Seagrass meadows are more than just coastal greenery. Beneath the waves, they capture carbon and bury it in the seafloor, storing it safely for decades or centuries. This makes seagrass a powerful ally in the fight against climate change

But there’s a catch. When humans flood coastlines with too much nutrient pollution – especially from wastewater – it doesn’t just mess with the water. It messes with the seagrass, too.

Scientists wanted to find out exactly how this works, and they didn’t take shortcuts. Over nine years, researchers observed what happened when they added extra nutrients to seagrass beds. What they found was a balancing act between helping and hurting.

The balance of seagrass and nutrients

In the warm, shallow waters of tropical bays, seagrass grows in sandy beds. Such places are usually poor in nutrients. That may sound bad, but it is one of the reasons why seagrass is good at capturing carbon.

The plants grow slowly and consistently, extending their roots deep into the sediment where carbon is stored and trapped.

But when researchers added phosphorus and nitrogen to these beds, something interesting happened. The seagrass responded with a burst of growth.

First, it strengthened its root systems. Then, it put more energy into its blades – the flat, grass-like leaves. As the roots grew quickly, they also died more quickly, sending extra carbon into the sediment. That should be good news, right?

The shadow of too much nitrogen

Here’s where things got complicated. When nitrogen levels went too high, the team saw a surge in phytoplankton – tiny algae that float in the water. These blooms blocked sunlight, putting the seagrass at risk.

“People thought excess nutrients were killing seagrass,” said Jacob Allgeier, associate professor of ecology and evolutionary biology at the University of Michigan.

“But we show that as long as there are not too many nutrients, which would also increase phytoplankton, the seagrass will just increase growth with excess nutrients.”

How the experiment worked

To figure out what was really going on, a team led by recent doctoral graduate Bridget Shayka went hands-on – literally. The team collected seagrass samples from test plots in the Bahamas.

Shayka and 17 undergraduate students separated each plant into parts: blades, sheaths, roots, and rhizomes (the stems that grow underground). Next, the parts were freeze-dried, crushed, and tested for nitrogen, phosphorus, and carbon.

The researchers found that nutrients from human sources had a bigger effect than those from fish.

“The systems we study are pretty low-nutrient systems, so adding nutrients can increase seagrass production,” said Shayka, now a program officer at Ocean Visions.

“But we also know that when you go too far and add too many nutrients, it really destroys these systems. It’s one of the leading causes of their destruction around the world and in coastal systems.”

Nitrogen isn’t the only problem

In another experiment, the researchers tried 21 varied nutrient ratios in bottles of seawater and phytoplankton. They were curious to know if the ratio of nitrogen to phosphorus mattered more than just the amount of each nutrient. The results surprised them.

In this system, phosphorus had a stronger positive effect on seagrass growth. Nitrogen, on the other hand, caused phytoplankton levels to spike. That meant more light-blocking algae in the water and more stress for the seagrass beds.

“When you grow tomatoes, you don’t just add nitrogen. You add a perfect ratio of nitrogen and phosphorus. That idea is replete in our society,” said Allgeier. “But because we tested the water column and because we tested the seagrass, we’re able to say that model doesn’t work in our system.”

Seagrass and carbon storage

It’s unlikely that we’ll stop nutrient pollution altogether. But these findings suggest we don’t need to. Instead, we need to manage it better.

“We’re not stopping nutrient enrichment. It’s just not going to stop,” said Allgeier. “But we can manage it. And how do you best manage it? We scrub it for nitrogen.”

If local communities can control nitrogen runoff – especially from sewage and agriculture – they might give seagrass a better chance to grow, store carbon, and stay alive. This matters, not just for marine ecosystems, but for climate action too.

Seagrass may not get the attention coral reefs or rainforests do, but when it comes to carbon storage, it’s one of the most powerful tools in nature.

The full study was published in the journal Global Change Biology.

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