Green vs. Blue: How Green Turtles Might Limit Blue Carbon Storage

Paper: Blue carbon stores in tropical seagrass meadows maintained under green turtle grazing, Johnson, R., Gulick, A., Bolten, A., Bjorndal, K., Nature Scientific Reports, 7, 2017,  doi: 10.1038/s41598-017-13142-4


Welcome back, turtles!

Recent conservation efforts have increased the global green turtle population but they are still classified as endangered. They have been overexploited by humans through “overharvesting of their eggs, hunting adults, being caught in fishing gear and loss of nesting beach sites” (World Wildlife Fund website, 2017).

Figure 1: Green turtle swimming off the coast of Egypt (Source: Flickr, Kris-Mikael Krister)

Green turtles live primarily in subtropical and tropical waters (Figure 1), where they eat seagrass as a large part of their diet. Typically they eat the seagrass blades down to the sediment surface and then will eat the new growth as it appears. Green turtles eat in “plots” where they pick an area and eat it all but they leave an untouched plot next to it (Figure 2).

Are you gonna eat that?

Recently, there has been concern in the scientific community that the increasing green turtle population will lead to a loss in blue carbon storage by seagrass meadows. Blue carbon storage is carbon buried by the marine vegetation system. Seagrass can store a lot of carbon because of the high production rate and subsequent burial of organic matter in the sediment. This sediment storage is an anoxic (no oxygen) environment, which allows the carbon to be stored for hundreds of years. Therefore, seagrass blue carbon storage could help mitigate the impacts of climate change. Plants take up carbon dioxide and a portion of carbon dioxide is eventually stored in the soil. Seagrass meadows store carbon dioxide deep in the sediment removing it from the atmosphere.

One current hypothesis is that it is impossible to increase the green turtle population and maintain seagrass meadows as blue carbon storage systems. Grazing reduces the photosynthetic capacity of the meadow.  In other words, the idea is that if conservation work continues to increase the green turtle population there will be a negative impact on the extent of blue carbon sequestration in seagrass meadows. If the green turtle population gets large they may resort to digging up seagrass in the sediment which will release the stored carbon back into the atmosphere.

Useful Definitions

This study focuses on the seagrass ecosystem metabolism. Ecosystem metabolism is the total production and respiration for an ecosystem (how much food/energy was made and consumed). Researchers studied different pieces of this ecosystem. They measured gross primary production which is the total amount of energy produced by vegetation (aka photosynthesis). This was compared to the respiration of the ecosystem which is a measure of the energy consumed for growth (e.g. green turtles eating and breathing). Finally, the difference between the gross primary production and the respiration is the net ecosystem production. Net ecosystem production is the remaining energy produced through primary production available after accounting for all respiration.

The study

Researchers clipped seagrass in a seagrass meadow off the Cayman Islands to simulate green turtle grazing for 12 weeks. They had an untouched plot next to this that served as a reference point for the clipping experiment.  They measured gross primary production, the respiration of the ecosystem, and the net ecosystem production to understand the overall ecosystem metabolism.  They measured the same things at a nearby meadow that had an ungrazed plot and a naturally grazed area that had been actively grazed on for at least a year by juvenile turtles next to each other (Figure 2).  These three would serve as comparisons of maximum blue carbon storage (natural ungrazed),  short term grazing (clippings plot), and long term grazing (naturally grazed) (the good, the bad, and the ugly).

Clipped vs. Natural

Researchers found that the ecosystem metabolism (total energy made and used in the system) was lower in the clipped plot compared to the reference plot.  The gross primary production was reduced by 77% and the respiration of the ecosystem was reduced by 75%. The net ecosystem production was reduced by 70%. With such a large decrease in the net ecosystem production researchers expected the seagrass wouldn’t be able to store carbon.  That remaining unused photosynthetic energy is the portion of carbon that can be stored in the sediment.  However, the clipped plot was still a net carbon sink!

Figure 2: A naturally grazed green turtle feeding plot (right) next to an ungrazed plot (left) off the coast of the Cayman Islands (Source: Gulick et al., 2017)

The net ecosystem production was reduced by 92% (wow!) in the naturally grazed area. Researchers found that the ecosystem metabolism was similar between the clipped and naturally grazed plots. This was surprising because the seagrass blades of the naturally grazed area were shorter and narrower meaning there was less surface area available for photosynthesis. It was assumed that the smaller blades wouldn’t be able to take up as much carbon dioxide as the wider clipped blades but both systems were similar. Surprise! It doesn’t really matter how big the blades are!

Production/Respiration Ratio

Researchers found that there was still a high production to respiration ratio in both the clipped and naturally grazed areas. This ratio indicates if an area if autotrophic (production is greater than respiration aka primary production dominates) or heterotrophic (production is less than respiration, aka primary production can’t keep up with consumption).  This is important because an autotrophic environment should stay oxic (oxygenated) which allows for more carbon sequestration If the ecosystem became heterotrophic the seagrass meadow would not stay as a carbon storage system. So a high production/respiration ratio means that the seagrass meadow was still a carbon storage system.

Moving Forward

We don’t really know what the “natural” relationship is between seagrass meadows and turtles because the turtle population was so low for a very long time. As conservation efforts succeed and populations of green turtles increase, they will graze more. We need to understand how seagrass carbon storage will change with this increased grazing since seagrass meadows are currently considered a large blue carbon storage system. These researchers found that the meadow can’t take up as much carbon when heavily grazed but they seagrass meadow will remain a blue carbon storage system. Naturally, green turtles and seagrass meadows could maintain a balance of consumption and carbon storage. However, we know that humans have impacted, and will continue to impact, this ecosystem and we don’t know what will happen in the future. Based on this study we don’t have to pick sides in the green vs. blue battle–yet.

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