Abstract
Pollution a major threat to global freshwater biodiversity and ecosystems. Compounding the issue are emerging organic contaminants (EOCs), such as pesticides and pharmaceuticals, typically detected at ng–μg/L concentrations. Their widespread and growing use raises significant concerns, not only for public health but also for their potential impact on freshwater ecosystems. Among the most frequently detected EOCs in surface waters worldwide are the pharmaceuticals carbamazepine (an anti-epileptic drug) and ibuprofen (a non-steroidal anti-inflammatory drug, NSAID), as well as the insecticide imidacloprid (a neonicotinoid).
Much of the ecotoxicological research on EOCs has been conducted in laboratory settings, often using unrealistically high concentrations over short exposure durations. This project aimed to help address this knowledge gap. Firstly, a static-renewal laboratory experiment was conducted to characterise the toxicity of carbamazepine (Chapter 2) at environmentally relevant concentrations (nominal 0.1–10 μg/L) with 21 days exposure duration using Deleatidium spp. mayfly nymphs. These are among the most common invertebrates in New Zealand streams and rivers and are sensitive to a wide range of pollutants. A second laboratory experiment (Chapter 3), using the same study design and model taxon, was conducted for ibuprofen (nominal concentrations 2.0–2147.5 μg/L). Mayfly endpoints used to assess toxicity responses were survival, moulting propensity, emergence, impairment, immobility, feeding and swimming. Results indicated that Deleatidium nymphs showed no concentration-dependent response to increasing concentrations of carbamazepine or ibuprofen in all endpoints. However, mayfly feeding rates in the highest carbamazepine concentration and the positive control (imidacloprid) were greater than in the solvent control, suggesting an adaptive response to stress. Moulting propensity was higher in the highest ibuprofen concentration than in the imidacloprid control, but both were comparable to the solvent control, indicating a dissimilarity in mode of action between the two toxicants. Longer exposure durations would help to clarify and reinforce these toxicity responses.
Another limitation of the current evidence base is the paucity of community-level assessments of ecological risks of EOCs, particularly regarding contaminant mixtures, which commonly occur in natural environments. To help address this gap, a 30-day, field-realistic experiment was conducted using outdoor stream mesocosms. Flow velocity (fast vs. slow), imidacloprid (nominal 0 vs. 0.7 μg/L), and ibuprofen (nominal 0 vs. 32 μg/L) were manipulated in a full-factorial design. Flow velocity modulated many of the macroinvertebrate (Chapter 4) and periphyton (Chapter 5) community responses by enhancing the negative effects of the toxicants at fast flow. Invertebrate responses were examined by sampling drift and benthic communities, and emerged insects. Imidacloprid led to increased drift of some sensitive taxa, reduced benthic diversity, reduced abundances of benthic Ephemeroptera, Plecoptera and Trichoptera (EPT) and Chironomidae, and altered community structure (based on the 12 most common invertebrate taxa). Imidacloprid also resulted in fewer emerged Chironomidae. By contrast, ibuprofen exerted hardly any adverse effects on the invertebrates. The combined imidacloprid x ibuprofen treatment caused fewer adverse responses compared to imidacloprid alone, and in most cases the adverse effects from the binary exposure were largely driven by imidacloprid.
Imidacloprid promoted the growth of thick algal mats, likely due to reduced macroinvertebrate grazing. Imidacloprid also altered community composition based on the 21 most common taxa, though not necessarily in relation to the pollution sensitivity of algal species, whereas ibuprofen had minimal impact on the periphyton community. In terms of functional traits, only imidacloprid caused notable shifts in algal motility and cell size, with no effect on substrate attachment—suggesting limited influence on algal disturbance resilience. Overall, these findings point to a top-down effect of imidacloprid on periphyton, consistent with its insecticidal mode of action.
The findings of this project were summarised and synthesised (Chapter 6), with a focus on how to improve the current evidence base. Overall, environmentally relevant concentrations of the studied pharmaceuticals were largely benign, likely due to the low sensitivity of the invertebrates tested. In contrast, the mesocosm experiment showed that imidacloprid had more pervasive effects on invertebrate and periphytic algal communities. Negative effects from the contaminant mixture were primarily driven by imidacloprid, highlighting the need to prioritise pesticides such as neonicotinoids in environmental regulation. The findings emphasise the importance of using biological models with sufficient sensitivity to reliably assess the true ecological risks of EOCs. The results also indicate that using a single representative compound from a chemical group (e.g. only ibuprofen from the NSAID group) in mixture studies may underestimate the toxicity of that group. A more accurate approach may involve combining multiple compounds from a single chemical group (e.g. based on a shared mode of action) into a single toxic unit and mixing that with other contaminant groups known to co-occur in the environment. These strategies would enhance both the efficiency and ecological relevance of toxicity testing by better reflecting the complex mixtures found in nature, ultimately supporting more balanced environmental policies for protecting freshwater biodiversity and ecosystem services.