Monitoring temperature in an intertidal zone
To better understand the effects of varying temperatures on sediment ecology, we have been monitoring sediment temperatures on mudflats in the Eastern Scheldt since 2020 (Figure 1). Using specially designed temperature loggers (see photo), measurements are taken continuously at different depths and at locations with varying drying rates. A record is therefore kept of the warming and cooling of the sediment under the influence of solar radiation, air temperature, water flow and dry periods.
Figure 1. Research sites with a transect of different dry periods at which sediments are exposed to air (since 2020), with the research sites with HZ/NIOZ (since 2024) shown in orange; the triangles are relevant water temperature measurement points from the Rijkswaterstaat monitoring network.
Measurements at the Roggenplaat and the Oesterdam in summer 2020 immediately produced a great many insights. During a strong heatwave in August, waterbed temperatures rose to 35 °C at a depth of 3 centimetres (see Figure 2), and the heat appeared to penetrate to 10 centimetres, where temperatures of 30 °C were reached.
Temperature extremes occurred at both locations, mainly in areas with longer dry periods. However, it was notable that the temperature progression for shorter dry periods was significantly higher at the Oesterdam (Figure 2). This seems to be mainly due to higher surrounding water temperatures, potentially in addition to local sediment characteristics (such as silt content).
The study shows that similar sites within the same system can nevertheless form very different environments, with benthic organisms in one location possibly being exposed to heat stress at a relatively short dry spell, while at another location this only occurs at longer dry spells.
Monitoring in later years confirmed that temperature peaks in the sediment depend on multiple factors, and that these can produce very different conditions for species each year. In the relatively cool and wet summer of 2021, hardly any extreme sediment temperatures were measured. In 2022, there was a milder heatwave where temperatures remained relatively moderate despite high air temperatures. Both 2023 and 2024 had no official heatwaves, but some warm periods. Temperatures also did not rise as high as in 2020 during these years.
Figure 2. During the August 2020 heatwave, sediment temperature increased significantly at both locations daily (solid lines), but temperatures at shorter dry periods were significantly higher at the Oesterdam (see dashed lines). Background colours: grey = the maximum temperature at the KNMI weather station in Wilhelminadorp; blue = average daily water temperature just outside the Eastern Scheldt storm surge barrier (= Rijkswaterstaat measuring point closest to Roggenplaat); red = average daily water temperature at Marollegat (= Rijkswaterstaat measuring point closest to the Oesterdam).
How do benthic organisms respond to higher temperatures?
The relationship between temperature progression and ecological effects is complex. How do highly variable temperatures affect species tolerance limits? Not only the level of temperature but also the duration of exposure plays a role, and both can vary greatly in intertidal zones.
Moreover, many studies on temperature effects on benthic organisms are conducted in sublittoral zones (which never run dry), where species are not exposed to the large temperature variations in an intertidal zone. Zones that run dry do not always have the temperature build-up measured in the field (duration and level). As a result, much information is still lacking on the effects of exposures in a highly variable system and standard tolerance limits cannot always be easily applied to tidal system species.
Nevertheless, with appropriate nuances, comparisons can be made between measured temperatures and results from previous research. For example, during the August 2020 heatwave, sediment temperatures in excess of 35 °C were measured for several days at some locations: well above the temperature at which mortality can occur in cockles according to previous research. In-house lab experiments also showed that during a simulation of the 2020 heatwave cockles showed mass mortality, while conditions more closely resembling the milder 2022 heatwave resulted in a far lower mortality rate. Furthermore, previous research has associated prolonged exposure of cockles to moderate temperatures (20–29 °C) with potentially significant sublethal effects, such as growth retardation and energy reserve depletion.
Field experiment with cockles and pullet carpet shells

Since survival experiments have failed to provide a complete picture of the ecological damage of extreme heat in intertidal zones, we have been conducting a field experiment on tidal flats in the Eastern Scheldt since 2021. In this experiment, we track the health and survival of two shellfish species throughout the summer. The species selected were the common cockle (Cerastoderma edule), a key species and food source for many birds, and the non-native Manila clam (Ruditapes philippinarum), an introduced species that is now widespread and seems to fill a similar niche to the cockle.
The condition index (a measure of shellfish meat weight and vitality) and survival on the mudflats are monitored by regularly collecting samples in quadrants. It is hoped that this field study will provide insight into both sublethal effects (do prolonged high temperatures in the sediment lead to a deterioration in condition?) and lethal effects (does mortality occur during or after heat spikes?). Other influences, such as food availability, also play a role. The combination of measurements and experiments is therefore essential.
