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Seascape connectivity increases carbon and nitrogen exchange across coastal ecosystems through flow of particulate organic matter (POM). However, there are still critical gaps in knowledge about the drivers that mediate these processes, especially at regional seascape scales. The aim of this study was to associate three seascape-level drivers which could influence carbon and nitrogen stocks in intertidal coastal seascape: connectivity between ecosystems, ecosystem surface area, and standing vegetation biomass of ecosystems. Firstly, we compared whether connected mangrove and seagrass ecosystems contain larger carbon and nitrogen storage than isolated mangrove and seagrass ecosystems. Secondly, we compared autochthonous and allochthonous POM in mangrove patches and seagrass beds, simultaneously estimating the area and biomass relative contribution to POM of the different coastal vegetated ecosystem. Connected vs isolated mangrove and seagrass ecosystems were studied at six locations in a temperate seascape, and their carbon and nitrogen content in the standing vegetation biomass and sediments were measured. POM contributions of these and surrounding ecosystems were determined using stable isotopic tracers. In connected mangrove-seagrass seascapes, mangroves occupied 3 % of total coastal ecosystem surface area, however, their standing biomass carbon content and nitrogen per unit area was 9-12 times higher than seagrasses and twice as high as macroalgal beds (both in connected and isolated seascapes). Additionally in connected mangrove-seagrass seascapes, the largest contributors to POM were mangroves (10-50 %) and macroalgal beds (20-50 %). In isolated seagrasses, seagrass (37-77 %) and macroalgal thalli (9-43 %) contributed the most, whilst in the isolated mangrove, salt marshes were the main contributor (17-47 %). Seagrass connectivity enhances mangrove carbon sequestration per unit area, whilst internal attributes enhance seagrass carbon sequestration. Mangroves and macroalgal beds are potential critical contributors of nitrogen and carbon to other ecosystems. Considering all ecosystems as a continuing system with seascape-level connectivity will support management and improve knowledge of critical ecosystem services., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Daniel Saavedra Hortua, Lucy Gwen Gillis reports financial support was provided by German Research Foundation., (Copyright © 2023. Published by Elsevier B.V.)

Tropical forests fix large quantities of carbon from the atmosphere every year; however, the fate of this carbon as it travels through ecosystem compartments is poorly understood. In particular, there is a large degree of uncertainty regarding the time carbon spends in an ecosystem before it is respired and returns to the atmosphere as CO2.We estimated the fate of carbon (trajectory of photosynthetically fixed carbon through a network of compartments) and its transit time (time it takes carbon to pass through the entire ecosystem, from fixation to respiration) for an old‐growth tropical forest located in the foothills of the Andes of Colombia.We show that on average, 50% of the carbon fixed at any given time is respired in <0.5 years, and 95% is respired in <69 years. The transit time distribution shows that carbon in ecosystems is respired on a range of time‐scales that span decades, but fast metabolic processes in vegetation dominate the return of carbon to the atmosphere.Synthesis. The transit time distribution integrates multiple ecosystem processes occurring at a wide range of time‐scales. It reconciles measurements of the age of respired CO2 with estimates of mean residence time in woody biomass, and provides a new approach to interpret other ecosystem level metrics such as the ratio of net primary production to gross primary production. [ABSTRACT FROM AUTHOR]