B. Walles1,2*, Qi Liu1,3, Oscar Franken1, Jim van Belzen1,4, Alicia Hamer4, Wouter Suykerbuyk4, Jaco de Smit5, Chiu Cheng5, Wietse van de Lageweg5
1NIOZ - Royal Netherlands Institute for Sea Research, the Netherlands; 2Delta Climate Center, the Netherlands; 3Utrecht University, the Netherlands; 4Wageningen Marine Research, the Netherlands; 5HZ University of applied Sciences, the Netherlands
* Corresponding author: brenda.walles@nioz.nl
Introduction
Grain size and water content of sediments influence thermal dynamics. Coarse sands drain quickly, dry out faster, and absorb more heat under direct sunlight, potentially creating thermal stress for burrowing benthic species. Shallow-burrowing organisms, such as bivalves, are particularly vulnerable to extreme surface heating, as they cannot escape rising temperatures. Prolonged heat stress can disrupt their survival, alter burrowing behavior, and reduce their availability as prey for higher trophic levels, such as oystercatchers.
The decline of estuarine ecosystems due to reduced sediment supply is a growing global concern, as these habitats are vital for biodiversity, coastal protection, and ecosystem services. To mitigate this loss, nature-based solutions like tidal flat nourishments are increasingly implemented. In the Oosterschelde, nourishments have been carried out since 2008, starting with Galgeplaat (0.13 million m³), followed by Schelphoek in 2011 (0.09 million m³), Oesterdam in 2013 (0.35 million m³), and Roggenplaat in 2019 (1.13 million m³), with the next large-scale nourishment planned for Galgeplaat-Dortsman in 2026 (3 million m³).
These engineered habitats are typically composed of coarser sediments than natural tidal flats. With climate change pushing temperatures in natural tidal flats toward lethal levels, could coarser sediment in nourishments unintentionally amplify heat stress in these man-made environments?
Objective and Methods
We investigated how sediment characteristics and inundation regimes influence thermal dynamics in tidal flat ecosystems. We deployed temperature sensors in 2024 at three contrasting sites: a muddy tidal flat (Paulinapolder, Westerschelde), a sandy tidal flat (Dortsman, Oosterschelde), and a sandy nourishment area (Roggenplaat, Oosterschelde). Sensors were placed at multiple sediment depths (0, -3, -5, -10, -20, and -40 cm) to monitor temperature fluctuations across varying inundation times (20%, 33%, 50%, 66% and 80%). The goal was to assess how sediment properties, particularly grain size and water content, contribute to temperature buildup and how this varies with different inundation regimes. Initial field observations suggest that nourished sediments dry out quickly, potentially enhancing heat accumulation, which could exacerbate thermal stress for benthic species.
Results
The data collected from various tidal flats revealed significant temperature fluctuations across sites and depths. At several points, temperatures exceeded 30°C for hours at a depth of -3 cm, the depth typically occupied by cockles.
An advanced sediment temperature simulation model developed at NIOZ (Liu et al., in prep) accurately replicates temperature dynamics but struggles to capture peak temperatures during warm days. Adjusting water content in the model improved predictions, suggesting that drying of the sediment alters thermal properties, leading to higher-than-expected temperatures during heatwaves.
In nourishment areas with low water content and coarse grain sizes, the potential for extreme temperatures is further amplified, potentially exceeding lethal thresholds for benthic organisms. Shallow-burrowing species like cockles are particularly vulnerable, as they cannot escape these elevated temperatures.
These findings emphasize the need to better understand thermal dynamics in these habitats, particularly how sediment properties influence temperature extremes. This knowledge is crucial for designing sustainable intertidal nourishments that mitigate the losses of bird foraging area. Without careful consideration of sediment properties, we risk creating unintended climate traps for benthic infauna undermining restoration efforts meant to protect foraging habitat for birds.
