**4. Hydrology and environment variables**

The freshwater discharge of the Madeira River is characterized by an annual unimodal cycle defined by four phases: low, rising, high and falling water level. The low water level phase is when the discharge is minimal and the river beach is exposed, usually between August and November, while the high water level phase is when the discharge is high and the river floods the marginal areas, it occurs generally between February and May. Other months are consid‐ ered transition phases when the water level is rising or falling. The rising water level phase is when begins the rainy season and the river discharge starts to increase, between December and January, and the falling water level phase is when the river discharge decreases and the flood retreats, between June and July. The diary discharge median estimated for the study period was 15 thousand m3 /s, which was used to separate the low and high discharge phases. Interannual variations of the hydrological cycle were observed during the period studied, with a short and intensive high discharge phase in 2010 (<160 days and >39 thousands m3 /s) in relation to other years (>190 days and <36 thousands m3 /s) (Figure 3).

to the discharge, and the highest values occurred during the low discharge period (rconductivity= -0.74, rpH= -0.46; p<0.01, n=36). The mean turbidity is also high in relation to the other rivers, varying from 84 to more than one thousand NTU, with the highest turbidity value occurring in the beginning of the high discharge period (Pearson correlations: r= 0.63, p<0.01, n=36). The dissolved oxygen mean was relatively high, which can be associated with the aeration process caused by the movement of water in the rapids. The monthly average ranged from 5.7 to 8.8 mg/l and there was an inverse relationship with the discharge (r= -0.41, p<0.05, n=36). The high river discharge is associated with the flooding of the wetland and decomposition of large amount of organic material, which consumes oxygen of the aquatic environment (Table 1).

Diversity and Abundance of Fish Larvae Drifting in the Madeira River, Amazon Basin: Sampling Methods Comparison

http://dx.doi.org/10.5772/57404

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**Month Conductivity Temperature Oxygen Ph Turbidity** 73 27.5 8.6 7.03 1.137 69 27.1 7.0 6.83 1.129 73 27.0 5.7 6.94 961 78 27.6 6.7 7.10 564 80 27.6 6.7 6.69 401 74 26.9 7.6 6.75 298 90 27.2 8.9 6.08 205 107 26.4 8.8 7.88 180 131 28.1 8.6 8.07 84 121 29.4 7.8 7.98 411 105 29.4 7.5 7.48 320 78 27.9 7.1 7.28 806 Mean 89.61 27.67 7.59 7.17 551.57 SD 22.68 1.14 2.20 0.87 421.58

**Table 1.** Average monthly of the physicochemical parameters obtained in the Madeira River.

During the three years, 4,148 individuals were collected by 432 samples of ichthyoplankton realized monthly in four transects, with an average of 9.5 samples by transect and method. The number of samples were similar for each combination of sampling methods and nets, however there were more samples in downriver (Jirau: 56%) than in upriver (Abunã: 44%). This difference in samples number was due to the river width, which is related to the number of sampling sites for integrating sampling method. A total of 21,665 larvae (99%) and 282 eggs (1%) were collected. The point sampling (PL) method was more efficient at collecting eggs than the integrating sampling methods. The PL collected 53% of the eggs, followed by the integrat‐ ing sampling method with juvenile net (IJ), that collected 29%, and the integrating sampling method with larvae net (IL), that caught the remains 17% of the eggs. The PL method also

**5. Ichthyoplankton abundance**

**Figure 3.** The hydrological cycle of the Madeira River Basin obtained at the Hydrological Station of Abunã, at the up‐ river of the study area (discharge- continuous line; water level- dotted line).

The Madeira River is a muddy river that receives a large amount of sediments rich in mineral salt, which comes from Andean region. Due to this, the monthly averages of the conductivity and pH are in general higher than the other rivers of the Amazon Basin, varying from 69 to 131 µS/cm and 6 and 8, respectively. The conductivity and pH values were inversely related to the discharge, and the highest values occurred during the low discharge period (rconductivity= -0.74, rpH= -0.46; p<0.01, n=36). The mean turbidity is also high in relation to the other rivers, varying from 84 to more than one thousand NTU, with the highest turbidity value occurring in the beginning of the high discharge period (Pearson correlations: r= 0.63, p<0.01, n=36). The dissolved oxygen mean was relatively high, which can be associated with the aeration process caused by the movement of water in the rapids. The monthly average ranged from 5.7 to 8.8 mg/l and there was an inverse relationship with the discharge (r= -0.41, p<0.05, n=36). The high river discharge is associated with the flooding of the wetland and decomposition of large amount of organic material, which consumes oxygen of the aquatic environment (Table 1).


**Table 1.** Average monthly of the physicochemical parameters obtained in the Madeira River.
