Circadian Rhythms and environmental changes

As we are now experiencing the shortest day of the year (5 h 43 min from sunrise to sunset in Turku), it is relevant to think how environmental changes affect naturally rhythmic responses.

This is especially important for temperate and arctic/antarctic areas, where the daylength variations are great. The most important biological rhythm is the 24-h or circadian rhythm, and associated day length changes. The day length and its predictable changes generate a cue for many biological phenomena such as timing of migration, reproduction, feeding and hibernation. However, recent disturbances that man has caused can mean that the rhythms, their lack and cues that result from adaptations of thousands of years become inappropriate. Below are a couple of examples:

We and others have recently observed that cold acclimation reduces rhythmicity in fish. This is appropriate, since all phenomena associated with rhythmicity such as spawning are energy-demanding, and should not take place in short day-length conditions, as primary production, which forms the basis of food chains, cannot occur in low  light. Now, with increasing temperature due to climate change, rhythms get more pronounced and oxygen consumption of fish increases, although food resources would not make it possible.

In eutrophic conditions, oxygen levels vary diurnally being highest at the end of the day and lowest in the early morning. In natural conditions fish have adapted to this appropriately with rhythmicity of hypoxia responses. However, the levels of some pharmaceuticals in natural waters have increased to concentrations that disturb or abolish the rhythm of hypoxia responses. Consequently, fish can succumb to conditions, which they could easily tolerate without the contaminant insult.

Despite the dark times, Merry Christmas