We employed laser-induced fluorescence (LIF) measurements of odorant plume structure and behavioral observations to examine how turbulence affects the three-dimensional structure of odorant plumes and subsequently mediates olfactory search efficiency and success in our model organism, the blue crab (Callinectes sapidus). The turbulent characteristics of two laboratory flumes (one for behavioral quantification, one for signalstructure quantification) were systematically varied by changing the bed substrate roughness to create smooth, transitional, and fully rough flow conditions. Generally, increasing bed roughness caused greater mixing, decreased the time-averaged odorant concentration and concentration variance, and increased the plume width and homogeneity. Foraging success and speed of blue crabs attempting to locate the odorant source both declined consistently with increasing bed roughness. The variation in signal structure at the height of the antennules among bed-roughness treatments explains the observed behavior differences in crab foraging speed. In contrast, steering (path linearity) appeared to be unaffected by bed-roughness-induced turbulence. The transverse correlation function for odorant concentration at sensors separated across the width of the plume was examined among bedroughness treatments, and, ultimately, the correlation function was found to be related to the spatial position of tracking blue crabs. The spatial arrangement of blue crab chemosensors combined with the three-dimensional structure of odor plumes account for the differential effects of turbulence on the speed and success of crabtracking behavior.

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The laser-induced fluorescence ͑LIF͒ measurement technique is discussed in the context of environmental fluid mechanics. The measurement equipment and procedures employed in our laboratory are described in detail. The technique is applied to an isokinetic chemical plume ͑neutrally buoyant͒ in a turbulent open channel flow. A nonuniform laser sweep rate is employed to take full advantage of the dynamic range of the digital camera over the entire spatially varying concentration field. Statistical measures of the concentration field, such as the average and variance, are presented and discussed. Comparisons are made with theoretical models and previous experimental observations. The plume width grows at a greater rate in the downstream direction than that predicted by the analytical model of a point source release into a uniform flow. Probability density function distributions do not resemble Gaussian distributions, which reflects the highly intermittent nature of the concentration time record. The current measurements suggest that LIF is a valuable technique for nonintrusively recording the scalar field evolution in turbulent flows.

This article reports on the usefulness of bilateral comparison of chemosensory information to an animal or agent tracking an odor to its source. Instantaneous concentration fields of a chemical plume diffusing in a fully developed, turbulent, open channel flow are measured using planar laser-induced fluorescence. The plume is released isokinetically 25 mm above the smooth bed (z ϩ ϭ 90), thus transport is mainly due to advection and ambient turbulence. A spatial cross-correlation function in the spanwise direction gives a strong indication of the relative position of the centerline and distance from the source. The relative direction of the plume centerline can be estimated from the instantaneous concentration, provided the sensors are separated by a distance larger than the spanwise integral length scale of the concentration field.