NOT ALL SEAGRASS MEADOWS ARE DECLINING: HIGH LIVE-DEAD FIDELITY OF SEAGRASS-ASSOCIATED MOLLUSK ASSEMBLAGES ALONG THE NORTHERN GULF COAST OF FLORIDA

Northern Florida’s gulf coast harbors one of the largest remaining seagrass habitats in the United States. Seagrass meadows are biodiversity hotspots that support many economically important organisms. In many areas of the world seagrass habitats are disappearing rapidly due to direct and indirect human impacts. However, the study region is relatively pristine compared to other coastal areas. This makes the region a potentially important study area to establish baseline conditions for seagrass habitats.

We assessed the ecological state of seagrass-associated benthic assemblages by comparing live mollusk assemblages and sympatric dead mollusk accumulations. The goal of this live-dead fidelity analysis is to assess if the current seagrass-associated mollusk assemblages differ notably in diversity and faunal composition when compared to a historical baseline represented by dead shell assemblages accumulated over multiple millennia. By comparing the dead subfossil populations with the extant populations of species in the same area, we can potentially detect faunal shifts representing tentative evidence that human-induced changes may have occurred. Due to the relatively pristine state of the region, we predicted live-dead fidelity (i.e., high compositional congruence between live and dead mollusk assemblages).

Live and dead mollusks were sampled along the northern gulf coast of Florida. Samples were collected at multiple stations, dead and live specimens were sieved into different size fractions, and all individual mollusks were identified to the lowest possible taxonomic level. A total of 17 live-dead stations were used in this study.

We found that rank order abundance agreement and taxonomic similarity were high when comparing live and dead mollusk assemblages (mean Spearman Rho = 0.58, sample adjusted Spearman Rho = 0.82; mean Chao = 0.78 n =17). Sample standardized species richness and evenness were also comparable between live and dead assemblages. The high live-dead fidelity of these seagrass habitats suggests that they are relatively unaltered by human impacts and so represent areas that should be a focus for conservation and protection. They can also serve as a model system for studying ecological functioning of unaltered seagrass habitats.