EPA - Ireland's Environment, An Integrated Assessment - 2020

Ireland’s Environment – An Integrated Assessment 2020 Figure 7.7  Chemical status of surface water bodies, with (top) and without (bottom) ubiquitous substances, 2013-2018 (Source: EPA) Chemical status ( n = 322) Chemical status ( n = 322) (without ubiquitous substances) 0 50 100 150 200 250 300 350 Good Fail Number of water bodies 242 319 3 80 Many of the chemical substances monitored are pesticides and, while none of the pesticides assessed exceeded their environmental quality standards (where applicable), some were detected in a high proportion of monitored rivers. 2,6-dichlorobenzamide, MCPA (2-methyl-4-chlorophenoxyacetic acid) and mecoprop (methylchlorophenoxypropionic acid) were the most widely observed substances: the first two substances occurred in over half of the rivers surveyed and mecoprop was present in over one-third (EPA, 2019a). All three substances are herbicides, which are used to control weeds. MCPA is widely used in agriculture to control rushes in grassland. The presence of these substances in water can harm very sensitive aquatic wildlife such as river insects and cause problems in drinking water supplies (see Chapter 14 on health and wellbeing). Fish Kills in Rivers, Lakes and Estuaries The increase in the number of fish kills in 2018 may have been indirectly linked to the drought-like conditions experienced in the summer of 2018. Lower oxygen levels associated with low flows and high water temperatures are likely to have increased the vulnerability of fish populations to water pollution. A fish kill is usually an indicator that serious pollution has occurred. There are many possible causes, but depleted oxygen levels in the water is the principle mechanism leading to fish deaths. Oxygen depletion can occur following the breakdown by bacteria of organic matter contained in algal blooms or from agricultural, municipal and industrial sources. After declining to a historical low of 14 fish kills in 2017, the number of fish kills increased to 40 in 2018 but decreased again to 20 in 2019. The low-flow conditions and higher water temperatures (warmer water holds less oxygen) in the summer of 2018 may have contributed to the increase that year by reducing ambient oxygen concentrations and increasing the vulnerability of fish to underlying levels of pollution or to acute pollution events. This highlights the importance of water body resilience: water bodies in good ecological health are likely to be more resilient than those in poorer ecological health, and therefore in a better position to withstand the additional pressures caused by climate change. 168