Back to the Future: Seagrass Meadow History Predicts Contemporary Carbon Stocks

Bijak estimating seagrass cover. Image courtesy of Alexandra Bijak.

Alexandra Bijak is a Coastal and Ocean Graduate Fellow and a Ph.D. candidate in the Soil, Water, and Ecosystems Department at the University of Florida. 

Anytime you go to the doctor, you’re usually asked about past conditions, surgeries, and vaccinations you’ve had—your medical history—because this information can provide your doctor with context for diagnosing and treating any symptoms you may have today. In natural landscapes, ecological histories can be useful in understanding the present condition of ecosystems and provide guidance on how to remedy degraded ecosystems or manage them for specific ecosystem services and benefits provided to humans by the natural environment.

Under the guidance of my PhD advisors, Dr. Laura Reynolds and Dr. Ashley Smyth, I set out to test whether ecological history is important to present-day ecological functions in seagrass meadows. Seagrass meadows are submerged marine plant communities that look like lush underwater prairies teeming with invertebrates and fish. In our recent study, we found that historical seagrass presence is a good indicator of contemporary carbon storage in a seagrass meadow along Florida’s Nature Coast.

Carbon storage in the marine environment, also known as blue carbon, is a valuable ecosystem function because carbon buried in oxygen-poor sediments can remain buried and largely unchanged for a long time. When carbon is buried, it can be like hitting a pause button on the carbon cycle: carbon is ‘locked away’ rather than being consumed by microbes and then eventually released as carbon dioxide to the atmosphere, where rising carbon dioxide concentrations are driving global climate change. Seagrasses store so much carbon globally that researchers, restoration practitioners, and policymakers are working to monetize the carbon stored in seagrass sediments in the form of carbon offset programs. Understanding how and why seagrass carbon storage differs across the world, regions, or even within meadows, is necessary to successfully implement such strategies.

Bijak sampling seagrass biomass with a metal corer. Image courtesy of Alexandra Bijak.

Seagrass meadows store large amounts of carbon in their underlying sediments by slowly accumulating small particles of decaying seagrass leaf and root tissue and other organic material at the sediment surface over hundreds to thousands of years. An extensive root network of seagrasses helps stabilize sediments so that accumulated carbon is retained within the sediments and not released to the water column. There are some physical and geographical factors, like wave energy, water depth, and proximity to rivers, that largely control how much carbon is delivered to and retained within a seagrass meadow, but site-specific properties, like seagrass shoot (stem) density, determine how efficiently the meadow canopy traps carbon given these constraints. Since carbon accumulation is such a slow process that occurs over many years, we expected historical rather than contemporary meadow cover to best predict the amount of carbon found in underlying sediments.

We were therefore unsurprised to find a strong relationship between meadow stability, defined as consistent seagrass cover over time, and carbon storage. In other words, in places where seagrasses have been observed year after year, there’s more likely to be higher amounts of carbon stored within those sediments.

Our results indicate undisturbed, stable meadows store the highest amount of carbon, and other researchers have found that disturbances that cause seagrass die-off, like heat waves and disease, can diminish carbon stocks.

Shoal grass are thinner seagrass species that store less carbon, but live under high energy conditions. Image courtesy of Alexandra Bijak.

We leveraged both historical and newly collected data to study past and present drivers of carbon storage in seagrass meadows near Cedar Key. The Florida Department of Environmental Protection provided us with long-term annual seagrass monitoring datasets spanning 2006 to 2019 so that we could generate historical predictors, like historical cover and changes in cover over time. We visited the monitoring stations in 2020 to measure surface sediment carbon stocks and contemporary predictors, such as seagrass biomass. We focused on carbon within the surface (0-10 cm depth) sediment because we assume the top sediment layer accumulated over the past few decades and was therefore influenced by seagrass cover recorded in the monitoring datasets.

Our major findings that sustained seagrass presence over time promotes carbon accumulation and storage, and that changing levels of seagrass presence over time has the opposite effect and reduces carbon storage, are in line with expectations: seagrasses need to be continuously present to maximize the benefit of trapping carbon within seagrass sediments. We also found that the most stable seagrass meadows were dominated by turtlegrass (Thalassia testudinum), a large, long-lived seagrass species typically found in climax, or ecologically mature, communities.

Turtlegrass have wider blades compared to other seagrass species, and are resistant to disturbances. Image courtesy of Alexandra Bijak.

Turtlegrass better resists disturbances like hurricanes because of its large rhizomes that weave and wind throughout the sediments, so the plant is less likely to be uprooted and damaged by wave action. In this way, turtlegrass helps create stable environmental conditions that allow it to persist over time. Meadows composed of smaller species, like shoal grass (Halodule wrightii) and manateegrass (Syringodium filiforme), store less carbon but live under higher energy conditions where turtlegrass cannot. The smaller species can also modify sediment conditions by increasing nutrient levels so that turtlegrass can eventually be established. In this way, the coexistence of multiple seagrass species in seagrass meadows may be important to sustaining carbon storage over time.

Our findings are important for seagrass ecosystem management. Our results indicate undisturbed, stable meadows store the highest amount of carbon, and other researchers have found that disturbances that cause seagrass die-off, like heat waves and disease, can diminish carbon stocks. Protecting seagrass meadows from disturbances we can directly control, like propeller scarring due to boating activity, is one way to manage meadows for carbon storage.

Restoring meadows can help regain the benefits of carbon storage where meadows have been lost, but the recovery of carbon stocks can take years to decades. While turtlegrass meadows contain the most carbon, protecting other seagrass species now is important, too, because they also store carbon, even if in smaller amounts, and can facilitate the future establishment of turtlegrass. Our study also highlights the utility of long-term monitoring: the Florida Department of Environmental Protection datasets allowed us to ask research questions that we could only answer because the agency had the foresight to document changes in Florida seagrass meadows over time.