Seagrass blue carbon stocks and sequestration rates in the Colombian Caribbean

Abstract

Seagrass ecosystems rank amongst the most efficient natural carbon sinks on earth, sequestering CO2 through photosynthesis and storing organic carbon (Corg) underneath their soils for millennia and thereby, mitigating climate change. However, estimates of Corg stocks and accumulation rates in seagrass meadows (blue carbon) are restricted to few regions, and further information on spatial variability is required to derive robust global estimates. Here we studied soil Corg stocks and accumulation rates in seagrass meadows across the Colombian Caribbean. We estimated that Thalassia testudinum meadows store 241 ± 118 Mg Corg ha−1 (mean ± SD) in the top 1 m-thick soils, accumulated at rates of 122 ± 62 and 15 ± 7 g Corg m−2 year−1 over the last ~ 70 years and up to 2000 years, respectively. The tropical climate of the Caribbean Sea and associated sediment run-off, together with the relatively high primary production of T. testudinum, influencing biotic and abiotic drivers of Corg storage linked to seagrass and soil respiration rates, explains their relatively high Corg stocks and accumulation rates when compared to other meadows globally. Differences in soil Corg storage among Colombian Caribbean regions are largely linked to differences in the relative contribution of Corg sources to the soil Corg pool (seagrass, algae Halimeda tuna, mangrove and seston) and the content of soil particles < 0.016 mm binding Corg and enhancing its preservation. Despite the moderate areal extent of T. testudinum in the Colombian Caribbean (661 km2), it sequesters around 0.3 Tg CO2 year−1, which is equivalent to ~ 0.4% of CO2 emissions from fossil fuels in Colombia. This study adds data from a new region to a growing dataset on seagrass blue carbon and further explores differences in meadow Corg storage based on biotic and abiotic environmental factors, while providing the basis for the implementation of seagrass blue carbon strategies in Colombia.

Megaherbivore exclusion led to more complex seagrass canopies and increased biomass and sediment Corg pools in a tropical meadow

“…At our site in 13 months within exclosures, the relative sediment level increased up to 4.4 cm, however long-term accretion rates at this site will be subject to compaction and they remain unknown at this time. C org content (%) in the surficial sediment at our tropical study site was similar to the reported 2% C org for seagrass sediment globally (Fourqurean et al, 2012), similar to the Pacific coast of Costa Rica (Samper-Villarreal et al, 2018a;Samper-Villarreal et al, 2020a;Samper-Villarreal and Corteś, 2020;Samper-Villarreal et al, 2022), and higher than meadows also dominated by T. testudinum in the Caribbean coast of Colombia (Serrano et al, 2021); thus indicating effective C org sequestration in seagrass sediment at our site. Similar to our findings, sediment C org content did not decrease at a simulated grazing experiment in the Bahamas which maintained canopy structural complexity low over 16 months in clipped plots (Moran and Bjorndal, 2005;Moran and Bjorndal, 2007).…”

“…More complex canopies decrease water flow and increase the deposition of suspended particles, thereby increasing vertical accretion of seagrass sediment (Kennedy et al, 2010). Short-and long-term sediment accumulation rates within seagrass meadows dominated by T. testudinum in the Colombian Caribbean range between 0.04 and 0.7 cm yr -1 (Serrano et al, 2021). Meanwhile, sediment accretion over the summer from experimental eelgrass transplantation in the Wadden Sea was 0.5-0.7 cm (Bos et al, 2007).…”

“…The effects of overgrazing on seagrasses have not been studied in Central America to date (Dahl et al, 2021) and blue carbon studies in tropical seagrass meadows have been scarce (Serrano et al, 2021). In this study, we aimed to assess the effect of megaherbivore exclusion over a 13-month time period at a tropical seagrass meadow that has shown decline in structural complexity over twenty years of monitoring.…”

Biomasa de pastos marinos y su rol como sumideros de carbono en las localidades de la isla de San Andrés y La Guajira, Caribe colombiano

“…A nivel del Caribe colombiano esto cobra mayor importancia, teniendo en cuenta los datos registrados en Serrano et al (2021), los cuáles muestran una aproximación al cálculo del carbono en los sedimentos de los pastos marinos del país, destacando que el valor de los stocks en los sedimentos de La Guajira y San Andrés superan los promedios globales, alcanzando los 240 Mg C/ha. Esto, sumado a lo registrado por Guerra et al (2020) para San Andrés, donde se calculó que el valor de carbono en los sedimentos osciló entre 91 y 144 Mg C/ha, y lo hallado por Congdon et al (2017) y Gullström et al (2018), donde se evidenció que el contenido de carbono en sedimentos es más alto en zonas donde la cobertura y la biomasa tiende a ser mayor, refrenda el hecho de que la protección del sistema de biomasa aérea y subterránea repercute directamente en los reservorios de los sedimentos, y por ende en la optimización de los procesos de captura de carbono.…”

“…At level of the Colombian Caribbean, this becomes more important, taking into account the data recorded in Serrano et al (2021), which shows an approximation of the carbon values in the sediments of the country's seagrasses, highlighting that the stocks in the sediments of La Guajira and San Andrés exceed global averages, reaching 240 Mg C/ ha. This, added to the study from Guerra et al (2020) for San Andrés, where the value of carbon in the sediments ranged between 91 and 144 Mg C/ha, and the information reported by Congdon et al (2017) and Gullström et al (2018), where it was shown that the carbon content in sediments is higher in areas where coverage and biomass tend to be greater, endorses the fact that the protection of the aerial and underground biomass system has a direct impact on the reservoirs of sediments, and therefore in the optimization of carbon capture processes.…”