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This year, Deltares is working with Wageningen University & Research (WUR) and the Directorate-General for Public Works and Water Management (Rijkswaterstaat) to publish an update of the Climate Scan, in the context of the European LIFE-IP NAS programme. The 2019 report [1, 2] looked at the main water systems, namely the IJsselmeer region, Rhine-Meuse delta, Wadden Sea and south-west delta. The updated report will also include the North Sea and place a greater focus on regional differences and seasonal aspects of climate trends. However, the most important addition is that the new Climate Scan will look more specifically at the physical, chemical and ecological changes caused by underwater warming. In this article, we discuss the effect of rising water temperature on oxygen availability and attenuation of temperature fluctuations in water.
Warming of water
Warming of shallow surface waters occurs at a similar level to that of the air above it, but has a more complex effect on flora and fauna. This is partly because water temperature is also related to other physical and chemical mechanisms, such as the solubility of oxygen and other substances, and because temperature fluctuations are attenuated in deeper water.
Standing water is a poor conductor and heats up primarily through solar radiation. As a result, the level of warming depends not only on weather conditions, but also on depth, turbidity and the presence of aquatic plants. During the day, the water warms up to where radiation can reach. This heat is then diffused by the mixing of the water (convection), creating a gradient of temperature and of increasing water density towards greater depths, which increases the likelihood of stratification. Finally, where stratification occurs, the bottom layer does not mix with the upper layers, creating an abrupt temperature difference.
In water that is several metres deep, the water often becomes more or less fully mixed again at night. During heat waves or in deeper water, however, this process can take several days to months. In such cases, the oxygen concentration in the bottom layer often decreases – sometimes to levels that are too low to support oxygen-dependent organisms like fish.
Fluctuations
Temperature differences over a 24-hour period and from day to day are attenuated in the water column. The deeper the water, the greater the attenuation. This can clearly be seen in Figure 1, which shows the development of average, minimum and maximum temperatures of air and IJsselmeer water at Stavoren per 24-hour period for 2022. Of a summer average amplitude of the air temperature by day, 8 °C, only an amplitude of 1 °C remains at a depth of 1.5 metres in the IJsselmeer. This can mean that extreme events (see below) are less likely to occur in slightly deeper water, so that sensitive species can avoid lethal temperatures.
However, this also affects the start of spring, since the Royal Netherlands Meteorological Institute (KNMI) specifies that spring starts when the 24-hour average air temperature permanently remains at 5 °C or above. Due to climate change, this shift now occurs three to four weeks earlier. The 24-hour average water temperature fluctuates less at depth, which means that, on average, this criterion is met earlier. Moreover, as the nights become shorter in spring, water gradually retains more heat per 24-hour period than the air, leading to higher 24-hour average temperatures in summer. The growing season is therefore longer and warmer in slightly deeper water, but with fewer fluctuations.
This is relevant from an ecological point of view. In early spring, for example, temperature can be a trigger for reproductive activity. There is already evidence that the European smelt is laying its eggs earlier. In addition, differences in the temperature response of prey or predators can lead to mismatching, resulting in shifts in food availability. A longer growing season also means a longer period with a chance of algal blooms and other events. Lastly, organisms such as certain insect species (like mayflies [3]) may produce an extra generation, or warm-water fish such as zander can become larger at the end of the season.
All of this has potential implications for the balance of the food chain.
Figure 1. Average, minimum and maximum 24-hour temperatures in the air at Stavoren (KNMI; orange) and in the water at a depth of 1.5 metres off the coast of Stavoren (MWTL measuring point at Vrouwezand, Rijkswaterstaat; blue).
Oxygen availability
Aquatic communities are also faced with the physical and chemical effects of increasing water temperature. These effects include poorer oxygen solubility, which increases the risk of oxygen shortages.
Warming stimulates algal blooms, leading to additional oxygen production during the day. At night, more oxygen is actually consumed, which can exacerbate problems for aquatic fauna. For a long time, measurements in the large lakes were only taken during the day, so in the years when there were high levels of nutrients (eutrophication in the 1980s and 1990s) the data series often show strong oversaturation in summer (upper part of Figure 2). Since the decline in nutrient levels in the 1990s, oxygen concentrations have been moving closer to 100% saturation for the relevant temperature. Algal blooms and fluctuations in oxygen saturation can also be found in the North Sea.
