Water turbidity helps protect coastal ecosystems

To secure the future of crucial intertidal areas and salt marshes along our coastlines, increasing the turbidity of the water is vital, according to a recent study conducted by a team of Dutch and Chinese researchers. 

“These natural areas outside our dikes are essential for nature and coastal defense. But because of how we are now building in the Delta and the hinterland, coastal defense is endangered in the long term,” said Tim Grandjean, a postdoctoral fellow in estuarine and delta systems at the Royal Netherlands Institute for Sea Research (NIOZ). 

Water turbidity and intertidal areas 

The experts utilized decades of satellite measurements of tidal regions globally, correlating them with turbidity data also obtained via satellite monitoring. This analysis helped them establish a critical minimum turbidity needed for delta waters worldwide. 

The findings revealed a clear long-term relationship: intertidal growth is contingent on sufficient water turbidity. “After all, turbid water means more sand and silt particles in the water, which can allow the coast to grow,” Grandjean explained.

Vulnerability of tidal flats 

Globally, the clarity of many coastal waters has been improving for decades, which paradoxically leads to the vulnerability of tidal flats. Moreover, with sea levels rising, maintaining these flats requires even higher turbidity. 

The current low turbidity levels pose a significant threat to critical biodiversity areas and coastal safety since tidal flats serve as vital foraging grounds for birds and contribute significantly to wave attenuation.

Human impacts on intertidal areas 

The study underscores the need for a reevaluation of human impacts on delta regions, stressing the importance of strategic planning in coastal development to safeguard these vital ecosystems. Thus, we have to think more carefully about how human activities affect the coast, on which we rely for our safety. 

Large-scale infrastructural projects like the Three Gorges Dam on China’s Yangtze River have drastically reduced downstream sediment supply, essential for the nourishment and maintenance of tidal flats. 

Similarly, in the Netherlands, constructions such as the Oosterschelde storm surge barrier have led to significant reductions in sand deposition, causing the water to become clearer and less capable of supporting intertidal zones, leading to erosion.

Balancing nature conservation and coastal protection 

“In the long run, this is not sustainable. When too much intertidal area disappears outside the dikes of Zeeland, the safety of the complete coast is at stake. So not only for nature, but also for safety, we must begin to protect the areas outside the dikes much more effectively,” said Grandjean.

“The water has to become murkier, so we may have to consider dismantling the Oosterschelde storm surge barrier in the future. A careful balance between nature conservation and coastal protection is essential, ensuring that both goals strengthen rather than counteract each other.” 

Dynamics of intertidal coastal areas 

The study also involves innovative techniques developed at NIOZ workshops for measuring the dynamics of intertidal zones, providing crucial insights into their short-term vulnerabilities. 

“The dynamics of intertidal areas can be measured with very expensive sensors or with labor-intensive, manual measurements. However, this way, we could never collect data in many different places at the same time,” said Grandjean.

“The technicians at NIOZ developed sensors that were cheap enough to record the height of the bottom to the millimeter, several times a day, at different locations along the Western and Eastern Scheldt for several years now.”

The precise data collected shows that an intertidal zone’s height can fluctuate by one or two centimeters within a single tidal cycle. “For you or me, a centimeter more or less under our boots may not matter, but if you are a worm or a mussel, or a seed of eelgrass, that is indeed too much dynamics,” he explained.

Spacing between groynes 

Grandjean’s further studies into the spacing between groynes – structures built out from coasts as part of coastal management or to protect against erosion – by the province of Zeeland show that these also enhance the habitat quality along the Westerschelde by reducing dynamic changes, allowing for better settlement of benthic animals. 

“Of course, we already knew that these groynes, along rivers and coasts, can protect, but our measurements show that benthic life along the Westerschelde also benefits from these measures. The dynamics decrease enough in time and space for benthic animals to settle better,” he said. 

Moreover, the flow reduction between the groynes could help stabilize intertidal coastal areas over the long term and mitigate rising sea level effects. However, continued research is crucial to fully understand these dynamics and their long-term impacts on coastal ecosystems.

The study is published in the journal Nature Geoscience.

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