Climatic and Anthropogenic Influence on Florida Bay Salinity over the Past Century

Topical Area: Paleoecology

T. M. Cronin, L. Brewster-Wingard, S.E. Ishman, U.S. Geological Survey, Reston, VA; R.B. Halley and C.W. Holmes, U.S. Geological Survey, St. Petersburg, FL; G.S. Dwyer, Department of Geology, Duke University, Durham, NC

Establishing the causal factors underlying 20^th century historical trends in Florida Bay salinity requires a firm understanding long-term variations in Florida Bay salinity and its causes. In particular, one important question pertains to what degree have human activities and natural climatic variability contributed to recent periods of hypersalinity. We conducted multi-proxy faunal (mollusk, foraminiferal, ostracode) and geochemical (stable isotopic, elemental [Mg, Sr, Na]) analyses of radiometrically-dated sediment cores from central and northeastem Florida Bay to establish semi-quantitative paleo-salinity estimates for the mid-19^th century to present.

Our results indicate that prior to major 20^th century canal building, central Florida Bay experienced wide interannual and decadal swings in salinity (from polyhaline to euhaline conditions). Although additional research is needed, results to date indicate an approximate 6-yr periodicity can be identified in some faunal and geochemical records for the period 1870 through 1940 that appear to be associated with well-known cycles of precipitation due to El Nino-Southern Oscillation (ENSO) Strong warm El Nino events bring heavy winter season precipitation and accompanying reduced Florida Bay salinity; cool La Nina events produce the opposite. In addition to high-frequency salinity oscillations, there is a low frequency period (~13 years) evident in some paleoecological salinity indicators suggesting decadal climatological processes might also influence the Florida Bay ecosystem.

Beginning about 1910, and accelerating until the 1940s, there is a directional shift towards higher salinities. By about 1940, there had been an apparent disruption of the typical 6 year salinity-related faunal and geochemical cycles and an increase in abundance of paleoecological indicators of hypersalinity (>40-45 ppt). Florida Bay essentially became "hypersensitized" to natural climatic variability, a process which led to several periods of extreme hypersalinity since 1940, such as the one that occurred during the late 1980s. The causes of this mid-century salinity and ecosystem shift will be discussed in terms of disruption of freshwater flow into Florida Bay and changing patterns of climate and rainfall.