Climate change causes rising air temperatures, changing precipitation patterns, sea level rise, and an increase in the occurrence of extreme weather events. These climatological pressures affect the water quality and hydrology of the IJsselmeer (Lake IJssel in English) itself, but the changing discharge dynamics of the rivers IJssel, Rhine, and other surrounding regional water bodies also impact the ecological water quality. These result in multiple ‘pressures’ for the water quality of Lake Ijssel. Higher air temperatures and lower water inflows could cause higher water temperatures and salinisation. These in turn increase the chance of stratification and anoxic conditions in Lake IJssel. Downpours could lead to increased runoff from agricultural land and discharges from sewage overflows. The organic matter and nutrients from these sources could cause eutrophication. Figure 1 gives a schematic overview of these processes.
Wageningen Environmental Research has assessed how these abiotic pressures could affect biotic pressures in Lake Ijssel. As a result of these changes and shifts of the ecological equilibriums, three important ecological risks have been identified [1].
Figure 1: Cross-section of the IJsselmeer showing the effects of climate pressures on biological water quality.
Risk factors
Blue-green algae and cyanobacteria are an increased risk, as higher temperatures, larger temperature fluctuations, increased nutrient concentrations and stratification all are favourable for algal and cyanobacterial growth, at the expense of macrophytes. This can result in cyanobacterial/algal blooms. Furthermore, cyanobacteria are efficient in the uptake of nutrients, and can grow very fast at higher temperatures. As they also have a relatively low density, they are able stay in the upper water layer of a warming water column. This allows them to outcompete other primary producers, by preventing the penetration of both light and CO2 into deeper layers of the water column. High concentrations of blue-green algae can result in the clogging of membrane filters, which increases the purification efforts necessary to produce drinking water. Cyanobacteria can also produce a plethora of cyanotoxins, which may be harmful to human health. Which cyanotoxins will be produced in higher quantities due to the changing climate and how well these can be removed during drinking water production, is difficult to predict.
Another ecological risk is posed by the inflow of organic matter into Lake IJssel. Increased irregularity of precipitation patterns with more downpours, increases in agricultural runoff and sewage overflows in upstream areas, could result in increased inflow of organic matter and nutrients. Combined with higher temperatures, this will result in a higher microbial activity. As a result, oxygen concentrations in the water will drop, and anaerobic degradation products will be produced. As stratification is also more likely at increased water temperatures, the risk of anoxic zones in deeper water layers is further increased. This does not only form a risk for the ecosystem, but may also increase the purification effort to produce drinking water. Organic compounds that consist of longer carbon chains (biopolymers), such as complex carbohydrates, proteins and fatty-acids, are difficult to remove during the purification process. The concentrations of these compounds may increase due to the above mentioned abiotic pressures.
The third identified risk revolves around the quagga mussel. This non-native species has spread quickly in Lake IJssel since 2017. These mussels have a large filtration capacity, which may have a positive effect, but they also produce large amounts of larvae. As higher temperatures may persist during a larger part of the year, the mussels may have a prolonged and intensified breeding season. The larvae could clog purification systems and thus increase purification efforts. Furthermore, risks of massive die-offs increase at water temperatures above 25°C [3]. Decomposing mussels could cause an increase in degradation products such as ammonium and other nutrients, which in turn could lead to an increase in algal growth.
Current correlations
To assess possible interactions between pressures that could impact ecological risks, existing water quality data from the PWN water intake point at Andijk were analysed for correlations between relevant parameters (Figure 2). After all, the climate has been changing for the past decennia. From the data set ranging from 1990 until 2023, the relevant physical, chemical and biological parameters were selected that were measured for at least ten years. These parameters were placed in a correlation plot, and clustered hierarchically.
Figure 2: Correlation plot of relevant parameters measured in the Andijk inlet water, 1990-2024. Parameters with fewer than 20 corresponding points in the correlation plot were not tested for significance.