Extension to other systems
The insights gained so far, particularly that temperature progression in the sediment can vary so much from location to location, highlight the importance of field measurements. In partnership with research institutes the Royal Netherlands Institute for Sea Research (NIOZ) and HZ University of Applied Sciences, the decision was therefore taken in 2024 to expand temperature measurements not only in the Eastern Scheldt but also in the Western Scheldt (see Figure 1) and the Wadden Sea. This makes it possible to identify the influence of local factors such as water temperature and sediment type.
The ultimate goal is to be able to predict where and when high temperatures will occur in intertidal systems in different coastal waters. The measured temperatures also provide a basis for future lab research on species-specific tolerances.
Climate change: influence on patterns and future scenario
According to Royal Netherlands Meteorological Institute (KNMI) scenarios, heatwaves will become more frequent and prolonged in the future. Extreme heatwaves as in 2020 are likely to become more frequent. This will mainly affect the build-up of temperature extremes of high-lying flats and other areas with long dry periods.
However, the more frequent occurrence of milder heatwaves can also lead to higher stress for species. Since 1980, water temperatures have risen in both the North Sea and Dutch coastal waters. Our research shows that temperature progression is clearly location-dependent, particularly in the case of shorter dry periods, with higher water temperatures presumably playing a role. A future rise in water temperatures or the more frequent occurrence of marine heatwaves (periods of abnormally high water temperature) may lead to higher exposure to medium-high temperatures, particularly in the lower intertidal areas.
Other species composition
Heat stress can potentially lead to species displacement. Field studies, literature and experiments all show that the Manila clam is resistant to exposure to high temperatures. This species therefore has a clear competitive advantage in a warming climate. In the longer term, this could lead to habitat loss on tidal flats, and thus a decline in cockles and increase in clams. Similar changes may also be expected in other species that are sensitive to temperature stress. Such a shift could have major implications for the wider ecosystem, particularly for bird species that forage on the mudflats and are highly dependent on tidal benthic organisms.
Policy and management
The observed temperature differences between locations and years clearly show that monitoring of tidal flats is essential. Standard monitoring of water and air temperature is not sufficient. Targeted monitoring, during various dry periods and at different locations, promotes a better understanding of the effects of temperature stress on ecosystems and species. Such data will make it possible to develop predictive models, which can help identify vulnerable locations, or high-impact heat events, early on. If managers take these factors into account in food stock estimates, they can better anticipate risks to benthic organisms and the food supply of birds.
When designing and landscaping intertidal zones, it is important to consider location-specific differences. In such nature-based solutions, change is not resisted but facilitated in a way that benefits conservation. In the case of replenishment, for example, choices of sediment type, dry period and precise location (renewal time and resulting water temperature) largely determine the thermal load.
Temperature monitoring also provides insight into the extent to which local changes in benthic organism communities are caused by climate change, and the extent to which these changes are caused by other stressors. This is important when assessing the effects of landscaping measures or other interventions.
It is also important to make the ecosystem more robust and resilient. Variation in habitats and species reduces the chances of everything failing at once in extreme conditions. This approach is in line with the four pillars of resilience from the central government’s Programmatic Approach to Large Water Bodies (Programmatische Aanpak Grote Wateren, PAGW): diversity, dynamics, connectivity and water quality.
Intertidal zones are experiencing increasingly extreme temperature changes. Monitoring in the Eastern Scheldt shows that sediment can become very hot during heatwaves. Since climate change is likely to mean that these kinds of heat extremes occur more frequently, Wageningen Marine Research and partners are studying these temperature patterns and how benthic organisms cope with them. Measurements show that sediment temperatures vary widely in time and space. The distinction between lethal effects and sublethal effects (or deterioration in condition) appears to be particularly important when interpreting the ecological effects of high temperatures and heatwaves.
Hamer, Alicia et al. (2023). Hittestress bij schelpdieren op intergetijdenplaten van de Oosterschelde in 2021 en 2022 (Heat stress in shellfish on intertidal flats of the Eastern Scheldt in 2021 and 2022). Yerseke: Wageningen Marine Research.
https://doi.org/10.18174/640397
Domínguez, Rula et al. (2021). Contrasting Responsiveness of Four Ecologically and Economically Important Bivalves to Simulated Heat Waves. Marine Environmental Research 164 (February 2021): 105229. https://doi.org/10.1016/j.marenvres.2020.105229
KNMI (2023). KNMI’23 klimaatscenario’s voor Nederland (KNMI’23 climate scenarios for the Netherlands). De Bilt, KNMI publication 23-03.