Demographic and morphological responses of intertidal eelgrass (Zostera marina) transplanted across sediment conditions

Eelgrass (Zostera marina) shows substantial spatiotemporal variation in morphological and demographic traits that can arise from phenotypic plasticity. These traits commonly shift across light and depth gradients, as well as seasonally, but less consistency has been demonstrated across sediment conditions. We conducted two field transplant experiments in which seedlings or overwintering shoots of Z. marina were moved among sand and muddy-sand sites within a bay (Willapa Bay, Washington, USA). Shoots were regularly re-marked and measured for size, asexual, and sexual reproduction for 1–1.5 year, and upon collection, for rhizome extension and root investment. Seedlings, which were sourced from one site, had poor survival in muddy-sand, but smaller size and more branching with less flowering in sand. Shape also exhibited phenotypic plasticity, with relatively wide leaves and longer rhizome internodes at the sandier site. Overwintering shoots, which were transplanted among four sites in a fully reciprocal design, had lower survival and size after 6 months at sand compared to muddy-sand sites, but with greater root investment at collection. Shoots from one muddy-sand site, regardless of outplant site, maintained relatively short internodes over 1.5 years. Overall, shoots tended to remodel in summer as a function of sediment type, while internode extension was phenotypically plastic in both experiments, as well as fixed by source. Because sediment correlated with water movement, eelgrass traits expressed in sand could reduce drag (smaller above-ground size) and provide anchorage (root investment). Shorter internodes at muddy-sand outplant sites and from one muddy-sand source site could influence space-filling and foraging strategies. Across experiments, poor survival of seedlings relative to vegetative shoots in muddy-sand may reflect low-oxygen conditions that are challenging for early life stages of eelgrass to overcome. The substantial trait variation in transplants across sediment type could be a response to below-ground chemistry and above-ground hydrodynamics, but these drivers were not distinguishable with the study design.