Can salt marshes keep up with sea level rise?

This summer we conduct field surveys across the Wadden Sea, where we measure sediment accretion or erosion at fixed positions, along with vegetation surveys. This way we can link patterns in sedimentation rates to vegetation dynamics.

Pioneer salt marsh consisting of (among others) cordgrass and Salicornia species. By Fee Smulders

The salt marshes at the mainland coast of Friesland and Groningen are semi-natural as they are accompanied by wave-reducing dams and drainage networks. Because of these past and current management practices in combination with an abundance of fine sediment near the coast, this area has high accretion rates of about 8 - 20 mm per year. However, the salt marshes on the barrier islands grow at an average rate of 1 - 4 mm per year, making them much more vulnerable to drowning at current sea level rise level of 2 - 3 mm per year. We cannot be sure to say when or how these salt marshes may drown in the future as this depends on sediment dynamics within the whole Wadden Sea area. But we know that salt marsh vegetation can be quite resilient once settled.

Picture of one of our permanent plots, with the sedimentation-erosion bar (SEB) which we use to measure sedimentation and erosion with a 1.5 mm accuracy in front, and the 2 by 2 m plot in which we assess vegetation cover and diversity in the back. Note the goose marks as proof of the important habitat and food function of salt marshes! By F. Smulders.

We were joined by Gert Elbertsen of Vroege Vogels for a radio report (in Dutch) of our work. You can listen to the radio fragment via the link below (it starts after 31.30 minutes).

New report out: State of Nature - Sea turtles in the Caribbean Netherlands

By Fee Smulders

After centuries of hunt, habitat loss, and disruption, some sea turtle populations in the Caribbean Netherlands appear to be slowly recovering. The report State of Nature Caribbean Netherlands shows that protection works. But it also warns of ongoing threats. The future remains fragile for these ocean dwellers.

Sea turtles have a remarkable life cycle. After hatching, the young turtles enter the open sea, where they remain out of sight for years. Only around their 25th to 35th year, as with the green sea turtle, do they return to their birthplace to nest. This long lifespan and strong site fidelity of sea turtles makes them especially vulnerable to disruption both at sea and on land.

Threats

In Dutch, the green sea turtle is also known as soepschildpad, which translates to 'soup turtle'. A name that refers to a time when sea turtles were a major food source in the Caribbean. In this day and age, sea turtles are protected, and it is forbidden to eat them. However, that does not mean these animals are free from human threats.

On beaches, coastal development, artificial light, erosion, and recreation cause problems for nesting. At sea, turtles face pollution, loss of seagrass beds, declining coral reefs, and the consequences of increasing recreational activities, such as boat collisions. As turtles forage in the same areas for years, they are vulnerable to environmental pollution. On Bonaire, in particular, heavy metals like cadmium and copper exceed safety thresholds in seagrass. Chemical pollution can cause turtles to be more susceptible to diseases like fibropapillomatosis, a herpes virus that causes tumors.

Important foraging areas

The waters around the Caribbean Netherlands have become increasingly important as foraging areas for turtles. On Saba, juvenile green turtles graze in seagrass beds, while hawksbill turtles are regularly seen on the reefs and the nutrient-rich Saba Bank. On Bonaire, green and hawksbill turtles are widespread, with high numbers of green turtles around Lac Bay, an important area for seagrass.

Lisa Becking, researcher at Naturalis and associate professor at Wageningen University & Research, sees reason to be cautiously optimistic about the future of sea turtles in the Caribbean Netherlands. “Twenty years of monitoring show that sea turtle numbers remain relatively stable. Genetic research indicates that juvenile green turtles, in particular, originate from recovering populations in the northwestern Caribbean. The influx of turtles from other regions is decreasing however. This suggests that population recovery is less strong in other parts of the Caribbean.”

By Milan van Boheemen

International cooperation

The threats to turtles are not limited to the islands themselves. During their long migrations between breeding and foraging grounds they face dangers as well. “Nicaragua, for example, is responsible for a large portion of green sea turtle catches in the Caribbean, by some estimates. And those just happen to be the waters where many nesting sea turtles from Bonaire like to spend their time,” Becking explains.

Sea turtles often travel thousands of kilometers between their nesting beaches and foraging grounds. Conservation on a single island is therefore not enough. According to Kaj Schut, manager of Sea Turtle Conservation Bonaire, international cooperation –even with countries where fisheries and habitat disruption are still major risks – is essential. “Thanks to satellite research, we know that sea turtles travel enormous distances. A turtle nesting in Bonaire can migrate to Nicaragua to forage. This means that protection does not stop at the island borders.”

