Tree Islands of the Everglades

Chapter 5.
Nutrient Geochemistry of Sediments from Two Tree Islands in Water Conservation Area 3B, the Everglades, Florida

William H. Orem, Debra A. Willard, Harry E. Lerch, Anne L. Bates, Ann Boylan, and Margo Comm
email: borem@ncrds.er.usgs.gov
U.S. Geological Survey, 956 National Center, Reston, VA 20192

From Tree Islands of the Everglades edited by Fred H. Sklar (South Florida Water Management District) and Arnold G. van der Valk (Iowa Lakeside Laboratory)

Abstract

Sediment cores from two tree islands (Nuthouse and Gumbo Limbo) located in Water Conservation Area 3B of the Everglades, Florida were examined for preliminary studies of their nutrient geochemistry and paleoecology. Cores were collected from sites on the head, tail, and surrounding slough/marsh at each of the islands, and sediments from these cores were analyzed for various chemical constituents. Porewater in cores collected from the head and surrounding slough/marsh was also analyzed for its chemical composition for the purpose of evaluating nutrient recycling from tree island heads. Intervals in selected cores from both tree islands were dated using ¹⁴C analysis. The major objectives of the study were to: 1) determine the concentrations and accumulation rates of nutrients (carbon, nitrogen, and phosphorus) in sediments of tree islands, 2) examine the role of nutrients (if any) in the development of tree island tails and 3) examine downcore trends in nutrient element concentrations for evidence of ecological changes through time.

Organic carbon data suggest different histories for the two tree islands, with the head of Gumbo Limbo being dry and the head of Nuthouse submerged for most of the last 2,000 years. The sediments from the heads of both tree islands have very high concentrations of total phosphorus (TP): up to 1,500 µg/g at Nuthouse and up to 3,000 µg/g at Gumbo Limbo. These concentrations are 6 to 12 fold higher than the concentrations of TP found in the surrounding pristine slough/marsh sediments, and approach concentrations observed in phosphorus-contaminated areas of Water Conservation Area 2A. The source of the excess TP on the tree island heads is uncertain, but may be from guano deposited by birds nesting in the canopy of the tree islands. Concentrations of TP in sediments of the tree island/marsh/slough complex decrease in the order:

head > near tail = head edge > far tail > slough/marsh.

This indicates the possibility of transport of phosphorus in runoff from the head to the surrounding slough/marsh. On Nuthouse head, peaks in TP concentration were observed at 0.31-0.28 ka (thousands of years ago) and 1.85-1.50 ka, based on ¹⁴C dates. These peaks may reflect peaks in nesting bird populations on the head. Based on TP profiles, the initial development of the far tails began between 2 and 2.4 ka, but with accelerated development at Nuthouse after about 0.9 ka. Downcore profiles of total nitrogen and atomic C/N values provide evidence of historical changes in the hydroperiod of the ecosystem, including a wetter period from 1.1 to 0.4 ka. These results point to the importance of tree islands in the geochemical balance of the ecosystem, and tree island sediments as records of the paleoecology of the Everglades.

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