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Suscptability to Ammonia in Temperate versus Tropical Organisms
Unionised ammonia (NH3) is highly toxic to freshwater organisms. Yet, most of the available toxicity data
on NH3 were predominantly generated from temperate regions, while toxicity data on NH3 derived from
tropical species were limited. To address this issue, we first conducted standard acute toxicity tests on
NH3 using ten tropical freshwater species. Subsequently, we constructed a tropical species sensitivity
distribution (SSD) using these newly generated toxicity data and available tropical toxicity data of NH3,
which was then compared with the corresponding temperate SSD constructed from documented
temperate acute toxicity data. Our results showed that tropical species were generally more sensitive to
NH3 than their temperate counterparts. Based on the ratio between temperate and tropical hazardous
concentration 10% values, we recommend an extrapolation factor of four to be applied when surrogate
temperate toxicity data or temperate water quality guidelines of NH3 are used for protecting tropical
freshwater ecosystems.
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Towards more Robust data on Tropical Species
Ammonia is considered as one of the most important pollutants
in the freshwater environment. It can enter natural aquatic systems
through direct means such as municipal sewage effluent discharges
and excretion of nitrogenous wastes from animals, and indirect
means such as natural nitrogenous organic matter breakdown in
contaminated sediments (Thurston and Russo, 1986). In freshwater
aqueous solution, unionised ammonia (NH3) exists in an equilibrium
with ionised ammonia (NH4), in which the concentration of
NH3 is positively correlated with water temperature and pH
(Emerson et al., 1975). Harmful effects of ammonia on aquatic organisms
have been extensively studied. In fish, for instance,
ammonia can disrupt the normal functions of internal organs such
as damage to the gill epithelium (Lang et al., 1987) and disruption of
normal metabolic functioning of the liver and kidneys (Arillo et al.,
1981).
To better control the release of ammonia and protect freshwater
organisms from the toxicity of ammonia, a number of guidelines or
criteria for ammonia have been established around the globe
To date, most of the documented acute toxicity test results for
ammonia were primarily generated from temperate species in
North America and Europe (Kwok et al., 2007). Acute toxicity data
on ammonia derived from tropical species are limited except for
two taxonomic groups of species namely, amphibians and fishes.
Most tropical WQGs and ecological risk assessment (ERA) studies
rely on extrapolation from temperate information, despite this the
regulatory standards are widely thought to be geographic-specific
(Kwok et al., 2007; Wang et al., 2014). However, there is a high
uncertainty on whether or not such an extrapolation from the
temperate information to generate tropical WQGs can adequately
protect tropical ecosystems. This practice remains controversial
and debatable in the tropics With limited acute toxicity data in terms of NH3 toxicity obtained
from tropical freshwater animals (n = 9 species), Kwok et al.
(2007) revealed that these tropical species were generally more
sensitive to NH3 than their temperate counterparts (n = 31 species).
Nonetheless, the results of their comparison of ammonia sensitivity
between tropical and temperate species remained uncertain, since
their tropical dataset was dominated by tropical fishes, and other
taxonomic groups such as crustaceans, molluscs, insects and
worms were underrepresented or missing. There is a need to
generate more tropical toxicity data of NH3 from a wider array of
taxa, allowing a sound comparison between the two geographical
groups, and derivation of tropical WQG for ammonia and an
ammonia-specific temperate-to-tropic safety extrapolation factor.
There were three main objectives in this study. First, we aimed
to determine the acute toxicity of NH3 on ten selected tropical
freshwater organisms. Second, we systematically compared the
potential differences in sensitivities of NH3 between temperate and
tropical freshwater species by comparing their species sensitivity
distributions (SSDs). Third, this study was also aimed at deriving
interim acute and chronic predicted no-effect concentrations
(PNECs) of NH3 for protecting tropical freshwater ecosystems using
the tropical SSD generated in this study and available temperate
acute-to-chronic ratios (ACRs), and determining a temperate-totropic
safety extrapolation factor for tropical countries or regions
which would borrow temperate information to derive WQGs of
NH3
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Invertibrates, fish and algae: Susceptibility to Ammonium Chloride solution
The test tropical freshwater species included two algae (Chlorella
vulgaris and Pseudokirchneriella subcapitata), three fishes
(Cheirodon axclrodi, Danio rerio and Oryzias sp.) and five invertebrates
consisting of two crustaceans (Daphnia magna and
Moina macrocopa), two molluscs (Pomacea lineata and Sulcospira
hainanesis) and one insect (Chironomus sp.). The source and culture
conditions of each test species are summarised in Table 2.
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Invertibrates, Fish and Algae were Increasingly Tolerant to Ammonia
Experimental dissolved oxygen, pH and conductivity remained
stable for each treatment throughout the test duration (i.e., at the
beginning, the prior to test solution renewal and the termination).
No differences in dissolved oxygen and pH were detected among
controls and treatments for each test, while an increase in electric
conductivity (ANOVA, p < 0.05) was noted with increasing NH4
concentrations of each test (Table S1). The concentration-response
relationships of NH3 on the ten test species are shown in Fig. 1.
Across all test species, molluscs P. lineata and S. hainanesis and insect
Chironomus sp. were sensitive to NH3, in which S. hainanesis
was the most sensitive one (Table 4). Fishes and crustaceans were
intermediately sensitivity to NH3, while algae were the least sensitive
organisms tested (Table 4).
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Toxicity of NH3 Used in this Study Differed Greatly Among Ttest Species
The results of the short-term toxicity tests showed that the
acute toxicity of NH3 used in this study differed greatly among test
species, with almost a 32-fold difference between S. hainanesis (the
most sensitive species) and C. vulgaris (the most tolerant species).
Molluscs were found to be highly sensitive to NH3, and these
findings were similar to several studies showing high toxicities of
NH3 to temperate snails (Helisoma trivolvis, Lymnaea stagnalis and
Potamopyrgus jenkinsi; Table S2). The tropical insect larva Chironomus
sp. was found to be more sensitive than its temperate
counterparts (e.g., Chironomus riparius: 2610 mg L1 NH3 and Chironomus
tentans: 3090 mg L1 NH3; Table S2). The test fishes and
crustaceans were intermediate in terms of sensitivities towards
NH3. In contrast to other test species, the algae were the most
tolerant group partly due to the fact that NH3 is an essential
nutrient, and readily available and energy-efficient source of nitrogen
to the aquatic plants, although NH3 can be toxic to the algae
through inhibiting their photosynthesis when it presents at high
concentrations (Kallqvista and Svenson, 2003).
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Based on the Current Results, Concentrations of 15 mg L-1 NH3 and 1.5 mg L-1 NH3 at pH 7.0 and 25 C are recommended as interim acute and chronic PNECs for tropical areas, respectively.
Through comparative comparison of tropical SSDs constructed
from our newly generated toxicity data and documented tropical
toxicity data of NH3 with the corresponding temperate SSD constructed
from temperate NH3 toxicity data, we found that tropical
species are generally more sensitive to NH3. Based on the ratio
between temperate and tropical HC10 values, an extrapolation
factor of four is recommended when using temperate information
of NH3 and related WQG as a conservative surrogate for protecting
freshwater ecosystems in tropical regions. Based on the current
results, concentrations of 15 mg L1 NH3 and 1.5 mg L1 NH3 at pH 7.0
and 25 C are recommended as interim acute and chronic PNECs for
tropical areas, respectively.
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