Differences between species and islands

There has been a huge increase in the number of observed nests of green sea turtles, hawksbill turtles and loggerhead turtles on Bonaire since 2003. This is in part thanks to the increased monitoring of nesting areas by Sea Turtle Conservation Bonaire. But an increase in the number of green sea turtle nests has also been observed on Sint Eustatius. For hawksbill and loggerhead turtles, the number of nests there remains the same. For the leatherback turtle, however, the situation is very worrying. The number of nests on Sint Eustatius has dropped significantly in recent years, to almost zero. Saba has no suitable beaches for sea turtles to nest on, and therefore, no nests have been observed in recent years.

The report also shows that the quality of monitoring differs per island. According to Eleanor Butler, marine park manager at St. Eustatius National Parks, this is primarily due to the available resources. “Bonaire has a robust, long-term dataset, which allows for reliable trend analysis. Due to a lack of capacity, sometimes data is lacking from Sint Eustatius and Saba, which makes it difficult to properly track local developments or make timely adjustments. This emphasizes the need for continuous monitoring and targeted protection, especially in places with a lot of human activity. Bonaire's long-term data provides a valuable basis for effective management.”

A perspective on recovery

Sea turtles are indispensable for healthy ecosystems. The recovery of some populations proves that conservation measures are effective – as long as they are sustained and scientifically supported. According to Becking, the Caribbean Netherlands offers a unique prospect on recovery. “By building on the successes on Bonaire, improving monitoring, and addressing threats in a targeted way, these extraordinary animals can continue to thrive in our coastal waters. But this requires sustained commitment – from policymakers, researchers, and the local community. This is the only way to ensure a future where turtles continue to return to our beaches.”

By Fee Smulders

Recommendations from the report

To better protect sea turtles, the researchers make four concrete recommendations in their report:

Protect the habitat: Restore and protect beaches and seagrass beds from construction, light pollution, and disruption, and control water quality.

Collect better data: Structural monitoring on all islands is needed, including research into chemical pollution in turtle tissues.

Increase awareness: Tourists and residents have a role to play in protecting turtles, for example, by keeping a distance and not disturbing nests.

Collaborate: A new, joint Sea Turtle Recovery Action Plan (STRAP) must strengthen cooperation within the Kingdom and with neighboring countries.

New paper out: Temperature drives recovery rates of seagrasses

by Casper Douma

The results of recent research reveal both the vulnerability and the surprising resilience of tropical seagrass, despite the growing threats posed by climate change. Led by Fee Smulders from Wageningen University & Research, international scientists investigated how seagrass responds to warming waters, grazing by sea turtles and fish, and nutrient pollution. This ecologically important ecosystem, which provides food and shelter for fish and turtles, protects our coastlines from erosion, and stores CO₂, is increasingly under pressure due to environmental changes.

Over thirty researchers conducted year-long experiments at ten locations along the western Atlantic coast, from the Caribbean to the United States. At each site, they first created gaps in the seagrass meadows and then monitored both above- and below-ground recovery. In addition, half of the plots were enriched with excess nutrients. In this way, they studied how seagrass recovers under varying conditions.

Warming waters: opportunity or threat?

Remarkably, warming seawater was found to even promote the recovery of seagrass in some areas. “Unlike corals, seagrass seems to grow better in slightly warmer waters,” says marine ecologist Fee Smulders. “But we must be cautious with this conclusion: there is a limit. If the water becomes too warm, for instance during a heatwave, entire seagrass meadows could disappear.”

By Owen O’Shea

Excess nutrients: not good news

“Ocean warming is driving herbivores, such as sea turtles and fish, into more northerly areas, where they eat more seagrass. Another important pressure is that in areas where many turtles or fish graze on seagrass, additional pollution – such as fertilisers from agriculture entering the sea – can severely hinder recovery. ‘Too many nutrients promote algal growth, which smothers the seagrass,’ explains Fee Smulders. ‘We found that especially the combination of high nutrient and grazing pressure decreases seagrass resilience.”

By Marjolijn Christianen

Time for action

According to co-author Marjolijn Christianen, who is also affiliated with Wageningen University & Research, the study emphasises the importance of good water quality for the preservation of seagrass meadows. “Seagrass is of great value – for marine life, for climate solutions, and for coastal communities. If we want these ecosystems to persist in a changing climate, we must prevent them from being further burdened.”

link to paper: https://doi.org/10.1111/gcb.70172

Teatales: why did I bury teabags in seagrass meadows?

For the citizen science project TeaBagIndex, I buried teabags in seagrass meadows to assess decomposition rate within these systems, and to learn what the impact is of turtle grazing on carbon cycling. This is a quantified method to study the carbon cycle across the globe in different habitats. See the video below to learn more about this easy and interesting method and some of my results! For other teatales & more information about the project visit www.teabagindex.org.

