A ~3,800-YEAR RECORD OF SEAGRASS COVERAGE IN SOUTH FLORIDA, BASED ON FORAMINIFERA

Marine waters flooded the southern tip of Florida ~3,800 YBP. Beginning in the 1880s, agriculture, bridges and canals changed circulation and caused increased nutrient runoff and periodic releases of freshwater. These anthropogenic changes alter water quality (salinity, turbidity, circulation), which is correlated with seagrass cover. We develop a seagrass-associated foraminiferal (SAF) proxy by assessing the effect of a large, well-studied seagrass die-off, then apply the proxy to a (C-14-dated) ~3,800-year record.

Thirteen species found in seagrass more than surrounding sediments of the region compose the set of SAF in six cores from four locations. Benthic foraminiferal assemblages from 203 sediment samples contained 76 identified species, 25-36 each. Trends were identified with Discriminant analysis, the Assemblage Turnover Index and the Conditioned-on-Boundary Index.

SAFs in 1970-1985 differed significantly from 1986-2001, and SAF percentages declined in 1986 (± 2 yr) with no recovery. The results agree with observations of seagrass coverage, which generally declined from 1987-2002 and began to increase thereafter. No large upset in total assemblages was identified. The analyses support use of SAF as a proxy for seagrass coverage for examining four intervals: flooding (~3,800 YBP), pristine conditions (until 1880s), human occupation (1880-1986), and seagrass die-off (1986-2001).

All four intervals had significantly different SAF assemblages. SAF were lowest (< 5% of the total foraminiferal assemblage) during flooding, increasing to an average of 18% during human occupation, with a large decrease ~1930-1940, possibly a result of a large salinity drop caused by three strong hurricanes followed by multiyear droughts and hypersalinity. Percentages during the die-off were comparable to those >3,000 YBP. It appears that naturally occurring events combined with anthropogenic alterations to coastal waters resulted in extremely low seagrass coverage 1986-2002.