Abstract
Sensitivity to acid water was assessed in three species of Amazonian fish that encounter naturally acidic blackwaters to differing degrees in the wild: tambaqui (Colossoma macropomum), matrincha (Brycon erythropterum), and tamoatá (Hoplosternum littorale), in decreasing order of occurrence in blackwater. Fish were exposed to a graded reduction in water pH, from pH 6 to 5 to 4 to 3.5, followed by return to pH 6. Fish were exposed to each new pH for 24 h. During these exposures, net transfers of ions (Na+, K+, Cl-, and Ca2+) and acid-base equivalents to and from the external water were used as physiological indicators of acid tolerance. Exposure to pH 5 had a minimal effect on net ion fluxes. Significant net losses of all ions (except Ca2+) were recorded in all three species during the first few hours of exposure to pH 4. However, ion balance was usually restored within 18 h at pH 4. Exposure to pH 3.5 caused even greater ion losses in all three species and proved to be acutely lethal to tamoatá. Matrincha sustained irreversible physiological damage at pH 3.5, as ion fluxes did not recover following return to pH 6 and there was some mortality. Tambaqui suffered the least ionoregulatory disturbances at pH 3.5 and was the only species to make a full recovery on return to pH 6. In all species, there was a tendency for ammonia excretion to increase at low water pH, but even at pH 3.5, there was no significant net uptake of acid from the water. Overall, there was a strong relationship between the magnitude of ionic disturbances and the lethality of exposure to low pH. The relative insensitivity of the ionoregulatory system of tambaqui to low pH indicates that this is a feature of fish native to blackwater systems rather than one that is common to all Amazon fish.
Highlights
The Rio Negro is a major tributary of the Amazon River and is characterized by its deeply colored appearance and paucity of dissolved salts
The Rio Negro and other blackwater systems of the Amazon basin are characterized by their acidity
The acidity reaches its peak in forest streams near the river source, but even the average pH for water in the main Rio Negro is low
Summary
The Rio Negro is a major tributary of the Amazon River and is characterized by its deeply colored appearance (often referred to as “blackwater”) and paucity of dissolved salts. Other tributaries of the Amazon River drain less nutrient-poor water, have a higher conductivity (150 mS cmϪ1) and ion concentrations (Na and Cl 1 100 mmol LϪ1), and are not acidic (pH ∼ 6.9; Furch 1984) These are often referred to as “whitewater” systems because of the amount of suspended solids present. 278 Wilson, Wood, Gonzalez, Patrick, Bergman, Narahara, and Val pattern, one would expect conditions within the dilute and acidic blackwaters of the Rio Negro to represent a relatively hostile environment for freshwater fish in terms of ion regulation Both the whitewater and blackwater regions of the Amazon support a remarkable abundance and diversity of fish life (Val and Almeida-Val 1995). It was decided to use the net fluxes of a variety of ions (Naϩ, Kϩ, ClϪ, and Ca2ϩ) and acid-base equivalents (as calculated from the ammonia and titratable acid fluxes) as physiological indicators of acid tolerance in these three Amazon species
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