Tiered Species Toxicity Assessment of Six Agricultural Pesticides on the Freshwater Environment
Citation:
ESMONDE, LOUISE MARGARET, Tiered Species Toxicity Assessment of Six Agricultural Pesticides on the Freshwater Environment, Trinity College Dublin.School of Natural Sciences.BOTANY, 2017Download Item:
Louise Esmonde_Final Thesis_ 02_11_2017.pdf (PDF) 4.024Mb
Abstract:
This research project aims to investigate the toxicity of six commonly used agricultural pesticides (formulations based on glyphosate, mecoprop-p, chlorothalonil, mancozeb, metaldehyde and chlorpyrifos) on three key functional groups of the freshwater environment. This was achieved by carrying out laboratory acute toxicity studies on 1) the bacterial community of a dehydrated microbial seed, 2) the submerged aquatic macrophyte species Myriophyllum aquaticum and 3) the aquatic invertebrate species Daphnia magna.
Firstly, the impact of pesticide exposure on the bacterial trophic level of a freshwater system was carried out. The effect of the pesticides on bacterial activity was measured by assessing the oxygen consumption of the population over time. This was achieved by using the standard BOD5 method and the Oxitop control measuring system. The inhibitory effect of the test substance on the bacterial population was measured in terms of percentage inhibition of BOD5, length of the lag phase, slope of the line during the exponential growth phase and EC50 values calculated at various time points on the growth response curve. Based on the results, the overall toxicity order of the pesticide formulations towards the bacterial population was as follows; glyphosate/mecoprop-p > chlorothalonil/chlorpyrifos > metaldehyde > mancozeb. Based on the EC50 value obtained, these pesticides are not thought to have a long term inhibitory effect on bacteria. All pesticides were degraded and used as a food source by the bacteria seed population over the test period. This indicates that freshwater systems have the ability to remediate against pesticide contamination.
A second toxicity bioassay was carried out on the submerged aquatic macrophyte Myriophyllum aquaticum, which represented the macrophyte trophic level of the system. While no standard procedure for toxicity testing of Myriophyllum aquaticum exists, the experimental procedure used was based on the Myriophyllum toxicity test method developed by the SETAC Aquatic Macrophyte Ecotoxicology Group (AMEG), with some minor modifications. Submerged rooted M. aquaticum shoots were exposed to the test substance, under controlled conditions, for a period of 14 days. Macrophyte inhibition was assessed by recording shoot length, biomass wet and dry weight, root length and root number, as well as noting physical changes of the plant over the course of the study. Based on mean toxicity values, the toxicity scale of the six pesticides examined was as follows; mancozeb > mecoporp-p > glyphosate > chlorpyrifos > chlorothalonil > metaldehyde. Five of the six pesticides examined exerted a toxic effect on M. aquaticum. Glyphosate, mecoprop-p, chlorothalonil, mancozeb and chlorpyrifos were all found to be toxic to the plant. Metaldehyde was the only active ingredient which did not stunt the plant growth. Due to the large range in sensitivity of parameters, it was concluded that a number of parameters should be used when measuring macrophyte toxicity. Roots are particularly sensitive to pollutants and where possible should always be included in toxicity tests.
Finally, the toxicity of plant protection products on the invertebrate trophic level of a freshwater ecosystem was examined by carrying out the Acute Immobilisation Test of Daphnia (OECD 202). The toxicity order of the pesticides towards daphnia was found to be as follows; mancozeb > chlorpyrifos > metaldehyde > chlorothalonil > glyphosate > mecoprop-p. Fungicide and insecticide products were the most harmful to daphnia while herbicidal products posed less of a risk. The OECD 202 Daphnia immobility study is a general protocol for the assessment of a wide variety of chemical substances, however, some modification for the assessment of plant protection products have been suggested. Stirring or aeration of the test solution may be required in cases where the test substance does not easily remain in solution. Secondly, distinguishing between immobility and mortality of daphnia, while more time consuming, can provide useful additional information on the effect of the substance on the daphnia.
By combining the above results, an overall assessment of the toxicity of pesticides on the freshwater environment can be evaluated. It was concluded that individual pesticides effect each trophic level differently. The invertebrate trophic level was observed to be the most sensitive to pesticide exposure, while the bacterial trophic level was the least sensitive. Bacterial populations have the potential to remediate against pesticide exposure and are an important defence against pesticide pollution events. It was concluded that of the six pesticide formulations examined here, mancozeb and in particular chlorpyrifos based formulations pose the greatest risk to the overall aquatic ecosystem. This is due to their toxicity on the three test species but also based on their potential for wider ecosystem effects. A tiered toxicity assessment, using a number of toxicity assays from different trophic levels, is therefore considered an effective way of assessing the risk posed by pollutants to the aquatic environment as a whole.
Sponsor
Grant Number
Earth and Natural Sciences (ENS) Doctoral Studies Programme, funded by the Higher Education Authority (HEA) through the Programme for Research at Third Level Institutions, Cycle 5 (PRTLI-5), co-funded by the European Regional Development Fund (ERDF), for funding this research
Higher Education Authority (HEA) PRTLI-5 postgraduate award
Author's Homepage:
http://people.tcd.ie/lesmondeDescription:
APPROVED
Author: ESMONDE, LOUISE MARGARET
Advisor:
GRAY, NICHOLASPublisher:
Trinity College Dublin. School of Natural Sciences. Discipline of BotanyType of material:
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