Based on this correlation plot, several clusters can be identified with parameters that correlate (strongly). For instance, different fractions of organic material are quite strongly correlated, including total organic carbon, dissolved organic carbon, larger organic compounds and humic acids. Most of these parameters are not correlated or negatively correlated with chloride and conductivity. This could indicate that the organic matter ends up in Lake IJssel via runoff during downpour events, when relatively fresh water enters the lake, and salt intrusion is therefore not possible.
In the middle of the plot a cluster is visible where higher chloride concentrations and conductivity correlate with higher concentrations of biopolymers, chlorophyll-a and sulphate. The correlation between chloride and conductivity with sulphate could be explained by salt water intrusion from the Wadden Sea during dryer periods. During these periods, the water of lake IJssel is relatively stagnant and warm. It is not possible to determine if the increase in biopolymers is related to the higher chloride concentrations or to the warmer, stagnant water in the lake. It is clear however that relevant parameters for drinking water production from the water of Lake IJssel, might deteriorate during periods of increased salinization. This highlights the importance of these correlation analyses, as desalinization equipment needs highly purified water to function.
Case study: Summer of 2023
Extreme weather events in recent years may provide an insight in the possible changes in the future. An example is the very warm summer of 2023, with high temperatures and sunny weather, with June being especially warm and sunny. This translated to multiple abiotic and biotic pressures on Lake IJssel: water temperatures were very high, and the concentration of blue-green algae also showed a large spike. September 2023 continued to have very high water temperatures and low concentrations of oxygen, although no spikes in algal concentrations were measured. Satellite images from September show that the eastern part of lake IJssel does change to a very green colour due to the high concentration of algae. These blue-green algae did not reach the western part of the lake, where the PWN intake point in Andijk is situated. However, if other wind directions or currents would have occurred during this event, these high algal concentrations might have reached Andijk. This shows the importance of the spatial heterogeneity of parameters in Lake IJssel.
Conclusion and recommendations
The climate-related ecological risks for drinking water production originate from a combination of physical, chemical and hydrological factors that play a role on different spatial scales. To reduce risks, multiple measures are possible. On a local scale, monitoring of relevant parameters could be extended, both at the current location and at other locations in lake IJssel, possibly also employing satellite data. These could be used, together with the application of hydrological and climatological models, to develop predictive ecological models that can function as early-warning systems.
Apart from a broader and more detailed monitoring infrastructure, a toolbox of mitigating and adaptive measures could be developed to adequately intervene when needed. Examples are: 1) increasing the capability to selectively take in water with a high quality, which could be achieved by enlarging buffer capacity, allowing for longer intake stops; 2) increased (nature based) pre-purification of Lake IJssel water, using basins and purifying landscapes [2].
Furthermore, measures on the catchment scale could be taken, as the effects of runoff and leaching are largely dependent on the amount of organic matter and nutrients that enter the surface water during rain events, before ending up in Lake IJssel. Measures in the hinterland to limit this inflow are, for example: circular agriculture to reduce runoff, buffer zones between agricultural areas and surface waters, and buffering of rainwater where it falls, by restoring soil water-retention capacity.
The effects of climate change on ecological water quality pose a real risk for the current drinking water production. There are options, however, to lower these risks. For now, current and future risks cannot be quantified in terms of likeliness and consequences. This scoping study provides a first glance at three types of potential risk: blue-green algae and cyanobacteria, organic matter, and mussels. An increase in algal concentration already hampered the production of drinking water in 2023. Mussel die-offs have not yet been observed in Lake IJssel, but when a tipping point is crossed, the consequences could be large. Mitigating these risks demands additional investments in the purification process, including energy, chemical additives and infrastructure. When mitigating these risks it is important to implement measures on multiple spatial scales: 1) the catchment area of the IJssel and the Rhine; 2) Lake IJssel; and 3) the technical purification and reservoir system.
Lake IJssel is an important source for the production of drinking water. Much is unknown about how climate change affects the ecological water quality. A deteriorating ecological water quality may affect multiple ecological risks, which in turn can hamper the production of drinking water. This publication discusses three potential risks: increased growth of cyanobacteria, increased organic matter inflow, and the reproduction and die-offs of Quagga mussels.