Quantifying chemical and hydrodynamic signal structure for aquatic ecology

 

D.R. Webster

School of Civil & Environmental Engineering

Georgia Institute of Technology

 

 

The use of modern experimental fluid mechanics techniques to quantify the chemical and hydrodynamic signals that aquatic organisms use to communicate and perform other functions will be discussed in the context of several on-going projects.  First, the spectacular success of blue crabs locating food and mates by tracking turbulent odor plumes will be discussed.  The instantaneous concentration fields are quantified using the planar laser induced fluorescence (PLIF) technique.  The results support the hypothesis that blue crabs employ an odor-gated rheotaxis strategy with bilateral comparison.  In other words, they move upstream when they sense food odor and adjust their lateral position (i.e. steer) by comparing the odor signal on the left and right sides of their bodies.

 

Second, the mechano-reception of copepods will be discussed in the context of isotropic turbulence and thin layers in the ocean.  Field observations indicate that certain copepod species migrate away from high turbulence regions.  We have constructed an apparatus to reproduce small-scale oceanic turbulence in the laboratory.  The apparatus facilitates an examination of copepod behavior response to specific flow motions at several turbulence intensities.  Copepods also have been observed to aggregate to thin layers in the ocean.  Thin layers consist of structured gradients of velocity and/or concentrated patches of chemicals or phytoplankton.  The vertical thickness of thin layers is typically on the order of a meter or less and they extend horizontally for hundreds of meters or kilometers.  We have constructed a low Reynolds number planar jet apparatus in the laboratory to mimic the structure and strain rate levels typically observed in thin layers.  The apparatus facilitates direct observation of copepod aggregation behavior to hydrodynamic signals.  In both cases, the instantaneous velocity fields are measured with particle image velocimetry (PIV) and the copepod kinetics are simultaneous recorded via shadowgraph images.