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U.S. Department of the Interior
Geochemical Productivity Monitoring in Florida Bay
The SHARQ consists of a 4.8(l) x 2.4(w) x 1.2(h) m. PVC frame covered with clear vinyl sheeting. A circulation system generates a current inside of the tent and carries water to the surface through a flow-through analytical system for geochemical analyses and water sampling. (Figures 1-3). We have successfully tested the SHARQ in both deep (> 40 feet) and shallow (< 4 feet) water in the Gulf of Mexico, the Bahamas, Hawaii and Florida Bay. Field trials have incorporated fluorescein dye injection studies to examine leak rates and mixing rates in the SHARQ, deployment of current meters to measure current characteristics generated by the circulation system, measurement of photosynthetically active radiation (PAR) attenuation by the clear vinyl used to construct the tent, and 24 hour monitoring of geochemical changes in seagrass beds.
Fluorescein dye was injected into the tent via a sample port on the flow-through analysis system during dark hours, and concentrations monitored through the duration of the experment using a Model 10-AU Digital Fluorometer (Turner Designs). Results indicate that approximately 1.6 to 2.3 hours are required to mix water in the tent and reach peak dye concentrations. There is a slight linear decrease in fluorescein concentrations through the night consistent with rates of dye absorption to sediments and organic material, and exponential decrease during the day consistent with rates of photochemical decay of fluorescein (Figure 4). Rates of absorption of dye to carbonate sediments (2% of total concentration) and organic material (17% of total concentration) (Smart and Laidlaw, 1977) were used to generate a theoretical concentration decay during the night. An average minimum photochemical decay coefficient of 3.1 x 10-2 calculated from values of Smart and Laidlaw (1977) was used to generate a theoretical concentration decay during the day. This theoretical decay curve was then used to correct raw fluorescence data to show decreases in dye concentration associated with leakage of water into or out of the tent. Results showed no decrease in corrected dye concentrations (indicating no leakage of water during the incubation period) and a water mixing rate of approximately 1.6 hours (Figure 4).
A Sontek current meter was placed on the bottom surface in seagrass beds covered with the SHARQ. Current speed and direction was measured along x, y, and z axes. Initial results indicate that flow is not laminar, and suggest that turbulent flow may occur within the tent. Additional measurements will be required to accurately characterize flow patterns.
The effects of clear vinyl sheeting were examined by measuring PAR attenuation with water depth using a LiCor 4P quantum sensor covered with a sleeve made from the same clear vinyl sheeting used to construct the tent and performing the same measurements with the sensor uncovered. No difference in PAR attenuation occurs between sleeved and unsleeved sensor data below a depth of 3 feet (Figure 5). Clear vinyl sheeting reduces PAR by 29% at 1 foot, 21% at 2 feet, and 4% at 3 feet water depth relative to measurements taken using an unsleeved sensor.
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U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:43 PM (KP)