Influence of pH on the acute toxicity of ammonia to juvenile freshwater mussels (fatmucket, Lampsilis siliquoidea)

Abstract

The objective of the present study was to evaluate the influence of pH on the toxicity of ammonia to juvenile freshwater mussels. Acute 96-h ammonia toxicity tests were conducted with 10-d-old juvenile mussels (fatmucket, Lampsilis siliquoidea) at five pH levels ranging from 6.5 to 9.0 in flow-through diluter systems at 20 degrees C. Acute 48-h tests with amphipods (Hyalella azteca) and 96-h tests with oligochaetes (Lumbriculus variegatus) were conducted concurrently under the same test conditions to determine the sensitivity of mussels relative to these two commonly tested benthic invertebrate species. During the exposure, pH levels were maintained within 0.1 of a pH unit and ammonia concentrations were relatively constant through time (coefficient of variation for ammonia concentrations ranged from 2 to 30% with a median value of 7.9%). The median effective concentrations (EC50s) of total ammonia nitrogen (N) for mussels were at least two to six times lower than the EC50s for amphipods and oligochaetes, and the EC50s for mussels decreased with increasing pH and ranged from 88 mg N/L at pH 6.6 to 0.96 mg N/L at pH 9.0. The EC50s for mussels were at or below the final acute values used to derive the U.S. Environmental Protection Agency's acute water quality criterion (WQC). However, the quantitative relationship between pH and ammonia toxicity to juvenile mussels was similar to the average relationship for other taxa reported in the WQC. These results indicate that including mussel toxicity data in a revision to the WQC would lower the acute criterion but not change the WQC mathematical representation of the relative effect of pH on ammonia toxicity.

An evaluation of the influence of substrate on the response of juvenile freshwater mussels (fatmucket, Lampsilis siliquoidea) in acute water exposures to ammonia

“…The 96-h EC50s ranged from 5.6 to 7.7 mg N/L (TAN) or from 0.47 to 0.58 mg N/L (NH 3 -N) at a mean pH of 8.4 across the four treatments with overlapping 95% confidence intervals (Table 1). The EC50s for ammonia in the four treatments were within the range of EC50s reported in previous 96-h water-only toxicity tests with juvenile fatmucket or other mussel species at similar pH [3][4][5]. These results indicate that the presence of the substrates did not influence the sensitivity of juvenile mussels to the acute ammonia exposure.…”

“…Mean TAN concentrations were slightly lower than the nominal concentrations, except for the highest TAN concentration, which was about 15% higher than the nominal concentration. The pH of the exposure water was about 0.2 units lower in high ammonia concentrations compared to the controls due to the addition of the ammonia stock solution at a pH of about 8.0; had the pH of the stock solution been adjusted to about pH 8.3, similar to the pH of the test water, this slight drop in pH would likely have been eliminated [5]. Mean concentrations of TAN or NH 3 -N and mean pH values were similar between inner chambers and outer beakers in the two treatments with substrate ( Table 1), indicating that ammonia concentrations and pH in the pore water surrounding the substrates were similar to those in the overlying waters due to the flushing of water through the substrates with each hourly cycling of the diluters.…”

“…The outer beaker had a 2.5-cm hole in the side covered with 50-mesh (279-mm opening) stainless steel screen, and contained 200 ml of water. The exposure units were held in two intermittent proportional flow-through diluter systems [5]. Each diluter system delivered five ammonia concentrations with a dilution factor of 0.5 plus a control.…”

“…The pH was determined using an Orion glass pH electrode and an Orion EA940 meter calibrated before using according to the manufacturer directions. To facilitate comparison of data in the present study to data in the literature, the concentrations of unionized ammonia nitrogen (NH 3 -N) were calculated based on measured TAN, pH, and temperature using the formula of Emerson et al [13], which was applied in the current U.S. EPA ammonia WQC [1] (note: like most other aquatic organisms, mussel sensitivity to ammonia reportedly increases with increasing pH [5]). In addition, dissolved oxygen, conductivity, hardness, and alkalinity were measured on composite samples of test solutions collected from the control, medium, and high ammonia exposure concentrations at the beginning and end of the test using standard methods [14].…”

“…Freshwater unionid mussels are generally more sensitive to ammonia than other tested organisms, and the current U.S. Environmental Protection Agency (U.S. EPA) water quality criteria (WQC) for ammonia [1] may not adequately protect freshwater mussels [2][3][4][5]. The U.S. EPA recently developed a draft update that substantially lowers the ammonia WQC primarily due to inclusion of data for freshwater mussels [6].…”