Is the Variation of the Temperature in an Intertidal Pool Diel or Tidal? A Paradigm for Studying Biological Rhythms in the Tidal Zone

This article analyzes the variations of the temperature recorded in a tidal pool near the half-tide level, in a semilunadian regimen. A model of the temperature variations of the pool, based on diffusional exchange with the sea at high tide and with the air at low tide, shows good agreement with the experimental results. Fourier transform analysis of the variations of the main components of the frequency spectrum during a lunar cycle and for different values of the parameters (temperature variations of the air, level of the pool relative to the tide, exchange coefficients with the air and the ocean) reveals a complex structure. When the tidal component exists, it is stable in power and frequency, but is, in most cases, much smaller than the die! peak. Barring the situations when the tide does not reach the pool for several days, the period of the pool is equal to 24.8 h (the first subharmonic of the tidal frequency), except for the part of the lunar cycle during which the high tide corresponds approximately to the extrema of air temperature: Then the period is lower than 24 h and the power decreases. Overall, the mean period is 24 h, which is coherent with the fact that the driving force of the temperature variations is the sun, while its input in the pool is modulated by the tide. The same results can be applied, with a lesser amplitude of the temperature variations, to larger bodies of water. From these results, it is difficult to decide whether the temperature variations constitute a zeitgeber for circadian or circatidal clocks. The present conclusion is that this parameter seems to be, at best, a very unreliable synchronizer.