Changes in salinity further contribute to these fluctuations, as oxygen is also less soluble in saline water.
In general, algal growth and the rate of organic matter decomposition increase with rising temperatures, as does oxygen consumption. In lakes, oxygen shortages can be exacerbated further by an increase in the frequency and duration of stratification. Aquatic organisms can avoid the water in the bottom layer, where the oxygen concentration is low, but may still die at times when the stratification dissipates and the oxygen-poor water mixes again with the upper layer.
Combined with factors such as chemical pollution, oxygen shortage can also occur during the day in any season, due to chemical oxygen demand. The lower part of Figure 2 illustrates this for three river locations.
Figure 2. Relationship between water temperature and oxygen concentration in the Meuse, Rhine and Scheldt rivers (bottom) and in the Eemmeer, IJsselmeer and North Sea (top) in the periods 1975–1996 (blue), 1997–2009 (red) and 2010–-2020 (green). The concentrations at 100% saturation (black) are provided for the sake of comparison. Rijkswaterstaat data.
Efforts to tackle chemical pollution have also reduced this under-saturation. In the Western Scheldt, concentrations are still somewhat lower than in the Rhine. However, this is partly due to higher salinity levels.
In rivers, algal growth is limited by flow. When that flow stagnates in summer, algal blooms also increase in rivers. This is most evident in the eutrophic years in the Meuse at Eijsden: as a pure rain river, the Meuse shows undersaturation of oxygen in winter, and oversaturation in summer (with sharply reduced flow rates). Since nutrient levels have fallen and water quality has improved, there has been relatively strong undersaturation. The influence of oxygen-poor seepage may increase in the Meuse in the event of a sharp decrease in summer run-off.
Meanwhile, climate change has also resulted in lower average oxygen concentrations, as the average water temperature has risen by around 2 °C. For a long time, the Rhine and Meuse rivers even saw higher temperature rises due to heat discharges. In conclusion, there are several factors that may lead to aquatic organisms facing low oxygen concentrations.
Events
For aquatic organisms, these gradual shifts are accompanied by other effects of climate change, such as changes in precipitation and evaporation and the growing likelihood of extreme weather. For example, conditions such as summer droughts are increasingly causing low river discharges, even in the Rhine. Extreme drought results in such low discharge levels that water comes to a standstill in parts of the rivers. In May 2011, this was accompanied by unprecedented algal blooms in the Rhine [4]. The reserve of dissolved phosphate was fully depleted, which is very unusual for river water. On 31 May 2011, oxygen oversaturation at Lobith reached the highest level ever recorded, at 142%, and biological oxygen demand reached the highest level since the resumption of the measurement series in 2002 at 5 mg/l.
This goes to show that dammed river areas have started to behave like a eutrophic, shallow lake: a situation that occurred twice more in 2018, and once more in 2022. The Meuse also experienced an exceptionally high level of algal bloom in 2022. In the Meuse, increasing oxygen demand in summer can have a different effect than in the Rhine due to structural undersaturation (Figure 2), resulting in a higher probability of mortality events.
Next steps and action perspectives
This analysis shows that the risks posed by a single climate factor – in this case, rising water temperatures – can vary greatly by water type. Factors such as depth, salinity or flow all play a role, as does combination with pressures other than climate change. The partnership of Deltares, WUR and Rijkswaterstaat is translating this knowledge into action perspectives through pilot projects in the Western Scheldt, IJsselmeer and North Sea. The new scan will make it easier to design area-specific measures. We have demonstrated above that water level management, hydrodynamics and reduction of water warming can potentially be used to control conditions in lakes. In specific terms, research will be carried out in areas such as the IJsselmeer region to determine whether shading with aquatic and riparian plants can provide shelter in shallow water through a combination of oxygen supply and cooler water.
A Climate Scan on the ecological impact of climate change in large bodies of waters was published in 2019. An updated Climate Scan is now in the pipeline, which will include data on the North Sea and a stronger focus on specific mechanisms and the risks for aquatic fauna that are associated with rising water temperatures, such as a decrease in oxygen availability. Combination with other pressures, such as pollution, can exacerbate the problem. On average, water can heat up more than air, but also with fewer fluctuations. Shelters (refuges) where the water is cooler can help. Research is being conducted to determine whether such areas can be created by encouraging aquatic plants.