The Risk of Mutualism Breakdown in Coastal Biogeochemical Hotspots

This project is a personal NWO-Veni grant.

Seagrass meadows and salt-marshes form the ecological and economic pillars of our Earth’s coastal zones. Over the last decades, both ecosystems have suffered unexpected large-scale sudden losses due to mass-mortality of the vegetation (“dieback”) of which the underlying causes are still largely unclear. Here, I hypothesize that seagrass meadows and salt-marshes can be considered biogeochemical hotspots driven by keystone mutualisms, pointing at basic processes that can explain their current dramatic worldwide losses. Biogeochemical hotspots are areas that show disproportionately high elemental cycling rates compared to their surroundings. Seagrass and salt-marsh plants engineer such hotspots, because they trap large amounts of organic matter by reducing currents and waves. This stimulates decomposition rates and nutrient release, causing these plants to facilitate their own growth. However, the enhanced anaerobic decomposition of organic matter also stimulates the production of toxic sulphides, counteracting the positive effects of increased nutrient availability for plants. My recent work demonstrated that keystone mutualisms between sulphide-oxidizer consortia and marine plants may strongly mitigate the effects of increased sulphide production, thereby breaking the potential negative feedback associated with organic matter accumulation. I hypothesize that anthropogenic impacts – specifically global warming and eutrophication – can disrupt these keystone mutualisms, leading to ecosystem collapse and degradation of biogeochemical coastal hotspots. In this project, I propose a combination of field surveys, laboratory and field experiments, and computer modelling to investigate these hypotheses. In order to achieve generally valid conclusions, I will explore these ideas in seagrass and salt-marsh ecosystems in both temperate (Wadden Sea, Netherlands) and subtropical (Florida Bay & Seahorse Key, Florida, USA) areas.