ECOSYSTEM DYNAMICS AND THE SEDIMENTARY RECORD: ASSESSMENT OF FAUNAL PATTERNS AND ENVIRONMENTAL HISTORY FROM CROSS BANK, FLORIDA BAY

Anthropogenic alteration of ecosystems is a persistent and growing problem in coastal marine settings. Nutrient pollution, initiated during a two-decade old study along Cross Bank, Florida Bay (Everglades National Park), instigated a transition in dominance of seagrass species from Thalassia testudinum to Halodule wrightii, changing the substrate for benthic animals. The objective of this research is to assess: 1) effects of changing environments on subfossil molluscan assemblages in the upper sedimentary veneer; and 2) potential for similar transitions to be detected within the deeper subfossil record. To this end, a transect of core samples, both surficial push cores (10 cm depth) and stratigraphic piston cores (~100 cm depth), were collected from nutrient enriched environments and corresponding unaltered settings to investigate the effect of this environmental change, and perhaps earlier transitions, on death assemblages.

Comparison of surficial cores from altered and unaltered environments on Cross Bank demonstrates that molluscan death assemblages responded compositionally to change in seagrass community dominance. In particular, Cerithium muscarum, a grazing gastropod, is highly abundant at Thalassia-dominated, unaltered sites but occur at greatly reduced abundances at Halodule-dominated, altered sites. Further analyses have shown an association between size attained by Cerithium and differences in blade width of the two seagrass species, with snails in Thalassia-dominated settings attaining greater size, suggesting a size threshold.

Here I present an assessment of the subfossil record with respect to ecosystem history and environmental transitions, as recorded in molluscan assemblages, using the results of push core analyses and the data set of Turney and Perkins (1972), a snapshot of subfossil occurrence and abundance from across Florida Bay during the early 1970's, as diagnostic tools and initial evaluation of near-surface piston core data suggest that the Thalassia to Halodule transition can be recognized stratigraphically at altered sites. Assembling a subfossil-based environmental history of Cross Bank can enhance our understanding of environmental perturbations, human-induced or natural, and their ecological effects for evaluation of future changes in coastal systems.