Mollusk shell assemblages as a historical tool for identifying unaltered seagrass beds

Seagrass meadows form structured habitats that support high biodiversity but are declining globally due to human impacts. Previous studies suggest that water quality conditions and seagrass extent in the northern Gulf Coast of Florida may have remained relatively unaltered, potentially representing a valuable baseline for intact seagrass ecosystems. Using seagrass-associated mollusks as a surrogate, we tested the ‘pristine seagrass’ hypothesis by comparing living fauna to the youngest fossil record archived in the surficial mollusk death assemblage accumulated over the last 3 millennia. Samples were acquired from 21 sites across 6 estuaries using a hierarchical sampling design to allow for live-dead comparisons at 5 observational scales: size fractions within quadrats, quadrats within sites, sites within estuaries, estuaries, and the entire study area. At all spatial scales, live and dead mollusks were concordant in compositional agreement as indicated by notably positive and statistically significant rank correlations. Sample-standardized species richness, species evenness, and spatial diversity patterns also were congruent between live and dead samples, and the pairwise Bray-Curtis similarities and non-metric multidimensional scaling ordinations suggest consistent patterns in the spatial distribution of past and present mollusk assemblages along a spatial productivity gradient. The high live-dead fidelity supports the hypothesis that seagrass meadows thriving along the north-central Gulf Coast of Florida have not been notably altered by human activities. The geohistorical lines of evidence presented here reinforce the need for continued conservation of the regional seagrass ecosystem and highlight its value as a benchmark for assessing shifts in seagrass ecosystems under more intense human stress.