Green turtle grazing and seagrass carbon capture across Caribbean meadows

Journal of Ecology recently published an exciting new research paper by Johnson et al.Seagrass ecosystem metabolic carbon capture in response to green turtle grazing across Caribbean meadows.”

Author Robert Johnson discusses this research in more detail and presents further insights into how green turtle grazing affects carbon dynamics, within seagrass ecosystems.

Green turtle abundance is increasing in many areas as a result of successful conservation efforts. As marine mega-herbivores that consume seagrass for a large part of their diet, an increase in green turtles will lead to an increase in grazed seagrass meadows – a naturally co-evolved system. However, protecting seagrass meadows is also a conservation priority, as they provide many beneficial ecosystem functions and services, such as high rates of carbon capture and storage. 

A seagrass meadow grazed by green turtles (left) and adjacent ungrazed seagrass (right) at a sampling site in Eleuthera, The Bahamas. (Photo: Robert A. Johnson)

Currently, it is not well understood how green turtle grazing affects carbon dynamics within seagrass ecosystems or if effects are similar across meadows. To work towards future management practices that will provide the greatest benefit for both seagrasses and green turtles, we need a better understanding of how grazing affects seagrass ecosystem function (e.g. carbon capture) across a range of meadows under differing conditions (for example, different environmental conditions or differing seagrass species composition).

In this study we measured differences in metabolic carbon capture between grazed and ungrazed areas of Thalassia testudinum dominated seagrass meadows, in five locations across the Greater Caribbean and Gulf of Mexico. All locations had actively grazing green turtle populations, and locations were specifically chosen so that meadows differed in biotic (e.g. aboveground seagrass biomass) and abiotic (e.g. light and depth) characteristics among sites. We used incubation chambers to measure net ecosystem production in the meadows and estimate carbon capture.

Collecting a sample from an incubation chamber in an area of ungrazed seagrass in Eleuthera, The Bahamas. (Photo: Nerine Constant)

We found that there is a consistent response in seagrass carbon capture to green turtle grazing among meadows across the Greater Caribbean. Areas of meadows grazed by green turtles consistently exhibit lower metabolic carbon capture than ungrazed seagrass. This is due to lower photosynthetic seagrass biomass in grazed areas. We found that aboveground biomass was a strong predictor of net ecosystem production across all meadows (grazed and ungrazed areas). However, grazing did not stimulate heterotrophic respiration (e.g. from microbes in the sediment), and carbon stored in the substrate of these meadows is therefore not lost as a result of grazing.

Net ecosystem production measured in grazed (open) and ungrazed (closed) areas of seagrass from meadows across the Greater Caribbean. Site information is given in the accompanying publication. (Locations L to R: Bonaire; St. Croix; Little Cayman; Eleuthera; Florida)

The results of this study add important information to our understanding of how green turtle grazing affects seagrass ecosystem function and how carbon capture dynamics may be expected to respond to a future increase in grazing. In areas of the Greater Caribbean where green turtle abundance is increasing, but where detailed information on local seagrass carbon dynamics is lacking, it may be expected that grazing will result in lower rates of carbon capture. However, increased grazing will not likely stimulate a metabolic loss of carbon that is already stored in the meadow.

Robert Johnson Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Iowa, USA

Read the full paper onlineSeagrass ecosystem metabolic carbon capture in response to green turtle grazing across Caribbean meadows

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