LIVE-DEAD FIDELITY OF MOLLUSCAN ASSEMBLAGES IN ANTHROPOGENICALLY IMPACTED SEAGRASS HABITATS: SILICICLASTIC VERSUS CARBONATE ENVIRONMENTS

Molluscan live-dead fidelity studies investigate the influences of anthropogenic activities on marine ecosystems by comparing the taxonomic composition of a living community to its corresponding death assemblage. Environments subjected to intense anthropogenic stresses tend to yield low fidelity (discordance) in rank-order abundance between living and death assemblages. This project assesses the sensitivity of the live-dead approach by applying the method to seagrass environments where anthropogenic impacts appear to be less intense and have not resulted in complete habitat degradation. Study sites include seagrass environments in siliciclastic (NC) and carbonate (FL) settings as well as within the siliciclastic-carbonate transition zone (FL). The dominant forms of human impact at these sites consist of increased freshwater runoff, sediment/terrestrial runoff, or substrate disturbance.

Based on bulk sediment samples, the rank-order abundance of genera within the living and death assemblages were compared at and among sites using Spearman’s rank correlation. Spearman’s results yield high fidelity at all localities, suggestive of little anthropogenic impact and modification to the environments. However, taxonomic comparisons between data from this project and results from previous studies indicate that high fidelity is a false assessment for the carbonate localities, and taphonomic inertia has been overcome. In contrast, live-live and dead-dead fidelity among siliciclastic sites reveals that NC death assemblages likely represent pre-impact assemblages. Additionally, siliciclastic sites yield slightly lower live-dead fidelity than the carbonate and transition sites, indicating that the siliciclastic living assemblages may represent ecological responses to more recent environmental changes. Differing fidelity values may also be influenced by sedimentological differences between siliciclastic and carbonate regimes.

Overall, high fidelity at all study sites despite observable and inferable evidence for anthropogenic impacts suggests that the live-dead approach may not be sensitive enough to detect ecological changes if the environment and resident faunal assemblages have not been dramatically altered by anthropogenic impacts or if taphonomic inertia has been overcome.