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Florida Lake Management Society Annual Conference, Naples, Florida, June 4 – 7, 2007

THE ROLE OF SEDIMENTS IN THE POST HURRICANE RECOVERY OF LAKE OKEECHOBEE

R. Thomas James1, Daniel R. Engstrom2, and Shawn P. Schottler2 South Florida Water Management District West Palm Beach, FL and 2Science Museum of Minnesota St. Croix, MN. 1

Introduction

The eye walls of three hurricanes, Frances and Jeanne in September of 2004 and Wilma in October 2005 passed very close to the center of Lake Okeechobee--a large shallow eutrophic lake in south Florida. The high winds created large waves, large standing waves (seiches), and strong currents that led to resuspension and movement of large amounts of sediments. There was a dramatic increase in suspended solids, total and soluble phosphorus, and dissolved inorganic nitrogen concentrations that have not returned to pre hurricane values despite the storm free year of 2006. This presentation evaluates monitoring data taken before and after these hurricanes to determine potential mechanisms influencing the recovery of Lake Okeechobee.

Methods

In-lake water quality was monitored on a monthly basis at 34 locations in the lake. Four of these locations included weather stations that measure wind speed. The monitoring program has been described by James et al. (1995). Secchi Disk depth, total suspended solids (TSS), total depth, chlorophyll a, nitrogen, and phosphorus species were measured.

Sediment cores were collected from the thirteen locations in Lake Okeechobee in 1988 (Brezonik and Engstrom 1998), three in 2003 (Schottler and Engstrom 2006) and eight in April 2005. Sediments were collected by piston corer, sectioned at 0.5-2 cm intervals in the field, and analyzed for water content; all 1988 and 2003 cores and two 2005 cores were also measured for

210Pb as described in Schottler and Engstrom (2006).

Discussion

The high TSS and nutrient conditions persisted after the hurricanes despite reduced wind speed and water levels, both related to improved water quality conditions in Lake Okeechobee (James and Havens 2005, Maceina and Soballe 1990). We hypothesize that the hurricanes produced an increased layer of more easily resuspended unconsolidated surface sediments, which contributed to the persistent TSS and nutrient conditions afterward. Three lines of evidence are presented: 1) 210Pb profiles in sediment cores taken before and after the storms show a marked change from an exponential decline with depth in the top 10 to 15 cm pre-hurricane to nearly homogeneous conditions after (Fig. 1A), 2) the slope of the relationship between wind speed and solids increased after the hurricanes (Fig. 1B), and 3) water content of surface sediments in four cores taken at the same locations before and after the hurricane was higher after the hurricanes than before (Fig 1C). As these sediments reconsolidate, water quality should improve in Lake Okeechobee.

Session 7A – Page 5

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