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BACKGROUND Evaluating effects of global warming from rising atmospheric carbon dioxide (CO 2 ) concentrations requires resolving the processes that drive Earth’s carbon stocks and flows. Although biogeomorphic wetlands (peatlands, mangroves, salt marshes, and seagrass meadows) cover only 1% of Earth’s surface, they store 20% of the global organic ecosystem carbon. This disproportionate share is fueled by high carbon sequestration rates per unit area and effective storage capacity, which greatly exceed those of oceanic and forest ecosystems. We highlight that feedbacks between geomorphology and landscape-building wetland vegetation underlie these critical qualities and that disruption of these biogeomorphic feedbacks can switch these systems from carbon sinks into sources. ADVANCES A key advancement in understanding wetland functioning has been the recognition of the role of reciprocal organism-landform interactions, “biogeomorphic feedbacks.” Biogeomorphic feedbacks entail self-reinforcing interactions between biota and geomorphology, by which organisms—often vegetation—engineer landforms to their own benefit following a positive density-dependent relationship. Vegetation that dominates major carbon-storing wetlands generate self-facilitating feedbacks that shape the landscape and amplify carbon sequestration and storage. As a result, per unit area, wetland carbon stocks and sequestration rates greatly exceed those of terrestrial forests and oceans, ecosystems that worldwide harbor large stocks because of their largemore »areal extent. Worldwide biogeomorphic wetlands experience human-induced average annual loss rates of around 1%. We estimate that associated carbon losses amount to 0.5 Pg C per year, levels that are equivalent to 5% of the estimated overall anthropogenic carbon emissions. Because carbon emissions from degraded wetlands are often sustained for centuries until all organic matter has been decomposed, conserving and restoring biogeomorphic wetlands must be part of global climate solutions. OUTLOOK Our work highlights that biogeomorphic wetlands serve as the world’s biotic carbon hotspots, and that conservation and restoration of these hotspots offer an attractive contribution to mitigate global warming. Recent scientific findings show that restoration methods aimed at reestablishing biogeomorphic feedbacks can greatly increase establishment success and restoration yields, paving the way for large-scale restoration actions. Therefore, we argue that implementing such measures can facilitate humanity in its pursuit of targets set by the Paris Agreement and the United Nations Decade on Ecosystem Restoration. Carbon storage in biogeomorphic wetlands. Organic carbon ( A ) stocks, ( B ) densities, and ( C ) sequestration rates in the world’s major carbon-storing ecosystems. Oceans hold the largest stock, peatlands (boreal, temperate, and tropical aggregated) store the largest amount per unit area, and coastal ecosystems (mangroves, salt marshes, and seagrasses aggregated) support the highest sequestration rates. ( D and E ) Biogeomorphic feedbacks, indicated with arrows, can be classified as productivity stimulating or decomposition limiting. Productivity-stimulating feedbacks increase resource availability and thus stimulate vegetation growth and organic matter production. Although production is lower in wetlands with decomposition-limiting feedbacks, decomposition is more strongly limited, resulting in net accumulation of organic matter. (D) In fens, organic matter accumulation from vascular plants is amplified by productivity-stimulating feedbacks. Once the peat rises above the groundwater and is large enough to remain waterlogged by retaining rainwater, the resulting bog maintains being waterlogged and acidic, resulting in strong decomposition-limiting feedbacks. (E) Vegetated coastal ecosystems generate productivity-stimulating feedbacks that enhance local production and trapping of external organic matter.« less

Abstract Despite international regulation, polychlorinated biphenyls (PCBs) are routinely detected at levels threatening human and environmental health. While previous research has emphasized trophic transfer as the principle pathway for PCB accumulation, our study reveals the critical role that non-trophic interactions can play in controlling PCB bioavailability and biomagnification. In a 5-month field experiment manipulating saltmarsh macro-invertebrates, we show that suspension-feeding mussels increase concentrations of total PCBs and toxic dioxin-like coplanars by 11- and 7.5-fold in sediment and 10.5- and 9-fold in cordgrass-grazing crabs relative to no-mussel controls, but do not affect PCB bioaccumulation in algae-grazing crabs. PCB homolog composition and corroborative dietary analyses demonstrate that mussels, as ecosystem engineers, amplify sediment contamination and PCB exposure for this burrowing marsh crab through non-trophic mechanisms. We conclude that these ecosystem engineering activities and other non-trophic interactions may have cascading effects on trophic biomagnification pathways, and therefore exert strong bottom-up control on PCB biomagnification up this coastal food web.