In the tropics, by contrast, light is always plentiful but because of that the nutrients get used up. The total biomass that can build up in the surface waters depends on the supply rate of nutrients, which may be very slow. Effect of mixing and the thermocline Now consider the effect of the thermocline on primary production. The phytoplankton, being more or less at the mercy of water motions, will be mixed up and down in the shallow mixed layer above the thermocline. If the thermocline is deep, the cells spend a lot of time at low light levels, and net pimary production may be low or even negative.
Respiration is more or less constant.
Phytoplankton can grow when the mixed layer depth is shallow relative to the compensation depth, because the phytoplankton are trapped in the sunlit surface waters. This means that, where the thermocline is not permanent but seasonal, there will come a time each spring when a seasonal thermocline is formed that rather quickly results in a bloom of phytoplankton. This spring bloom is the big event of the year in the N. Atlantic and other temperate-zone waters. Nutrient Cycles Where sunlight is plentiful and the water is stratified, nutrients tend to be in short supply because of uptake by phytoplankton. Animals that graze on the phytoplankton use some of the food for growth and some in respiration.