HISTORICAL ECOLOGY OF SEAGRASS MEADOWS ALONG THE GULF COAST OF FLORIDA: BIVALVE DEATH ASSEMBLAGES POINT TOWARD LONG-TERM ECOLOGICAL STABILITY

Seagrass meadows are structured ecologically, rich habitats that support ecosystems of great economic value but are declining globally due to human impacts. The Gulf Coast of Florida is one of the largest relatively unaltered seagrass habitats (~2.2 million acres), making it a model system for acquiring baseline data to better understand and monitor seagrass meadows. This project utilizes the shells of common bivalve species to investigate the historical ecology and multi-millennia dynamics of seagrass-associated benthic mollusk assemblages.

Live and dead mollusks were bulk sampled at 31 stations across a hierarchy of observational scales: multiple samples within stations, multiple stations across estuaries. Radiocarbon dating of individual mollusk shells (n = 90) indicated that the surficial mollusk assemblages represented a multi-millennia accumulation with a median shell age of 1760 AD (i.e., roughly half of the shells predated the industrial revolution). The samples were separated into multiple size fractions (1-2mm, 2-4mm, 4-8mm, 8+mm) and identified to the finest possible taxonomic level if feasible. Length and width of each valve was measured to the nearest 0.01mm, traces of predation (drill holes) were recorded, and each specimen was categorized in terms of taphonomic grade.

The pilot results focused on the bivalves Transennella spp. and Crassostrea virginica collected from six stations, suggests that predation frequency, shell size, and taphonomic grade vary notably across stations and estuaries. There was also a strong positive correlation between predation frequency and the productivity proxies, total dissolved phosphorus and chlorophyll-a. Moreover, shell size, a proxy for body size, correlated positively with total dissolved nitrogen. These pilot results indicate that the ecological, biological, and taphonomic attributes archived within the multi-millennial mollusk accumulations correlate with the current environmental conditions of the studied meadows. The spatial congruence between present-day environmental gradients along with the ecological and taphonomic proxies derived from time-averaged shell accumulations suggest that eco-environmental patterns have remained stable over multi-centennial timescales.