Saltmarshes are one of the most significant blue carbon sinks but there is a paucity of information regarding saltmarsh carbon stocks globally, consequently these habitats are not included in the global carbon budget. The aim of this study is therefore to better understand the spatial and temporal variation of saltmarsh sediment total organic carbon (TOC) content. Therefore, three saltmarshes along the salinity gradient of the Colne Estuary were studied. The effect of the study sites’ locations along the estuary, higher plant species distribution, above-ground biomass and aerobic respiration on TOC content was investigated. The spatial and temporal variation of sediment TOC content was investigated by monthly sampling from two habitats and three zones at each study site. There was a significant spatial variation in plant species distribution which could be due to zonation and the location of the sites along the estuary. Saltmarsh plants were the important driver of spatial and temporal variations in sediment TOC content. The sediment TOC content at the study sites in the lower (Colne Point) and the mid (Brightlingsea) estuary was significantly higher than the upper estuary (Wivenhoe) (P < 0.001, range: 9-25 Kg C m-2). The range of sediment TOC content of the studied saltmarshes was between 88% - 290% higher than other UK studied saltmarshes and between 4% -169% higher than the majority of the studied marshes in the Northern Hemisphere. Therefore, if the sediment carbon content of the similar saltmarshes to the Colne estuary were taken into account it would suggest that the UK and global saltmarsh sediment TOC estimate would increase. It will take possibly about 100 years for the realignment saltmarshes at Essex to reach the carbon storage capacity of Colne Point. Therefore, the Colne Point natural saltmarsh is a very significant carbon reservoir that has been overlooked.