The fine fraction (<63 µ) from 190 Florida Bay surficial sediment samples was analyzed for 30 elements using ICP (induction coupled plasma spectrography). Contour maps of the elemental data show concentrations of phosphorous, iron, aluminum, and chromium, at least one order of magnitude greater in those areas where seagrass mortality began in the late 1980s. The zones of elevated trace element content also coincide with areas of reduced circulation, i.e., shallow basins in the west-central bay, remote from both Gulf and Atlantic tidal influence. These areas are known to experience periodic hypersalinity during periods of reduced rainfall. To determine if the increase is of recent origin, additional samples were taken every 2 cm down-core from two different mudbanks and analyzed for trace elements. One core was taken in the area of elevated trace elements (Whipray basin) and the other was taken outside (Russell bank). The core from Whipray basin, the zone where seagrass mortality has been most pronounced, showed an upward increase of P, Fe, Al, and Cr through time. Pb-210 dating showed the upward increase in trace elements began about 30 years ago. Three explanations for the localized increase in trace elements have been considered. First, the trace elements were transported and deposited by longshore currents moving southward along the west side of Florida. Second, the trace elements precipitated directly from hypersaline waters that periodically form in these restricted basins. Third, the trace elements were deposited from the atmosphere but accumulated preferentially in the poorly circulated basins. Although arguments can be made for all three origins we favor the atmospheric dust hypothesis because the period of concentration coincides with a well-documented increase in transatlantic dust transport that began in the early 1970s.

African dust transportation and deposition reached maximum levels in Miami and throughout the Caribbean between 1983 and 1987 and is still rising in concert with desertification of northern Africa. In addition to trace elements, especially iron, African dust contains numerous fungal spores, one of which is the cause of a Caribbean-wide seafan disease. We speculate that death of seagrass in the west-central bay basins was caused by a combination of hypersalinity, elevated trace elements/nutrients, fungal spores, and anoxia related to reduced circulation. Regardless of the origin of elevated trace elements, the large elemental dataset should be useful to others conducting biological and geological research in Florida Bay.

Sediment ICP data also revealed a north-south central-bay zone of elevated copper adjacent to but not coincident with the zone of elevated P, Fe, Al, and Cr. The north end of the north-south zone of elevated copper extends northward into the Taylor River Slough, suggesting run-off, possibly from agricultural areas further north.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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