The Toxicological Effects of Bifenthrin on Urban Aquatic Fauna and Its Direct and Indirect Effects on Threatened Litoria raniformis Populations

Synthetic pyrethroids generally have low toxicity to mammals but are highly toxic to aquatic macroinvertebrates, fish, and frogs at very low concentrations. Bifenthrin is currently the most common synthetic pyrethroid used for termite control in new housing estates, contaminating Melbourne’s waterbodies and the increasing concentrations of bifenthrin in urban aquatic ecosystems in Melbourne is threatening the local aquatic fauna. Bifenthrin is usually found in high concentrations in sediments, especially in the vicinity of new housing developments where it is widely used as a termite control. Litoria raniformis (Growling Grass Frog) and Galaxiella pusilla (Dwarf galaxias) are threatened species that inhabit wetlands in the greater Melbourne area. These species could be affected by direct exposure to bifenthrin or indirectly through reduction of their food sources (zooplankton and some macroinvertebrates). A pilot survey was conducted in the aquatic habitats of 17 waterbodies inhabited by L. raniformis and 14 waterbodies inhabited by G. pusilla in February 2021 to see whether bifenthrin was present in sediments. Bifenthrin was found in 47% and 42% of wetlands inhabited by L. raniformis and G. pusilla respectively. The bifenthrin concentrations were mostly exceeding the toxic value for aquatic fauna (0.011 mg/kg) and the concentrations could be associated with presence of new residential developments in the vicinity of these sites (p = 0.004). Surveys were conducted in 26 wetlands across the greater Melbourne area over a three-year period (2021, 2022 and 2023) to determine whether bifenthrin concentrations change over time in response to urban development in their catchments. Bifenthrin was detected in 22 (87%), 24 (92%) and 22 (88%) wetlands in 2021, 2022 and 2023 respectively. Zinc (Zn) was detected in high concentrations in older wetlands (>10 years) and Zn concentrations were positively correlated with the age of the wetland. (rs = 0.623, p = 0.003), 2022 – (rs = 0.464, p = 0.017), 2023 – (rs = 0.426, p = 0.034). Stormwater runoff is a major way of transporting bifenthrin into aquatic habitats. Bifenthrin was detected in surface water samples collected in March 2023 from the wetlands and bifenthrin was detected in 61.5% of surface water samples collected with a positive correlation (p = 0.045) with turbidity. Rainfall also had a positive correlation with the turbidity (p = 0.027). Zooplankton and macroinvertebrate bioassessments were conducted in the same 26 wetlands where the sediment monitoring was carried out to determine what effects bifenthrin contamination may have on them. Zooplankton sampling was carried out in every season (i.e. 5 sampling events) from October 2021 to March 2023. Macroinvertebrates were sampled in October 2021 (Spring) and May/June 2023 (Autumn). Zooplankton communities tend to differ greatly in abundance between sites and between seasons. It was observed that by the last sampling event (March 2023), many sites had greatly reduced zooplankton abundance compared to the previous summer in 2022. Heavy rainfall was observed 7-days prior to sampling events in October 2021 (30-55 mm) and September 2022 (22-45 mm), which could be contributing to decreased numbers in those sampling rounds. The macroinvertebrate composition was similar in all wetlands in spring 2021 and autumn 2022. The dominant taxa were Oligochaeta, Hygrophila, Hypogastropoda and Chironomidae. The sediment-bound bifenthrin had no correlation to macroinvertebrate numbers, but all the animals present were pollutant tolerant families with low SIGNAL biotic index scores. Sensitive taxa were only observed at two wetlands in very low numbers (Hydrobiosidae and Telephlebiidae were found at two sites). Zooplankton is commonly used in ecotoxicological tests as they are highly sensitive to toxic substances and because they are good indicators of aquatic ecosystem health. The local species Daphnia carinata was used in laboratory tests to determine its sensitivity to bifenthrin. Low concentrations of bifenthrin in surface waters and sediments were toxic to D. carinata. The 48h LC50 for acute exposure was 0.0071 µg/L and the 48h LC50 for sediment was 1.603 mg/kg. Water exposures were more toxic to D. carinata in in concentrations as low as 0.010 µg/L showed significant responses. Therefore, current concentrations within the environment are likely to affect D. carinata and this indicated that the surface water concentrations present in the wetlands would be at concentrations that would potentially be toxic to other zooplankton species as well. This may have a negative effect on higher orders like threatened L. raniformis tadpoles that can rely on these organisms as a source of food. To determine the effects of direct exposure of bifenthrin on L. raniformis tadpoles, 48-h acute water and sediment toxicity tests were carried out in laboratory conditions. The tadpoles showed signs of stress (swimming on sides, not feeding and not active) in the highest concentration (0.5 µg/L) in water exposures. One-way analysis of variance with Tukey post hoc test conducted showed the highest concentration (0.5 µg/L) being significantly different to the control (33.20±0.2, p=0.016) in development stages after the 48-hr acute water test. The control also had a significantly greater weight compared to the 0.5 µg/L treatment (0.6700±0.1928, p=0.05 Biomarker acetylcholinesterase (AChE) expression did not significantly vary between treatment (p≥0.05). With the results of this study, it is very clear that bifenthrin is very commonly occurring in Melbourne wetlands and in concentrations above the guideline value (0.011 mg/Kg). Therefore, it should be potentially harming the biota in the wetlands including the zooplankton, macroinvertebrates, and L. raniformis tadpoles. Bifenthrin is certainly contributing to wetland pollution, and it could be having direct and indirect effects on the biota and their communities. Declining populations of zooplankton, macroinvertebrates, and L. raniformis confirms that wetlands are affected by the pollution. More work is needed to identify how communities of aquatic fauna would react to bifenthrin in their habitats. Special attention should be given to threatened L. raniformis tadpoles and its life stages that would mostly be vulnerable to bifenthrin toxicity. Control measures should be identified and applied to bifenthrin usages to protect the aquatic fauna in urban wetlands.<p></p>