PhD Defence: Tropical Blue Carbon Cascades

PhD Candidate: Sara Pino Cobacho

Defence date: 5-12-2023
Time: 13:30
Institute: Wageningen University & Research
Location: Omnia, Hoge Steeg 2, Gebouw 105, 6708 PH Wageningen

PhD supervisors: 
prof. dr. ir. Marten Scheffer
dr. ir. Miquel Lurling

Title thesis: Tropical Blue Carbon Cascades


Mangroves are essential components of interconnected tropical ecosystems, offering numerous benefits to nature and society. However, mangroves are facing the impacts of global changes due to shifts in climate and human activities, which are expected to intensify in the coming years. To effectively preserve and restore mangroves, it is crucial to understand mangrove functionality and their responses to global change drivers. While their role in climate change mitigation and physical protection has been extensively studied, the biogeochemical processes driving mangroves’ responses to global drivers remain poorly understood.

This thesis begins by exploring the broader topic of cross- ecosystem connectivity in tropical coasts, with a focus on biogeochemical connectivity between mangroves, seagrass, and corals. The results reveal that human actions can directly or indirectly disrupt the biogeochemical interactions that link these habitats, shedding light on various aspects regarding the functionality of the biome.

The next part of the thesis investigates the potential of adding iron to mangrove sediments to counter the impacts of sargassum brown tides (large seaweed inundations), which pose a significant and timely threat to tropical coastal ecosystems, including mangroves. This chapter examines the effectiveness of iron in mitigating the sulfide produced during sargassum decomposition, while also assessing the health of mangroves. The findings indicate that the presence of sargassum severely impacts mangrove health, and iron does not prevent their death, making this mitigation approach unfeasible. Moreover, sargassum’s presence transforms mangrove habitats from carbon sinks to sources of greenhouse gasses, nullifying their role in climate change mitigation.

The thesis also explores the potential of mangroves to help regulate low water pH to benefit corals and other sensitive organisms. Results from a mesocosm experiment show that black mangrove seedlings do not rise water pH. In fact, their presence leads to lower pH levels, alkalinity, and dissolved inorganic carbon (DIC). This finding indicates that black mangrove seedlings have a negative impact on water pH.

Furthermore, mangrove restoration is vital in counteracting the global loss of mangroves, yet our understanding of how mangroves respond to various drivers of change at different scales is limited. This thesis also examines the impacts global warming and eutrophication on mangrove functionality. The findings reveal that when these two drivers act together, they alter the biomass allocation of mangrove seedling, potentially making them more susceptible to uprooting due to lower stability in sediment. These findings stress the importance of considering local nutrient status and hydrodynamic energy levels when selecting restoration sites for black mangroves under future global warming conditions.

Finally, the thesis shows that black mangroves also exhibit adaptive reproductive strategies in response to their environment. It describes the discovery of polyembryony in black mangroves, a rare phenomenon where a single propagule produces two seedlings.

In summary, this thesis shows that mangroves are significantly impacted by global change drivers such as sargassum brown tides, rising temperatures, and local eutrophication. These impacts not only affect their functionality but also create a feedback loop of mangrove decline. Additionally, mangroves play a role in decreasing local water pH and display various forms of reproduction as adaptive responses to their surroundings. The various mesocosm experiments conducted throughout this thesis have been instrumental in addressing key questions about mangroves functionality and the underlying biogeochemical processes. The knowledge generated in this thesis fills critical gaps in understanding mangrove ecology and can inform management practices for more efficient restoration and conservation of mangroves in interconnected tropical ecosystems.