SS3.06 Large Scale Change in Prominent Ecosystems
Date: Wednesday, June 12, 2002
Time: 4:30:00 PM
Location: Lecture Theatre
 
KenneyWF, University of Florida, Gainesville, USA, kenney@ufl.edu
Schelske, C, L, University of Florida, Gainesville, USA, schelsk@ufl.edu
Shumate, B, C, University of Florida, Gainesville, USA, shumate@ufl.edu
 
SEDIMENT RECORDS OF LARGE SCALE CHANGES IN THE HARRIS CHAIN OF LAKES, FLORIDA, USA
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We analyzed 210Pb dated sediment cores from the Harris Chain of Lakes, Florida, USA. We hypothesize that changes in sediment characteristics may indicate past large-scale changes in this prominent ecosystem. We characterized sediments by physical parameters, phosphorus (P) content, total carbon to total nitrogen ratios (TC:TN), biogenic silica content and stable isotope ratios of organic matter (C and N). Historically these lakes produced macrophyte-derived sediments (high TC:TN, low diatom silica) with low P concentrations, but now produce phytoplankton-derived sediments (low TC:TN, high diatom silica) with larger (5- to 9-fold) P concentrations. Phytoplankton dominance began after ~1950 and results from anthropogenic disturbance in the watersheds. We found evidence for earlier, small-scale changes in sediment characteristics (decreased TC:TN, increased diatom silica) that are transient and are not associated with changes in sediment total P. Because these changes indicate increased phytoplankton contribution to sediments in macrophyte-dominated lakes, pre-date major anthropogenic disturbance and are independent of changes in sediment total P, we conclude they result from alternating stable states as defined in recent theoretical models of shallow lakes.