Illustrations by Carolina Olid @Carolina_Olid

News article Trouw

De verdediging van mijn proefschrift vond plaats op 1 maart: Wereld Zeegrasdag! Om dit te vieren en zeegras zo meer in het zonnetje te zetten werd ik geinterviewd over mijn promotie onderzoek voor het nieuwsblad Trouw. Link naar het artikel

Ook Bionieuws besteedde aandacht aan mijn promotie en dan specifiek mijn keuze om de gehele commissie uit vrouwen te laten bestaan. Link

Becoming a Dr.! // My PhD defense

Friday 1st of March I defended my PhD thesis titled: 'Herbivores shape the seascape - Cascading effects of herbivores on the functioning of tropical seagrasses in a changing world' at Wageningen University & Research. It was an incredible day filled with lots of nerves, where I first got the chance to present the main results of my work to a general audience (you can access the recording here) .

Next, there was a lively discussion between me and my committee, consisting of Petra Visser Marieke van Katwijk, Juul Limpens and Nicole Esteban. I am very thankful for their time in assessing my thesis and the challenging questions they asked me.

After this, I got my diploma and was awarded the distinction cum laude, which was a huge surprise. Many thanks to my paranimfs Sara Pino Cobacho and Marisha Smulders who were by my side this whole time, and my co-promotors Liesbeth Bakker and Ingrid van de Leemput for their supervision throughout the years.

And lastly all my thanks to the one and only best supervisor of them all: Marjolijn Christianen! I'm very proud that you are the promotor of my thesis and so thankful for everything you have done for me in the past (almost!) 10 years since we started working together. What a wonderful journey it has been. On to many new exciting projects!

Marjolijn and me doing seagrass restoration work on Bonaire

The celebration after my PhD defense with colleagues, family and friends

Finishing my PhD thesis and maternity leave!

This summer, I focussed on finishing my PhD thesis. I had a strict deadline: mid September I would go on maternity leave! Writing my introduction and discussion and thinking about the design of my thesis took up all my time.

One day before my leave started, I finally handed submitted my thesis to the university system (see picture). I will be back in January from my leave, and my PhD defense is scheduled for 1st of March (World Seagrass Day!).

Universiteit van Nederland

What makes seagrass so special? It may look insignificant, and hard to distinguish from other green stuff on the ocean floor such as seaweed. For the university of the Netherlands I was asked to share my enthusiasm on the importance of seagrasses. I took them with me to the northern most part of the Netherlands, to one of our last remaining eelgrass meadows. I also shared parts of my PhD research with footage from the Caribbean.

Tipping point ahead: educational clip

See below our educational clip (in Dutch) that was made for educational purposes (SchoolTV and others) and is centred around my PhD research

New paper out: Invasive seagrass and native upside-down jellyfish are battling for space

Photo credit: Erik Wurz

Researchers from Wageningen University and the University of Amsterdam report on a fascinating case of competition between an animal and an invasive pIant. In tropical ecosystems, photosynthesizing organisms are continuously competing for space and light. The invasive seagrass Halophila stipulacea has been very successful in new habitats both in the Mediterranean and Caribbean seas. It can quickly colonize these habitats because small fragments break off, remain viable and spread via currents.

In a new paper published in Ecology, the researchers report on their discovery that the invasive seagrass uses little mounds - created by burrowing animals as shrimp or seacucumbers – as a new habitat to settle and expand from. The mounds provide new space with sufficient light, opening up the dense meadows of native seagrass where the invasive seagrass otherwise cannot settle. From there, they observed that the invasive seagrass can spread.

But the researchers found that this can cause problems for native species. ‘The upside-down jellyfish lies upside down because it has photosynthetic algae in its tentacles. Therefore, these organisms also need light and prefer open spaces such as these mounds created by burrowing animals. WUR Msc student Naomi Slikboer recorded the presence of both invasive seagrass and upside-down jellyfish on many of these mounds on the island of Curaçao and found that often, the invasive seagrass pushes the upside-down jellyfish out of these habitats over time’, according to the lead author of the study, Fee Smulders.

Photo credit: Erik Wurz

This probably increases the energetic costs for the jellyfish as it has to move more often due to rapid overgrowth of H. stipulacea. Additionally, the authors hypothesize that the interplay between invasive seagrass and burrowing mounds will lead unstable, dynamic seagrass meadows, unfavorable for valuable native seagrass species. Fee Smulders: ‘We need to keep a close watch on this invasive seagrass and investigate the impact on both native species as well as the seascape patch dynamics in Caribbean seagrass meadows.’