Plasticity and adaptation of northern California eelgrass in response to sediment conditions

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.