SEAGRASS ABUNDANCE OF FLORIDA BAY, USING FORAMINIFERA AS A PROXY

In coastal areas such as Florida Bay, which drains Everglades National Park, seagrass abundance is well correlated with water quality, and whether humans or natural events have had a greater effect has been much debated. Using seagrass as a general indicator is not possible because it is rarely preserved in sediments. To examine historical seagrass abundances of Florida Bay, we used as a proxy the fossil record of benthic foraminiferal species that have been more abundant on seagrass than in sediments.

Two hundred samples from six sediment cores from across the Bay were dated with ²¹⁰Pb and ¹⁴C methods, and the percentages of seagrass-associated foraminifera (PSAF) assessed. To test whether PSAF indicates general seagrass abundance, we compared its 1970-2001 record to the timing of Florida Bay’s 1987-1994 great seagrass dieoff that caused sediment resuspension, decline of water clarity, phytoplankton blooms, a sponge die-off and declines in fishery harvests. All cores had lows around 1984-1994 (±2 yr), after which 4 of 5 showed increases, indicating a satisfactory correspondence between PSAF and seagrass abundance.

PSAF was studied at two time scales: from 1880-2001 (±5 yr) it was compared to known anthropogenic and environmental events; and for the last 4 ka (since Florida Bay’s origin) general patterns were examined. From 1910 to 1920, when a railroad was built that reduced water exchange, PSAF decreased in the central bay. The largest change, ca. 1940 and seen in 5 of 6 cores, corresponds to a multi-year drought and hypersalinity; a 1950s drop in PSAF agrees with another drought/hypersalinity. A decrease in PSAF from ca. 1965-1975 to ca. 1985, just before the great seagrass dieoff, occurs in 5 of 6 cores and may be related to the anthropogenic alteration of freshwater input from the Everglades. Over the last 4 millenia, seagrass abundance fluctuated with amplitudes as great as the last 100 years, but generally increased until the large ca. 1940 drop. Overall, seagrass abundance/water quality was apparently little affected by the initiation of agriculture (ca. 1905), was greatly affected by multiyear droughts, gradually decreased after ca. 1965-1975 (presumably a result of construction of canals and water retention areas), and has remained at relatively low levels since the 1987-1994 seagrass dieoff.