Molly S. Wood¹ and Robert M. Sheridan^2
1U.S. Geological Survey, Altamonte Springs, FL, USA
²ETI Professionals, Inc., Altamonte Springs, FL, USA

Lake Okeechobee is the heart of South Florida’s water supply and flood control system and is a major source of water for the Everglades. Agricultural development and canal construction in the watershed have resulted in more efficient delivery of stormwater to the lake, increased nutrient inputs, and a concomitant decline in ecosystem health. The U.S. Geological Survey is operating a 10-year (2003 – 2013) water-quality and streamflow monitoring program at 18 sites to help assess the effects of future restoration efforts in the watershed.

Many streams in the Lake Okeechobee watershed have substantial amounts of aquatic vegetation growth, likely owed to high nutrient concentrations combined with low flow. In-stream nutrient concentrations might vary due to diurnal changes in vegetation growth and associated nutrient uptake. As a result, water-quality data from the monitoring program may be biased if samples are collected only during daytime hours. To test this hypothesis, water samples were collected hourly for 24 hours from a monitoring site with substantial aquatic vegetation, Williamson Ditch, on March 15-16, 2005. Samples were spatially-composited and analyzed for phosphorus species, nitrogen species, and total suspended solids. In addition, a continuous water-quality monitor was deployed to measure dissolved oxygen concentration, water temperature, pH, turbidity, and specific conductance.

Statistical analysis (t-test on transformed data, alpha=0.05) showed that phosphorus and total suspended solids concentrations were significantly lower during daytime hours than nighttime hours. Dissolved ammonia concentrations were higher during daytime hours; however, this may be due to a change in equipment cleaning techniques in the latter part of the sampling event. Total nitrite plus nitrate did not exhibit a diurnal trend. Total organic plus ammonia nitrogen data could not be used due to holding time exceedance at the laboratory. Streamflow did not change significantly during the study. Dissolved oxygen, pH, and turbidity were lower and specific conductance was higher during daytime hours.

The observed diurnal fluctuations have important implications for water-quality sampling in the Lake Okeechobee watershed and perhaps in other similar watersheds. To avoid biasing water-quality sampling results, an effort is now made to randomize sample collection times, and more emphasis is placed on water-quality sample data collected using flow-triggered, automated samplers, which are less influenced by a time bias.

Contact Information: Molly S. Wood, Center for Aquatic Resource Studies, Florida Integrated Science Center, U.S. Geological Survey, 224 W. Central Parkway, Suite 1006, Altamonte Springs, FL, USA 32714, Phone: 407-865-6725, Fax: 407-865-6733, Email: mswood@usgs.gov

(This abstract is from the 2006 Greater Everglades Ecosystem Restoration Conference.)

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