STRENGTHS AND LIMITATIONS OF PROPOSED RESEARCH 



The strengths of the proposed methodology include the capability for time-series 

 observations of fine-scale distributions of pigment fluorescence using relatively inexpensive 

 hardware. Time-series observations are essential for characterizing episodic events that may 

 contribute significantly to lateral fluxes on the shelf. The low power demand of our instrument 

 should allow for extended autonomous operation. Another strength is the ability of our 

 instrument for precise sampling of fluorescence in the benthic boundary layer. Estimates of flux 

 of particulate pigments in the boundary layer require knowledge of the vertical integral of the 

 vector velocity at each height times the pigment concentration. Since both the velocity and 

 concentration vary with height, it is necessary to measure both at enough heights to resolve the 

 variability. The use of fiber optics allows for precise placement of fluorescence sensors at 

 multiple depths that will minimally interfere with other sensors. 



One of the potential weaknesses of this and any optical sensor is the potential for 

 bio-fouling. Bio-fouling will be considered as a potential factor influencing the fluorometer 

 signal response. This will be especially critical during long deployments. Previous strategies for 

 dealing with fouling problems have included both mechanical (Whitledge and Wirick, 1986) and 

 chemical (Whitledge and Wirick, 1983; Dickey et al., 1991) methods. In addition to use of an 

 anti-fouling clear organo-metallic polymer (tributyl tin), designs are being studied for construction 

 of a hydraulic system for periodic scrubbing of the optical sensors as well as "self calibration" 

 of the instrument. 



Calibration of the autonomous instrument represents a major challenge to the research 

 effort. A considerable proportion of our efforts will focus on instrument calibration and factors 

 which may modify this. We are currently evaluating instrument calibration and drift in controlled 

 laboratory conditions. Factors to be considered include effects of ambient light and temperature. 

 In addition, the possible influence of changes in particle concentrations and size distributions will 

 be examined empirically and through the use of Monte Carlo simulations. In field situations, the 

 instrument will be cross-calibrated on the basis of contemporaneous measurements of 

 fluorescence from other fluorometers and of organic matter and extracted pigments from water 

 samples. In addition, the development of a "self calibrating" capability during deployment using 

 a stable fluorescence reference is being explored. 



STATUS OF RESEARCH 



A successful field test of the multi-sensor instrument in the benthic boundary layer was 

 recently conducted aboard the R/V Pelican in May 1994. Observations were conducted in an 

 area near the Mississippi River outflow plume where significant accumulation of pigments in 

 sediments has been observed (Turner and Rabalais, 1994). Preliminary results revealed large 

 variations in near bottom fluorescence in 10-15 m of water. The variations appeared to be too 

 large to be explained on the basis of in situ growth, and will be evaluated in the context of other 

 measurements including acoustic doppler velocimetry, transmissometry and water column pigment 



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