Functional traits are moderate predictors of above‐ and below‐ground biomass in multispecies seagrass habitats

Abstract Seagrasses are important ecosystem engineers that maintain biodiversity and modify the abiotic and biotic environment. At present, we are lacking a wider understanding of the functional traits that predict seagrass biomass stock, whether trait‐biomass associations vary across multispecies seagrass habitats, and which biodiversity mechanisms explain variation in ecosystem functions in seagrass ecosystems. To explore which traits predict biomass, we conducted a field survey along the 1500 km coastline of Western Australia, where species‐rich seagrass meadows are common. We sampled multispecies meadows at 14 sites in coastal embayments or estuarine habitats and measured seven morphological and biochemical traits of multiple species. Our aim was to explore the functional structure of seagrass communities in coastal embayments and estuaries and investigate how various components of diversity (species richness, community‐weighted mean traits (CWM) and functional dispersion (FDis)) predict above‐ and below‐ground biomass in multispecies seagrass habitats by using piecewise structural equation modelling. Trait‐biomass associations ranged from strong (standardized path coefficient 0.5) to weak (<0.2). More traits predicted below‐ground than above‐ground biomass, and the total explained variance was higher when conducting separate analyses for coastal embayments compared to including both seagrass habitats. Site‐level variation accounted for the largest part of the explained variation in biomass stock, as the overall explanatory power of traits to biomass was low ( r 2 < 0.3). For individual traits, mass ratio effects (CWM) primarily predicted biomass in both coastal embayments and estuaries, and species were functionally similar (low FDis). Our study concludes that functional traits act as moderate predictors of biomass stock across multispecies temperate seagrass habitats, but environmental context is of more importance. Our results further demonstrate that the main biodiversity mechanism driving biomass allocation in multispecies seagrass communities is through dominance rather than complementarity, and co‐existing species show similarity in their functional traits. The predictive strength of individual traits to biomass varied between different seagrass habitats, indicating context dependency in trait‐biomass associations. More research is needed to understand how patterns in functional diversity are regulated by the environment and how such patterns relate to other ecosystem properties and services sustained by these important ecosystems. Read the free Plain Language Summary for this article on the Journal blog.