Spatial distribution of odors in simulated benthic boundary layer flows.

Many animals orient to odor sources in aquatic habitats where different flows and substrates affect the hydrodynamics of benthic boundary layers. Since the dispersal of chemicals is due to the fluid mechanics of a particular environment, we quantified the changes in the fine structure of an odor plume under different hydrodynamic conditions in the benthic boundary layer of a laboratory flume. We sampled turbulent odor plumes at 10 Hz using a microchemical sensor (150 µm diameter) under two flow speeds: 3.8 and 14.4 cm/sec, and at 1, 8, 50 mm above the substrate. These distances above the substrate occur within different flow regions of the boundary layer and correlate with the location of crustacean chemosensory appendages within boundary layer flows. The high flow velocity exhibited a greater level of turbulence and had more discrete odor pulses than the low flow velocity. In general, odor signals showed a high level of temporal variation in fast flow at heights 1 and 8 mm above the substrate. In slow flow, temporal variation was maximal at 50 mm above the substrate, exhibiting more variance than the same height at the fast flow. These patterns of odor signals resulted in part from differences in the height above the substrate of the main axis of the odor plume at the two flow speeds. Our results imply that animals chemically orienting to an odor source will need to compensate for varying hydrodynamic properties of odor transport and dispersal. The method by which animals extract spatial information from odor plumes will need to account for changing flow conditions, or else it will not be equally efficient in extracting information about chemical spatial distributions.