Salt marsh plants carbon storage in a temperate Atlantic estuary illustrated by a stable isotopic analysis based approach

•Carbon concentrations were higher in the warmer seasons.•Bellowground organs had higher carbon concentration than the aboveground organs.•Isotopic analysis results were similar to Z. noltii and S. maritima.•A part of the exported carbon remains buried in the sediment.•The study species act as a sink and source of carbon simultaneously.

The biomasses, carbon standing stocks, and exportations of three saltmarsh species – Scirpus maritimus, Spartina maritima and Zostera noltii – were determined and their isotopic composition analyzed to illustrate their role in carbon storage in a temperate Atlantic estuary (Mondego, Portugal). Biomass values were higher in the warmer seasons than in the cold seasons, with carbon contents following the same trend. Carbon content ranged from 27–39% in S. maritimus and S. maritima to 30–39% for Z. noltii. S. maritimus had the highest carbon production in the aboveground organs and had similar results with S. maritima in the belowground carbon production. These three species together occupied about 50% of the salt marsh area and they stored in 21 months of study 24,000 kg of carbon in their aboveground and belowground organs. Z. noltii presented highest carbon concentration in the sediment and S. maritimus the lowest. Stable carbon isotopic analysis showed that apparently, the sedimentary organic matter is composed by a mix of terrestrial sources, macro and microalgae. Regard the high carbon exportation, S. maritima and Z. noltii are constantly accumulating carbon. The studied species have both a sink and source behaviour simultaneously.