Practice question

Compensation depth is the depth at which a photosynthetic organism or community fixes carbon at the same rate that it loses carbon through respiration. At this point, gross photosynthesis equals respiration, so net photosynthesis and net biomass production are approximately zero over the specified period. Above that depth, stronger light usually permits photosynthesis to exceed respiratory demand and supports net primary production. Below it, light is insufficient; respiration exceeds photosynthesis, and organisms must use stored or imported organic carbon. The depth varies with water transparency, phytoplankton physiology, temperature, season, cloud cover, and the time scale over which rates are integrated. It should not be confused with complete darkness: some light remains at the compensation depth, but it is only enough to meet metabolic costs. In aquatic ecology, the lower boundary of the productive or euphotic layer is often approximated using a percentage of surface irradiance, yet the physiological definition is the balance of photosynthesis and respiration. This equality explains why neither “no light” nor a photosynthetic surplus correctly defines the boundary.