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U-M research uncovers the effects of nutrients on coastal seagrasses

Seagrass possesses the capability to be one of the planet’s most efficient sponges for absorbing and storing carbon, yet we still lack understanding of how nutrient contamination influences its capacity to sequester carbon.

In a duo of investigations, U-M scholars assessed the effects of nitrogen and phosphorus on seagrass, short, turf-like vegetation thriving in shallow coastal regions. By analyzing data collected from a plot of seagrass enriched with nutrients over nine years, the researchers discovered that nutrients have the potential to boost seagrass’s carbon storage capacity. Nevertheless, in a subsequent study, they also showed that an excess of nitrogen could promote increased phytoplankton growth, which may overshadow and eliminate seagrass.

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

Jacob Allgeier, an associate professor of ecology and evolutionary biology, investigates fish and seagrass as well as coral ecosystems in the bays of The Bahamas and Dominican Republic. He observed that seagrasses in bays inundated with nutrients, primarily from human sewage, rapidly perished. Light was unable to penetrate through the phytoplankton, which also thrived under the nutrient-rich conditions.

However, in other bays that experienced nutrient runoff but lacked light-obscuring phytoplankton, he noted that seagrass flourished. Tropical coastal regions often experience nutrient scarcity. When seagrasses in these areas are exposed to nutrients, their growth accelerates, provided there aren’t excessive phytoplankton blocking sunlight, Allgeier states.

The researchers, spearheaded by recent U-M doctoral alumna Bridget Shayka, discovered that in comparatively nutrient-deficient seagrass beds, phosphorus and nitrogen promoted seagrass growth. As the seagrass flourished, its initial investment was in underground growth or root systems, storing carbon within its roots. Following that, the grass focused on above-ground structures—their blades. Consequently, the roots developed swiftly but also withered quickly, diverting more carbon into the sediment near the roots and sequestering it at an elevated rate.

“People assumed that excess nutrients were harming seagrass,” Allgeier remarked. “But our findings indicate that as long as there aren’t excessive nutrients, which would similarly enhance phytoplankton, the seagrass will merely enhance growth with extra nutrients.”

To analyze nutrient influences, Shayka and Allgeier collected samples of seagrass from test plots in The Bahamas that had been nutrient-treated for nine years. Shayka, along with a cohort of 17 undergraduates, meticulously untangled the seagrass, categorizing the grass into components: the blades that grow above water but submerged, the sheath from which the blades arise, the roots, and the rhizomes—essentially an underground stem from which additional seagrass can emerge.

They subsequently freeze-dried each segment, ground them into powder, and tested for nitrogen, phosphorus, and carbon. In addition to discovering that increased nutrients in the system elevated carbon turnover in the plants, the team also found that nutrients originating from human sources had a more significant impact on seagrasses than those provided by fish.

“The ecosystems we are studying are quite low-nutrient systems, hence adding nutrients can amplify seagrass productivity,” stated Shayka, now a program officer at the nonprofit Ocean Visions. “However, we also recognize that overdoing it with nutrient addition can significantly damage these systems. It’s one of the primary factors in their decline globally and within coastal regions.”

In the second investigation, the team examined which nutrient, nitrogen or phosphorus, exerted the most significant influence on seagrass, as well as whether the nitrogen-to-phosphorus ratio or the total quantity of each nutrient had a greater impact on the system. They also analyzed the effects of nutrients on phytoplankton.

To accomplish this, they established 21 distinct ratios of nitrogen to phosphorus and added nutrients to test plots of seagrass thriving in another section of the same bay, as well as to phytoplankton contained in bottles. They discovered that phosphorus had a more substantial positive effect on seagrass growth compared to nitrogen in the nutrient-scarce conditions.

Longstanding ecological principles suggest that the nutrient ratio is most influential in a system—but the researchers discovered that in this case, phosphorus had a greater effect on seagrass, whereas nitrogen significantly impacted phytoplankton growth. Specifically, they found that the introduction of nitrogen resulted in the phytoplankton growth rates in the bottles soaring, indicating that increasing nitrogen in the natural habitat could generate phytoplankton levels that would overshadow the seagrass.

“When you cultivate tomatoes, you don’t merely add nitrogen. You create an optimal ratio of nitrogen and phosphorus. That concept is pervasive in our society,” Allgeier noted. “However, since we assessed both the water column and the seagrass, we can assert that this model doesn’t hold true in our system.”

This insight could guide local communities in managing nutrient effects on seagrass.

“We won’t halt nutrient enrichment. It’s simply not feasible,” Allgeier acknowledged. “However, we can regulate it. And the best way to accomplish that? We filter it for nitrogen.”

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