Abstract:
Herbicides destroy, subdue or inhibit the spread of weeds and are widely used across the world. Their use for agricultural or golf course maintenance is a major source of pollutants to fresh waters; however, studies which assess the ecological effects of herbicide mixtures are uncommon. Therefore, there is an urgent need for research to better understand the ecological relationships (e.g. synergistic) caused by multiple stressors. The aim of my study was to assess the response of aquatic communities when exposed to combinations of two of the most commonly used herbicides, 2, 4-Dichlorophenoxyacetic acid (2, 4-D) and Dicamba and to compare their response against the isolated response of each herbicide. Three experiments based on stream microcosms were conducted. The first was representative of the detritus food chain (including biofilms and amphipods), the second was representative of the algal food chain (including algae and Potamopyrgus antipodarum snails) and the third was a combination of both food chains in a single test system. Within each experiment the isolated and mixture effects of Dicamba and 2,4-D were determined over a range of concentrations through the measurement of the respiration rate, chlorophyll a concentration, pheophytin concentration and organism mortality. Within the detritus food chain, no synergistic relationships were found between the herbicides. However, exposure to a high Dicamba concentration resulted in a decline in biofilm respiration rates. Increasing concentrations of Dicamba and 2, 4-D increased the mortality of amphipods. Similarly, there were no synergistic relationships within the algal food chain; however, as Dicamba concentrations increased, chlorophyll a concentrations also increased. Furthermore, as the concentrations of Dicamba and 2, 4-D increased, the mortality of the snails increased. Results from the detritus food chain suggest that declines in biofilm respiration rates are related to high Dicamba concentrations triggering an oxidative stress response leading to DNA, RNA and protein damage which subsequently inhibited bacterial respiration. However, results from the algal food chain suggest that additions of Dicamba, which is a synthetic auxin, resulted in an increase in cellular expansion and subsequent algal biomass. The mortality of amphipods and snails observed in all three experiments may have been caused by oxidative stress, whereby DNA damage, RNA damage, protein damage, lipid peroxidation, endocrine disruption or reduced energy levels lead to organism mortality at the higher concentrations of the herbicides. Future studies should focus on combining Dicamba and 2, 4-D with other commonly used pesticides with different modes of action to gain a more comprehensive understanding of ecosystem responses. Studies could also add the toxicity data from this study to current data to advance the development of mixture predication models. The significant impacts of Dicamba and 2, 4-D on detrital and algal food chains suggest that regular monitoring of herbicides as pollutants should be carried out in New Zealand freshwaters. Keywords: Dicamba, 2, 4-Dichlorophenoxyacetic acid, freshwater, mixture toxicity, detritus and algal food chains.