Research

Here you find an overview of current and past research projects. The majority of my work has been on feedback interactions between organisms and their changing environment, particularly those in tropical marine ecosystems and shellfish reefs. In my research I combine experimental methods and technology from multiple disciplines. This includes ecology, biogeochemistry, physiology, remote sensing (drones), hydrology, food web analysis, mathematical models and telemetry.

Global defaunation and plant invasion: cascading effects on seagrass ecosystem service

Ecosystems are currently altered by human-induced changes in large herbivores and invasive species abundances. We investigate how green sea turtles, as large grazers, and invasive seagrass interactively affect seagrass ecosystem services. With Fee Smulders. Funding: NWO-VENI grant 2017

We found that megaherbivores, green turtles, may impact expansion of invasive seagrass in the Caribbean, Christianen et al. 2018

Tropical carbon cascades: mapping connectivity feedbacks, and tipping points. 

In this this project we aim to identify situations where feedbacks may lead to tipping points in connected tropical coastal ecosystems, so we can develop early warning signal recognition. This work package is part of NESSC, the Netherlands earth system science centre. Funding is provided by a Gravitation grant from the Dutch Ministry of Education, Culture and Science, which supports excellent research. With Marten Scheffer, Ingrid van de Leemput and others.

“Cross section of a tropical coast with a coral reef at the seaward edge, seagrass meadows in the shallows, and a mangrove forest at the landward edge. The arrows indicate a multitude of reciprocal positive interactions via wave attenuation (blue), nutrient capture and sediment trapping ( green), and nursery functions ( pink)” van de Koppel et al. 2014

Population connectivity and habitat use of mega-herbivores.

In this NWO funded research program we integrate data of sea turtles satellite tracking, stable isotope analysis, habitat mapping, animal physiology, and molecular tools to map sea turtle habitat use and population connectivity for the wider Caribbean to provide scientifically sound, ecosystem-based management strategies.

Back to the sea. Adult female green turtle on Zeelandia Beach St. Eustatius ready to rest after 1.5 hr of satellite tag processing.
Equipped with a satellite transmitter, an adult female green turtle is crawling back to sea in St. Eustatius.

Food web structure and restoration of foundation species – shell fish reefs – Wadden Sea

In this project we assessed the importance of foundation species for food web dynamics and biodiversity in the Wadden Sea (Waddensleutels). Foundation species such as mussel beds and seagrass meadows have strongly declined due to human interference in the Wadden Sea. In this Wadden Sea wide food web study we used stable isotope data of 10.000+ samples to develop new food web metrics to monitor nature conservation succes. Nature conservation agencies and scientific institutions have collaborated to study the importance of ecosystem engineers for the Wadden Sea food web

WaddenSeaFoodweb_penyu_nl_
Wadden Sea food web based on actual d15N and d13C stabile isotope values

Multi-scale seagrass patterning in Shark Bay, Western Australia

The seagrass beds of Shark Bay, Western Australia, are among the most extensive and pristine in the world. In the shallows of the Faure Sill, seagrasses (Amphibolis antarctica) forms distinct multi-scale patterns which can be seen from the air. We are currently trying to find out what factors drive this unique self-organization of seagrasses.

Seagrass bands in Shark Bay

Biodegradable ecosystem engineering elements

We are testing establisment structures for seagrass restoration. These structures temporarily facilitates this foundations species by temporarily simulating positive feedback mechanisms (reducing hydrodynamics, predation, macroalgae cover etc.) that often naturally occur in seagrass meadows. After bridging the establishment threshold and by expanding through the structure, the BESE should break down and new seagrass shoots can facilitate eachother and colonize the area.

 

 

 

 

Spatial Seagrass Patterns as indicators of seagrass resilience

UAV imagery showing seagrass patterns initiated by turtle grazing & hydrodynamics, Derawan

 

Aerial photography airplane launch (photo: Steve Oakley)

PHD

During my PhD project I studied the interactive effects of eutrophication, grazing and hydrodynamics on seagrass ecosystems. As a model system I studied tropical seagrass ecosystems, in Indonesia (Derawan Archipelago) that is also a global green turtle hotspot.

a) Eutrophication, Grazing & Resilience: Simultaneous effects of top-down control by megaherbivore grazing and bottom-up control by nutrient input (N&P) was experimentally tested in exclosure experiments. We found that intermediate levels of green turtle grazing increase the tolerance of seagrass ecosystems to eutrophication by the stimulation of seagrass production and concomitant nutrient uptake, the increased export of nutrients. Christianen et al. 2012.

Cage experiment on the interactive effect of grazing & nutrient addition

b) Habitat collapse in Marine Protected Areas: We reported severe seagrass degradation in a decade-old MPA where hyper abundant green turtles adopted a previously undescribed belowground foraging strategy (digging for roots). These results reveal an unrecognized need to consider interactions of protected species with their habitat for sustainability. In this project, we used UAVs (drones) for counting turtles.

c) Coastal protection services: We found that short (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments and hence lower beach erosion rates. We used manipulative field experiments (under water bunkers and exclosures) and wave measurements.

Installing sandbags as wave-barriers

d) Toxicity: Seagrasses are declining at a global scale due to light reduction and toxicity events caused by eutrophication and increased sediment loading. In lab-experiments we found interactive effects of light reduction and toxicants (reduced nitrogen, NHx) on seagrass distribution and species succession  By understanding the drivers and interactions responsible for the functioning and degradation of seagrass systems we were also able to develop indicator tools and to provide insight for the management of seagrass ecosystems, and, at a larger scale, for marine protected areas (MPAs).

Laboratory experiment on ammonia toxicity in tropical seagrasses

 

Analyse the 1000’s of samples in Nijmegen

Sea turtle population studies Indonesia

in-water surveys to gather (previously non-existing) baseline data green turtle populations in Indonesia

In the air: capturing green turtles rodeo-style

Population genetics of green turtle foraging aggregations

 

Impact on grazing on seagrass fauna (Derawan vs Spermonde archipelago, Indonesia)

 

 

River nutrient and sediment load impact on coastal ecosystems.

Sampling seagrass to look at possible effects of sedimentation & eutrophication of the Berau river (blog)

Restoring fen water bodies; effects for aquatic macroinvertebrates 2006

schermafbeelding-2016-10-12-om-23-05-16

 

Habitat connectivity of mangroves, coral reef, seagrass meadows for reef fish (Zanzibar, Pemba, Tanzania) 2003

schermafbeelding-2016-10-12-om-23-03-00

 

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1 Comment

  1. What an excellent experience!

    I work with seagrasses in Florida, micropropagation and see appkications of your work down the line!

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