Short-term toxicity of ammonia, nitrite, and nitrate to the aquatic snail Potamopyrgus antipodarum (Hydrobiidae, Mollusca)

Abstract

Nitrogen compounds, such as ammonia, nitrite and mtrate, are present naturaily in freshwater ecosysterns as biological degradation products of organic matter. These nitrogen compounds are interdependent through the nitrogen cycle. Ammonia may be present as two different chemical species, NH3 (unionized) and NH/ (ionized), whose equilibrium is affected both by water temperature and by pH (Emerson et al. 1975). Ammonia is normally oxidized by aerobic chemoautotrophic bacteria in a two-step process (Stumm and Morgan 1996), firstly to nitrite (by Nitrosomonas) and subsequently to nitrate (by Nitrobacter). Unfortunately, major anthropogenic sources, such as animal farming, agricultura! runoff, industrial wastes and sewage effiuents, are introducing huge amounts of organic matter and nitrogen compounds into surface waters. As a result, the concentrations of ammonia, nitrite and nitrate in aquatic ecosysterns have significantly increased over background natural levels, causing a world-wide environmental problem (Scott and Crunkilton 2000). At present, it is accepted that ammonia is more toxic than nitrite and nitrate to aquatic animals (Russo 1985; Scott and Crunkilton 2000), though the toxicity of nitrite and nitrate have been comparatively less studied than that of ammonia. The toxicity of ammonia is related to the unionized chemical species (Alabaster and Lloyd 1982; Russo 1985; Williams et al. 1986). In fish, NH3causes an increase in gill ventilation, hyperexcitability, convulsions and, finally, death (Alabaster and Lloyd 1982; Russo 1985). Nitrite turns hemoglobin to methemoglobin, which is unable to carry oxygen to cells, causing anoxia and death. Toxicity of nitrate may be related to in vivo reduction of nitrate to nitrite and a later conversion of hemoglobin to methemoglobin (Scott and Crunkilton 2000). As the sensitivity of aquatic organisrns to these toxics usually differs among taxa, specific toxicity testing is necessary to know the relative tolerance of a particular taxon to the toxicity of them. Potamopyrgus antipodarum Gray (=P. jenkinsi Smith) (Hydrobiidae, Mollusca) is an aquatic snail native ofNew Zealand, but since long established in Australia and Europe and North America as an exotic/invasive species. lt is abundant in freshwater ecosystems, from fast rivers to trickles, and also in brackish water, being the most common freshwater gastropod in Britain and other regions of Europe (Wallace 1985). Furthermore, it has been found in waters with organic pollution (Mouthon and Charvet 1999). The ecological success of P. antipodarum would be due to its fast parthenogenetic reproduction (Wallace 1985) and to its probable tolerance to organic pollution and nutrient enrichment. The aim of this study was to examine the tolerance of P. antipodarum to the toxicity of nitrogen compounds (NH3-N, N02-N and N03-N). Our hypothesis is that this aquatic snail will have a higher tolerance to these nitrogen compounds than other aquatic invertebrates reported in bibliography.