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

Chapter 7: Water Quality Topic Box 7.2 Nutrient Enrichment and Ecological Impacts One-third of rivers and lakes and one-quarter of estuaries have too much nutrient in their waters, and there is also evidence that nutrient concentrations in our rivers and nutrient inputs to our marine environment are increasing (EPA, 2019a). At least one-quarter of river sites monitored have increasing nutrient concentrations, while nitrogen and phosphorus loads to the sea have increased by 16 per cent and 31 per cent, respectively (see Figure 7.10a and b). These nutrients cause excessive plant and algal growth in our rivers and increase the likelihood of harmful algal blooms in our lakes and estuarine waters. In rivers, for example, too much plant growth uses up oxygen, particularly during the hours of darkness. This can lower oxygen concentrations in water to levels that harm other animals such as certain species of river insects (e.g. the larval stage of stoneflies and some mayfly species), which are sensitive to low oxygen levels. In some lakes the proliferation of phytoplankton blooms can reduce the depth to which light penetrates and this in turn can restrict the growth of bottom-dwelling lake plants. In our estuaries, nutrient enrichment can cause the extensive growth of opportunistic green seaweed mats, which can blanket intertidal areas and smother the animals living in the underlying sediments. These are just some examples of how nutrient enrichment can harm the natural balance of different categories of water and their ecological functioning. The trend of increasing nutrient concentration in our rivers is continuing with the latest EPA assessment of data on water quality (for a water indicators report that is due out shortly) is showing that nitrate concentrations have increased in nearly half (44%) of river sites surveyed between 2013 and 2019 (EPA, 2020b). The data assessment also confirms that nitrogen loads to the sea are also continuing to rise. A bloom of green seaweed in Youghal harbour, Co. Cork, as a result of too much nutrient in the water. 173