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Abiotic factors as dissolved oxygen are a reference in measuring a river’s biogeochemical cycles (Ríos-Villamizar et al., 2014). Dissolved oxygen (DO) enters the water through the air (diffused across the water’s surface) or autotrophs as a result of photosynthesis. DO is an important chemical parameter due to its effect on fishes in lotic ecosystems (rivers) and mainly lentic ecosystems (lakes). In lakes, DO is directly affected by the thermal stratification and its distribution is heterogeneous (Stefan & Fan, 1994; Bell et al, 2006; Elci, 2008).
DO can affect fishes behavior in many ways (Biertwell & Kruzynski, 1987; Xu et al., 2006). Kramer (1987) observed changes in fishes activity under low oxygen conditions, such as an increase of the respiratory rates and higher displacement to the water surface to obtain atmospheric oxygen. The hypoxia effect in fishes has been recorded by several authors (Junk et al., 1983; Kramer et al., 1983; Chapman et al., 1995; Anjos et al, 2008). Several species of Amazonian fishes exhibited behavioral changes under hypoxic conditions such as lower predation (Anjos et al, 2008). In addition, fishes of the Amazon (Colossoma) exposed to different experimental conditions also showed a physiological adaptation to hypoxia, such as changes in ventilation rates and increased circulation of red blood cells (Val et al., 1998). Conductivity and pH are also important environmental factors in limnological studies.
Several studies have shown the effect of the pH and the conductivity on fishes species richness (Matuszek & Beggs, 1988; Barros et al., 2013). Gonzalez et al. (1998) examined the effects of the pH on several species of fishes from Rio Negro, Brazil and they concluded an exceptional acid tolerance of fishes. Oliveira et al. (2008) also recorded acid tolerance of fishes from Rio Negro. Conductivity (K25) is the capability of water to pass the flow of electric current and is directly proportional to the concentration of the ions available in the water as inorganic matter. Almiron et al. (2000) recorded an possible indication of the decrease of fish density K25 in floods. Conductivity and pH directly affect the water coloration, which in its turn affects the fish species biodiversity (Sioli, 1965, 1968, Junket al., 2011). Ríos-Villamizar et al. (2014) emphasized the Amazonian river’s three types of water coloration: black, clear, and white. According to the authors, blackwater such as Negro river, generally presents a more acid pH due to its decomposing organic matter, low quantity of suspended material, with conductivity that can reach up to 20 µS/cm. White waters such as the Madeira river, usually present high conductivity, between 40 and 150 µS/cm, and a high index of suspended material. Finally, clearwater such as the Roosevelt river can show a low quantity of suspended material, a neutral pH (6-7), and conductivity between 10 and 20 µS/cm.