From distant shores to an emerging island: the role of surface and subsurface transport in modeling seed dispersal

Long‐distance seed dispersal by ocean currents plays a critical role in structuring coastal plant communities. As direct observations of this phenomenon are infeasible, numerical modeling is a valuable tool. Here, we use Lagrangian particle tracking to examine the colonization of the newly forming island Norderoogsand (German Bight, Wadden Sea) by five rare coastal plant species via ocean‐mediated seed transport. Using backward‐in‐time particle tracking driven by velocity fields from a numerical ocean model for the years 2012 to 2017, we compare seed dispersal pathways, travel times, and the connectivity of Norderoogsand with potential source regions along the German and Dutch coasts. In contrast to most modeling studies, which assume that seed transport occurs only at the sea surface, we include simulations driven by depth‐averaged currents, as recent experimental studies have shown that with time, the buoyancy of seeds and other propagules decreases, which could lead to subsurface transport. Depending on the carrying flow field (surface or depth‐averaged velocities), our simulations suggest significantly different sources. For one rare species, Euphorbia paralias , only the simulations driven by depth‐averaged currents reproduce the hypothesized source in the Netherlands, whereas surface‐only transport fails to connect Norderoogsand with any known occurrences of the species. Across all species, transport with depth‐averaged currents yields shorter median travel times, which implies less exposure to saltwater. Our results highlight that subsurface transport can substantially alter connectivity estimates and, for some species, is essential to explain observed colonization patterns. We conclude that future models of ocean‐mediated seed transport should incorporate seed buoyancy dynamics and vertical mixing, and that empirical studies on buoyancy decay and viability of seeds in saltwater are needed to refine predictions of range expansion under environmental change.