Seagrass | Theory: The underwater ecosystem you've never thought about (but probably should)

In this episode of Big Ideas Only, host Mikkel Svold sits down with Kasper Elgetti Brodersen, Associate Professor at Roskilde University, to explore the science behind one of our most overlooked coastal ecosystems: seagrass meadows. What looks like an annoyance when it brushes against your legs while swimming turns out to be a biological powerhouse — nursery grounds for fish, carbon storage systems, and water quality filters all in one. Kasper explains how seagrass interacts with sediments and bacteria, why nutrient runoff from farmland is suffocating Danish fjords, and the surprising discovery that stressed seagrass can flip from climate helper to greenhouse gas emitter. The conversation covers what makes restoration so difficult, why seeds might be better than transplants, and what still needs solving before we can successfully garden the sea.

In this episode, you'll learn about:

• Why seagrass provides four times the ecosystem services of coral reefs (measured in economic value)
• How seagrass creates its own oxygen supply to survive in toxic, oxygen-free sediments
• The mechanism that turns nutrient pollution into plant-killing hydrogen sulfide
• Why stressed seagrass meadows can start producing methane and nitrous oxide instead of capturing carbon
• What makes restoration in Danish fjords so challenging — and why seeds might work better than transplants
• The bacterial partnerships happening underground that help seagrass access nitrogen

Episode Content
01:13 Why seagrass is a big idea
03:11 Global distribution and the 70% loss in Danish waters over the last century
05:07 The main stressor: eutrophication from agricultural nutrient runoff
07:15 How seagrass survives in anoxic sediment by pumping oxygen through internal channels
08:56 Epiphytes explained
10:17 What healthy conditions look like
15:09 The eureka moment: discovering seagrass provides more ecosystem value than coral reefs
19:26 Below-ground interactions: how oxygen release acidifies sediment and mobilizes nutrients
20:22 The greenhouse gas twist
30:32 Why restoration is still so hard
41:04 What's happening in the rhizosphere

This podcast is produced by Montanus.