A review of the historic and present ecological role of aquatic and shoreline wood, from forest to deep sea.

The ecology of forests, their losses, and terrestrial wood decomposition dynamics have been intensively studied and reviewed. In the aquatic realm, reviews have concentrated on large wood (LW) in rivers and the transition from freshwater to marine environments in the Pacific Northwest of North America. However, a comprehensive global synthesis of LW dynamics, including decomposition processes and human influences across the freshwater-marine continuum, is lacking. Here, we review the role of LW and its fate across the entire freshwater-to-marine gradient and synthesise our findings in an integrative conceptual overview. LW has been exported by rivers to sea for hundreds of millions of years. During this journey, LW acts as an ecosystem engineer by modifying its environment and the landscape. In rivers, LW alters hydrodynamics, resulting in sediment retention and changes to riverbed and shoreline morphology. Along coastlines, driftwood initiates dunes, prevents erosion, retains moisture, and provides lignocellulose-based nutrients. Important habitats provided by floating rafts and sunken 'islands' of wood are found across estuarine, shelf and open/deep seas. Wood degradation gradually transitions from mechanical to biomechanical and chemotrophic. In rivers, degradation is primarily mechanical due to abrasion and impacts. In estuaries, salinity increases, allowing marine wood borers to begin biomechanical degradation; their activity remains the main degradation cause across marine environments. On the seafloor, chemotrophic micro-organisms finalise decomposition of small fragments. LW accumulations act as biodiversity hotspots across the freshwater-to-marine gradient. River communities rely on induced abiotic changes such as meanders, pools, and riffles, while log jams and dams serve as velocity and predation shelters, and create pools with cooler, deeper water. The wood itself acts as attachment substrate for eggs and larvae. From estuaries seaward, the focus fully shifts to LW itself: driftwood provides lignocellulose for wood-boring organisms and stable substrate for sessile animals and macroalgae. In shelf seas and open oceans, floating LW rafts provide shade, shelter, and attachment substrate. Humans have greatly decreased export of LW from river to sea by clearing forests for agriculture and urbanisation, damming rivers, and removing LW 'debris' that is often deemed a hazard or nuisance in developed areas. Indeed, the annual export of LW >3 m long to marine environments has decreased by 5,000,000 m