HIGH LIVE-DEAD FIDELITY OF SEAGRASS ASSOCIATED MOLLUSK ASSEMBLAGES ALONG THE NORTHERN GULF COAST OF FLORIDA AT BOTH REGIONAL AND LOCAL SCALES

Seagrass meadows harbor biodiversity and provide ecosystem services. They are declining globally, mostly due to human impacts. Our study region in Florida is an exception. It is relatively pristine in comparison to seagrass habitats in other places, which makes it a key place to assess ecological functioning and baseline conditions for seagrass ecosystems.

We studied 31 sites across 6 estuaries in the northern gulf of Florida. Sites were bulk-sampled for seagrass-associated benthic mollusks, including both live fauna and sympatric dead assemblages. Mollusks were identified to the lowest possible taxonomic level (species when possible). The resulting data were integrated with environmental variables recorded at the sites monthly over the last ~30 years. Radiocarbon dating of mollusk shells (n=60) indicated that surficial shell assemblages have accumulated over the last three millennia, with half of the specimens predating the industrial revolution.

By comparing the fidelity (compositional agreement) of live and dead mollusks from the same sites, we can potentially measure temporal trends in mollusk associations. Changes from the historical baseline represented by mollusk shell assemblages to the modern live mollusk associations may potentially measure ecosystem shifts brought on by human impacts. As this is a relatively pristine ecosystem, we hypothesized that fidelity would be high. Our results support this hypothesis. Rank-order abundance agreement and taxonomic similarity were high when comparing live and dead mollusk assemblages. Sample standardized species richness and evenness were also comparable between live and dead mollusks. Beta diversity was similar as well. Nearby sites displayed higher fidelity in diversity and rank order abundances, and this spatial relation is similar for both live and dead mollusk assemblages.

The high fidelity between the live and dead mollusks suggests that, over the recent centuries, the seagrass meadows have remained relatively unaltered in terms of spatial structuring and faunal composition of mollusk associations. The study area may provide a useful reference point for assessing changes in other seagrass ecosystems that have been subject to more intense human impacts.