| Sociedade Brasileira de Zoologia | www.sbzoologia.org.br | www.scielo.br/zool All content of the journal, except where identified, is licensed under a Creative Commons attribution-type BY-NC.Dissolved organic matter, an integral part of all ecosystems, results from the decay of plant and animal debris (THURMAN 1985). It comprises humic, fulvic, and other organic acids, and is usually quantified as dissolved organic carbon (DOC) (WOOD et al. 2011). DOC is known to positively regulate several biotic/abiotic processes (STEINBERG et al. 2007, WOOD et al. 2011. In blackwaters, such as those found in forest streams in the Amazon, coastal lagoons in southeastern Brazil, Finnish and Swedish lakes, and Canadian wetlands, DOC may range from 10 to 300 mg CL -1 , while its average content in freshwater systems elsewhere is 0.5-4.0 mg CL -1 (THURMAN 1985, KÜCHLER et al. 2000, FARJALLA et al. 2009). The high levels of DOC account for the acidity of the aquatic environment. In order to thrive in acidic environments, organisms need a certain degree of specialization in their osmoregulatory organs (MATSUO & VAL 2007).Low pH (pH 4-5) induces ion loss (ZAIONS & BALDISSEROTTO 2000, 2003, BOLNER & BALDISSEROTTO 2007, MATSUO & VAL 2007, DUARTE et al. 2013) and the interference with gill ionoregulatory mechanisms may also trigger hematological disturbances. Ionic dilution, potentiated by the plasma acidosis prompted by H + entry, affects body fluid distribution. This could promote reduction in plasma volume, swelling of erythrocytes or splenic contraction, resulting in elevation of the hematocrit (MILLIGAN & WOOD 1982). Despite being highly responsible for the acidic nature of blackwaters, there is evidence that DOC protects native fish from the deleterious effects of low pH, reducing ion loss , 2003, MATSUO & VAL 2007. According to some studies, the occurrence of various charged functional groups in the heterogeneous compounds of DOC may change fundamental properties of the gill epithelium, such as the transepithelial potential, thus altering membrane permeability and stimulating ion uptake (WOOD et al. 2011). Humic acid also reduces respiratory stress in fish exposed to slightly acidic water, but increases it at more acidic waters (HOLLAND et al. 2014) and decreases lipid peroxidation and modulates the antioxidant system (RIFFEL et al. 2014). In opposition to the several findings regarding the effects of DOC on ionoregulatory disturbances, respiratory stress and antioxidant system, no evidence has been documented about its influence on the hematology of fish subjected to acidic pH. 4.2, erythrocytes of fish not exposed to HA were smaller, an effect that was partially offset by the presence of HA, since the values at pH 7.0 were higher. Although HA showed some positive effects changes in hematological and plasma K + ª in silver catfish caused by exposure to acidic pH, the overall findings suggest that HA does not protect this species against acidic pH because it increased mortality and Cl -loss at pH 4.0.
Effect of humic acid on su...