The Editor’s Choice article for Journal of Ecology’s latest issue Volume 108 Issue 3 is “Seagrass ecosystem metabolic carbon capture in response to green turtle grazing across Caribbean meadows” by Johnson et al. Associate Editor Randall Hughes explores this paper in more detail and explains what makes this paper so novel and valuable.

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Vegetated marine ecosystems such as mangroves, salt marshes and seagrasses are increasingly recognized for the ecosystem service of sequestering carbon at high rates, often referred to as “blue carbon”. Seagrasses, in particular, are among the most productive ecosystems on the planet, and much of that production can be stored belowground in low-oxygen sediments for long periods, leading to calls for seagrass conservation and restoration as a climate mitigation strategy. However, the degree of variation in seagrass carbon storage through space and time remains open question that needs to be addressed to inform policy options.

In the Caribbean region, there are vast sub-tropical and tropical seagrass meadows that provide habitat for a diverse suite of species. Although seagrass food webs are often described as detritus-based, there are key species that consume live seagrass, including green sea turtles (Chelonia mydas). In fact, the dominant tropical seagrass species in this region, Thalassia testudinum, goes by the common name of “turtle grass”. Green sea turtle populations are increasing in the Caribbean due to successful conservation efforts, and thus grazing on seagrass is expected to increase as well. The limited data available (from a single site) suggest that grazing will reduce seagrass metabolic carbon capture, potentially leading to the remineralization of stored seagrass carbon. However, it is unclear whether this response to grazing is consistent across sites and environmental conditions.

The study by Johnson et al. addresses this gap in our understanding, measuring net ecosystem production (NEP) in grazed and ungrazed Thalassia testudinum meadows at 5 locations in the Caribbean and Gulf of Mexico spanning a wide range of meadow sizes and environmental conditions to test the hypothesis that grazing consistently leads to reduced NEP. Johnson et al. used light and dark benthic incubation chambers made of PVC cylinders, ~ 6L in volume, to generate snapshot measures of metabolic carbon dynamics at each site. Metabolic rates were corrected for water column metabolism and converted to estimates of daily NEP, which indicates whether a system is a carbon sink (positive values) or source (negative values).

Measured rates of NEP were highly variable across sites, but consistently positive, indicating that both grazed and ungrazed areas acted as carbon sinks. However, NEP was also consistently lower, and sometimes dramatically so (up to 96% lower), in grazed areas relative to ungrazed areas at the same site. Meadow NEP was positively correlated with both aboveground seagrass biomass and with canopy height, suggesting that these measures can be used as proxies for metabolic carbon dynamics. In contrast, NEP was not correlated with environmental temperature or irradiance, even when corrected for differences in aboveground biomass, potentially due to the relatively uniform warm, high-light conditions of the study region.

In addition to comparing grazed and ungrazed T. testudinum meadows, Johnson et al. also provide the first measures of NEP in seagrass meadows dominated by the introduced species Halophila stipulacea, which was first reported in the Caribbean in 2002. H. stipulacea appears to spread more quickly in areas of T. testudinum that have been grazed, suggesting that grazing could have both direct and indirect effects on seagrass metabolic carbon dynamics. By sampling invaded meadows at 2 of the 5 same locations where T. testudinum was examined, Johnson et al. show that NEP values in H. stipulacea were inbetween those of grazed and ungrazed T. testudinum meadows. Despite these similarities, additional research is needed to determine whether overall carbon sequestration and storage will also be similar between the two species given the reduced allocation to below ground rhizomes in H. stipulacea.

In summary, this study provides valuable data on metabolic carbon dynamics at multiple sites across a broad geographic region, adding to our understanding of the role of seagrass in carbon sequestration. The consistently positive values of NEP, even in grazed areas, suggest that grazing will not result in a large remineralization of carbon stored in seagrass habitats. Future work on temporal variation in NEP will aid our understanding of the effects of grazing on carbon dynamics and inform potential climate mitigation strategies involving seagrass.

Randall Hughes
Associate Editor
Journal of Ecology

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You can also read a blog by the authors of this paper here: Green turtle grazing and seagrass carbon capture across caribbean meadows

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Read the full article online: “Seagrass ecosystem metabolic carbon capture in response to green turtle grazing across Caribbean meadows” by Johnson et al. (2020)