Coastal wetlands (seagrass meadows, salt marshes and mangroves) play a fundamental role in global marine carbon cycles. Although it may seem that seagrass meadows have no direct link to climate change, the carbon dioxide sequestration in these ecosystems often exceeds the estimated capacity per surface area of terrestrial forests, which is why these plants are recognized as one of the most effective carbon sinks on Earth. Specifically, it has been estimated that the sediments located under seagrass meadows retain 20% of the total carbon sequestered in marine sediments, with layers of carbon-rich sediment more than 10 m thick and 6000 years old. In addition to being considered one of the ecosystems globally with the greatest capacity to capture and retain carbon, seagrass meadows provide critical ecosystem services that contribute to climate change mitigation and ecosystem resilience in coastal environments, such as sediment stabilization that prevents coastal erosion, nutrient recycling, water quality regulation, etc. and also constituting the breeding habitat for larvae and juveniles of many species.
Despite their importance, these ecosystems are declining globally at alarming rates, with a 30% reduction in their habitat since 1980, mainly due to declining water quality, aquaculture, dredging activities and other large-scale impacts resulting from climate change. This decline not only means a cessation of carbon capture, but also reverses the role of seagrass meadows, which are changing from carbon sinks to sources of carbon that is retained in sediment.
Carlota Barañano Carrión, a graduate in Marine Sciences, studied the Zostera marina meadow on the east side of Toralla Island (Ria de Vigo) as part of her master’s thesis project, with the aim of investigating the effects of physical disturbance caused by clam harvesting using manual traction gear on the capacity of an underwater Zostera marina meadow to accumulate organic matter and sequester carbon. To achieve this objective, Carlota, under the direction of Emilio Fernández Suárez (researcher in the Biological Oceanography group of the Singular Center for Marine Research of the University of Vigo, CSIM-ECIMAT) and Gonzalo Méndez Martínez (researcher in the Strategic Environmental Assessment group of CSIM-ECIMAT), characterized the spatial distribution, abundance and density of the Zostera marina meadow and the organic matter content, density and grain size distribution of the sediment, subsequently analyzing the carbon density and the total carbon and nitrogen content of the sediment to finally compare the carbon reserves in areas exposed to different degrees of physical alteration.
The researchers observed that the meadow exposed to shellfishing activity had a lower capacity to sequester carbon, reducing the permanent content of the meadow and the preservation of carbon in the associated sediments. The physical alteration resulted in a significant reduction in shoot density (63%) and biomass (64%) in the affected area compared to the adjacent area unaffected by the disturbance, while the sedimentary carbon content was reduced by 50%.
With these results, the study concludes that shellfishing activity not only erodes the carbon accumulated over decades, but also endangers the potential contribution of the meadow in the years to come. Therefore, the authors of the study recommend incorporating into marine resource exploitation plans not only the sustainability of exploited bivalve stocks, but also the loss of seagrass goods and services that are currently compromised.
In addition to this study, these researchers, in collaboration with Jesús Souza Troncoso (Coastal Ecology group of the CSIM-ECIMAT and director of ECIMAT), addressed the study of the recolonization capacity of said meadow, by monitoring the temporal evolution of various population variables of this marine phanerogamous and the structure of the bivalve community associated with it during the closure period. This study, recently published in Marine Biology Research (DOI:10.1080/17451000.2017.1307989), concludes that the closure period established for the recovery of exploited bivalve populations allows the recovery of the meadow biomass and density levels in areas with outbreaks that survive said impact, but causes an alteration in the characteristics of the population affected by the disturbance with respect to the unaffected population.
