In this lecture we will discuss the concept of primary production, including where the energy comes from and how it gets into the plants and animals. It is what allows plants to grow and therefore to feed all animals, bacteria, etc. Variation in primary production is closely tied to the ocean's physical and chemical conditions, and in turn influences production of fish and other species of interest to humans. When it is negative the plants basically stop growing, shut down, and some may sink out of the photic zone.
Light influences the seasonal abundance of phytoplankton and also their vertical distribution.
The amount of nutrients present in the water at the beginning of the growing season limits the extent to which plant biomass can build up. Nutrients are generally plentiful because there are no plants capable of taking them up without light. This growing season can be very intense. 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.