Shells in the seagrass: Holocene mollusks as a tool for identifying unaltered habitats

Surficial accumulations of marine mollusk shells archive information about ecosystems from previous centuries and millennia and can be used not only to measure recent human impacts but also detect habitats that have remained relatively unaltered. In this case study, we applied this near-time conservation paleobiology approach to assess the status of seagrass meadows that form structured habitats along the northern Gulf coast of Florida. Previous studies suggest that seagrass habitats in the study area may have remained relatively unaltered. We tested the “pristine seagrass” hypothesis by comparing living mollusks to the surficial mollusk accumulations time-averaged over the last three millennia. Samples were collected hierarchically at six estuaries (21 sites total) and live-dead comparisons were carried out at five observational scales: (1) size fractions within quadrats, (2) quadrats within sites, (3) sites within estuaries, (4) estuaries, and (5) the entire study area. At all scales, the species rank abundances of live and dead mollusks were positively and significantly correlated suggesting concordance in faunal composition. Similarly, local species richness and species evenness were congruent when comparing live and dead samples. Non-metric Multidimensional Scaling (NMDS) ordinations and pairwise Bray-Curtis similarities indicated consistent trends in the distribution of past and present mollusk faunas along a spatial gradient in productivity. The results support the hypothesis that seagrass habitats in the study area have not been notably modified by human activities and reinforce the urgency for continued conservation of the seagrass ecosystem of the northern Gulf coast of Florida. The results also suggest that the studied seagrass system can serve as a comparative benchmark for evaluating changes in other seagrass ecosystems that have been more strongly affected by human activities.