, in this case ANOVA could not be reliably used to assess the Chl-a distribution over time.

Among the sites, depth-weighted averaged Chl-a show that the Chl-a variation were similar at all sites (Table 18). Relatively higher were concentrations at shallower Sites C and D in the end of July in comparison with deeper sites A and B.

Table 18. Depth-weighted average Chl-a and Standard Deviations (STD), 2010

date (mo/day/yr)

Chl-a (average ± STD), µg/L

Site A Site B Site C Site D

6/4/2010 11.34 ± 2.39 12.99 ± 0.50 14.32 ± 1.64 16.56±2.77 6/18/2010 27.59 ± 3.54 30.09 ± 1.21 22.73 ± 1.64 19.05±16.41 7/9/2010 6.33 ± 5.32 7.67 ± 1.46 19.59 ± 13.27 13.05±5.08 7/23/2010 50.21 ±12.39 49.48 ± 19.60 97.96 ± 0.27 158.42±0.46 8/6/2010 41.77 ±18.29 51.37 ± 21.42 49.43 ± 14.33 52.58±18.11 8/20/2010 33.18 ± 1.08 40.08 ± 15.85 61.16 ± 21.70 89.23±30.61 9/3/2010 54.90 ± 4.40 72.26 ± 2.62 56.70 ± 4.90 76.56±7.56 9/17/2010 52.80 ± 2.96 51.86 ± 16.97 58.16 ± 9.11 57.16±9.51 10/10/2010 29.29 ± 2.37 26.71 ± 11.43 34.17 ± 14.93 26.07±5.66 10/15/2010 8.55 ± 2.65 2.65±3.36

Similarly to 2010, in the beginning of 2011, when the reservoir was artificially aerated, the surface Chl-a at Site A increased rapidly from 2.67 to 32.08 µg/L (Figure 31). A

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week after aeration was stopped and stratification was developed, Chl-a rapidly increased on the surface layers from 32 µg/L to 79 µg/L, but was lower in deeper part of the water column, indicating that phytoplankton accumulated on the surface. ANOVA results

confirmed that a significant differences of Chl-a concentrations among the sampling depths occur after the aeration was stopped (p<0.01, Table E58). The Tukey’s test show that Chl-a

concentrations were significantly different at the surface and at the Secchi depth than at the thermocline, 1.5m from the bottom and 0.5m from the bottom. No significant difference was found between the surface and Secchi depth, which confirms that phytoplankton accumulated in the surface layers (Table E59). Similar significant differences with the depth were found at Site B (p=0.03) (Table E60). Based on p-values for Sites C (p=0.06) and D (p=0.08) (Tables E62 and E63, respectively.) we cannot conclude that there were a

significant across sampling depths with the same confidence.

Figure 31. Chl-a concentrations at Site A (2011). Shaded area indicate period without aeration.

It is important to note, that although nutrient concentrations on the surface decreased, stopping of aeration did not affected phytoplankton growth immediately. Increase of surface Chl-a concentrations indicates that the phytoplankton population

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16-Jun 16-Jul 15-Aug 14-Sep 14-Oct 13-Nov 13-Dec

Ch l- a (µg /L ) date (day-month) At Surfece At Secchi depth At 2 × Secchi depth At 1.00m below Thermocline At 1.50m from the bottom

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continue to grow for the first two weeks after stopping of the mixing (Table 31). The depth- weighted averaged Chl-a concentrations (Table 19) also indicate that the Chl-a

concentrations have similar distribution at all sites. Two weeks after stopping of aeration the Chl-a rapidly decreased indicating a collapse of the phytoplankton population.

Table 19. Depth-weighted average Chl-a and Standard Deviations (STD), 2011

date (mo/day/yr)

Chl-a (average ± STD), µg/L

Site A Site B Site C Site D

6/30/2011 3.15±2.30 7.19±5.16 15.07±2.61 12.70±1.78 7/13/2011 21.30±12.29 13.31±6.74 25.55±10.61 9.46±12.39 7/20/2011 31.58±33.73 28.22±22.73 19.59±17.15 32.87±26.05 7/27/2011 34.73±39.13 40.89±37.75 40.48±24.19 54.89±2.79 8/3/2011 20.58±15.50 35.89±12.01 34.40±17.58 29.26±18.82 8/17/2011 28.15±7.58 30.01±9.98 33.54±5.12 60.02±31.79 8/30/2011 37.82±13.19 23.70±13.94 48.07±1.22 58.47±3.07 9/20/2011 21.65±1.66 18.63±8.73 26.26±5.99 27.16±9.36 10/4/2011 2.11±0.48 2.61±0.13 1.10±0.77 2.23±0.14 10/18/2011 5.88±0.50 5.48±0.16 7.062±0.16 4.74±0.44 11/8/2011 30.59±1.61 31.09±0.78 32.48±2.12 23.08±1.07

Note: bolded values indicate period without aeration

Comparison of DWA Chl-a concentrations for both years (Figure 32) show that Chl-a

concentrations were higher when lake was aerated. The Wilcoxon Mann-Whitney's test confirmed that a significantly difference between aerated and non-aerated period occur when lake was aerate (p=0.04, Table E109). Higher Chl-a concentrations during aerated period were coincident with higher TDIN and SRP (Figure 16 and 21).

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Figure 32. Depth-weighted average concentrations Chl-a at Site A: 2010 with aeration during entire period and 2011 without artificial aeration in shaded area.