Plasticity and Adaptation of Northern California Eelgrass in Response to Sediment Conditions

Abstract Considerable research describes the interactions between seagrasses and their sedimentary environment, but there is little information on how populations differ in their innate versus plastic responses to these differences. Here, we test whether sediment contributes to eelgrass population differentiation and the nature of plastic responses to different sediment environments. We do this via a 15-week, fully crossed common garden experiment with two populations and their native sediment types. Plants from the warmer-temperature, clay-dominated site (90% silt + clay, 10% sand) consistently maintained greater biomass than plants from the cooler, sand-dominated site (60% sand, 40% silt + clay). Plants from both populations were highly plastic for root length and clonal shoot size, with both increasing when planted in clay-dominated compared to sand-dominated sediment. Plants from the clay-dominated site grew longer rhizomes in foreign sediment while plants from the sand-dominated site had no change in this plant trait, indicating some measure of home site advantage with respect to sediment conditions. Porewater sulfide also exhibited this pattern where concentrations were very low in clay-dominated sediment for all plants, but in the sand-dominated treatment, only plants native to sand-dominated sediment maintained porewater sulfide concentrations below toxic levels. These patterns may be mediated by microbiome differences between populations as roots from plants native to clay-dominated sediment had more fixed microbiomes between treatments compared to plants native to sand-dominated sediment. These results support that sediment type partially mediates home site advantage in eelgrass populations and suggest differential population responses may be mediated by the associated microbiome.