Seagrass as a carbon sponge?

(Press-News.org) ANN ARBOR—Seagrass has the potential to be one of the world's most effective sponges at soaking up and storing carbon, but we don't yet know how nutrient pollution affects its ability to sequester carbon.

In a pair of studies, U-M researchers evaluated the impact of nitrogen and phosphorus on seagrass, short, turf-like grasses that live in shallow, coastal areas. Examining data gathered from a plot of seagrass enriched with nutrients over a period of nine years, the scientists found that nutrients can increase seagrass's ability to store carbon. However, in a second study, they also found that an overload of nitrogen could lead to increased phytoplankton growth, which can shade out and kill seagrass. 

Both studies, published in Global Change Biology and Conservation Letters, respectively, were supported by the National Science Foundation and the David and Lucille Packard Foundation.

Jacob Allgeier, associate professor of ecology and evolutionary biology, studies fish and seagrass and coral ecology in bays of the Bahamas and Dominican Republic. He noticed that seagrasses growing in bays overloaded with nutrients, mostly from human wastewater, quickly died off. Light couldn't penetrate through the phytoplankton, which also proliferated under the high-nutrient conditions.

However, in other bays that experienced nutrient runoff but didn’t have light-shading phytoplankton, he saw that seagrass grew well. Coastal tropical areas are often nutrient-starved. When the seagrasses growing there are bathed in nutrients, their growth takes off as long as there is not too much phytoplankton blocking out light, Allgeier says.

The researchers, led by recent U-M doctoral graduate Bridget Shayka, found that in relatively nutrient-poor beds of seagrass, phosphorus and nitrogen helped seagrass grow. As the seagrass grew, it first invested in its underground growth, or root systems, storing carbon in its roots. Then, the grass invested in above ground structures—their blades. The end result was that the roots grew quickly but also died quickly, which shunted extra carbon into the sediment surrounding the roots and sequestered it at a higher rate.

"People thought excess nutrients were killing seagrass," Allgeier said. "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." 

To study nutrient impacts, Shayka and Allgeier took samples of the seagrass from test plots in the Bahamas that had been treated with nutrients for nine years. Shayka and a raft of 17 undergraduates painstakingly untangled the seagrass, separating the grass into parts: the blades that grow above ground but under water, the sheath from which the blades emerge, the seagrass's roots and the seagrass's rhizomes—basically an underground stem from which other seagrass plants can grow.

They then freeze-dried each part, pulverized them and tested them for nitrogen, phosphorus and carbon. In addition to finding that increasing nutrients in the system increased carbon turnover in the plants, the researchers also found that nutrients supplied by human sources had a greater impact on the seagrasses than those supplied by fish. 

"The systems we study are pretty low-nutrient systems, so adding nutrients can increase seagrass production," said Shayka, now a program officer for the nonprofit 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."

In the second study, the researchers tested which nutrient, nitrogen or phosphorus, had the greatest impact on seagrass, as well as whether the ratio of nitrogen to phosphorus or the total amount of each nutrient had the greatest effect on the system. They also examined nutrient impacts on phytoplankton.

To do this, they created 21 different ratios of nitrogen to phosphorus and added nutrients to test plots of seagrass growing in a different part of the same bay as well as to phytoplankton in bottles. They found that phosphorus had a bigger positive effect on seagrass growth than nitrogen in the nutrient-poor conditions. 

Longstanding ecological theory suggests that the ratio of nutrients has the greatest impact on a system—but the researchers found that in this particular case, phosphorus had a greater effect on seagrass while nitrogen had a greater effect on phytoplankton growth. In particular, the researchers found that the addition of nitrogen caused the rates of phytoplankton in the bottles to skyrocket, showing that increasing nitrogen in the natural landscape could lead to levels of phytoplankton that would shade out the seagrass.

"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," Allgeier said. "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."

This finding could inform how local communities control for nutrient impacts to seagrass.

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

 

Study 1: Nutrient Enrichment Increases Blue Carbon Potential of Subtropical Seagrass Beds

Study 2: A New Conceptual Model of Tropical Seagrass Eutrophication: Evidence for Single Nutrient Management 

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