The carbon budget at Earth’s surface determines Earth’s climate; this is because the partitioning of carbon among various surface reservoirs determines how much CO2 is in the atmosphere, where it acts as the dominant greenhouse gas. Seagrass meadows play a key role in the global carbon cycle as they store over 15% of accumulated carbon storage in the ocean. Furthermore, seagrasses are considered ecosystem engineers by reducing wave flow, improving water quality through trapping suspended sediment particles and nutrients, and stabilizing the sediments. In recent years, seagrasses are threatened by global and local changes (e.g., global warming, coastal development, and eutrophication). Seagrass loss results in reduced carbon sequestration and higher CO2 emissions back into the water column through their decomposition in the sediments. However, the full extent of seagrass loss and the feedback loops it might generate on the carbon cycle is still poorly quantified.

In this talk, I will demonstrate how seagrass disappearance alters sediment biogeochemistry in the tropical seagrass Halophila stipulacea at the Gulf of Aqaba, using field observations and laboratory experiments. Following the die-off of seagrass, the coupling between plants and sediments is disrupted, leading to enhanced sulfate reduction and the accumulation of hydrogen sulfide. This toxic compound can cause further seagrass mortality, creating a positive feedback loop that accelerates meadow decline. At the same time, the absence of seagrass weakens nutrient uptake and retention. Nitrogen and phosphate that were once recycled within the meadow are released into the water column, where they reduce water quality and promote further eutrophication. Our findings demonstrate that when seagrass meadows disappear, they shift from functioning as long-term sinks of carbon and nutrients to becoming sources, undermining both ecosystem health and climate regulation.