This chapter discusses the fate of allochthonous particulate organic carbon (C) (POC), either sorbed organic C (OC) onto clay particles, as detrital organic particles, and dissolved OC (DOC) in modern coastal blue C ecosystem (BCE). BCEs, or the C stored in coastal systems such as mangroves, salt marshes, and seagrass meadows, form some of the largest global C reserves, and are distributed throughout numerous latitudinal zones. Coastal tidal marshes are largely found in temperate zones but do extend into the tropics, mangroves are confined to tropical and sub-tropical regions due to their freeze intolerance, and seagrasses are broadly distributed from cold polar waters to the tropics. The fate of allochthonous blue C released into coastal waters and the open sea or sequestered in the coastal BCEs is highly specific to the coastal margin setting. Coastal aqueous POC and DOC forms constitute the major pool of organic matter providing C and energy to heterotrophs in estuaries.