HISTORICAL ECOLOGY OF SEAGRASS MEADOWS ALONG THE GULF COAST OF FLORIDA: ENVIRONMENTAL TRENDS IN BODY SIZE AND PREDATION ARCHIVED IN SEAGRASS-ASSOCIATED MOLLUSK DEATH ASSEMBLAGES

Seagrass meadows are structured, highly productive habitats which support diverse ecosystem services, but are declining globally due to human-related activities. Florida’s gulf coast represents one of the largest surviving seagrass ecosystems, making the region a suitable system for establishing historical baseline conditions and assessing ongoing ecosystem changes in the context of previous centuries and millennia. The goal of the project is to evaluate the historical ecology of dominant mollusk species to improve our understanding of the long-term dynamics of Florida’s seagrass meadows.

We sampled live and dead mollusks using a hierarchical sampling approach: 4 samples per station, multiple stations per estuary, multiple estuaries (31 stations total). Radiocarbon dating indicated that mollusk shells represented multi-centennial to multi-millennial accumulations. The material was sieved, and all specimens were identified to species level (when possible). In the pilot study, dead specimens were measured in terms of body size to the nearest 0.1 mm and traces of predation (drill holes) were recorded. The pilot results suggest that predation intensity (as measured by drilling frequency) varied from 3% to 18% (Chi-Square = 15.5, p = 0.0007) between two coastal areas that differ in their salinity regime and phytoplankton production characteristics. In contrast, the body size of dominant mollusk species, estimated as median valve length, was remarkably invariant: median values varied by less than 1 mm between these two environmentally distinct systems (W=1487, p=0.28).

These pilot results indicate that mollusks from the two study areas have experienced varying predation pressures, with patterns persisting over centennial-to-millennial time scales. Conversely, the body size of the dominant species, as estimated indirectly by size frequency distributions of time-averaged death assemblages, may have been remarkably static across habitats and study areas despite differential predation pressures. Although the preliminary results reported here are still too limited to offer causative explanations, they illustrate the potential of the historical approach for studying long-term history of ecological interactions and functional traits in the environmental context of present-day seagrass habitats.