The growth of planktonic plant life requires light, as well as the presence of inorganic elements (carbon, nitrogen, phosphorus, silicon) which are incorporated into the plants as part of the process of PRIMARY PRODUCTION. The major macro-nutrients are: nitrogen in the form of nitrate (NO3-), phosphate (PO43-) and silicate (usually written as SiO2, although more usually appearing in the form of silicic acid Si(OH)4).
When these macronutrients are present in abundance, primary production is usually limited by the available light, which is typically the case in winter. During summer, phytoplankton can grow until one or more of the macronutrients are exhausted, primary production is then limited by the rate at which nutrient rich waters at (dark) depths can be mixed up to the surface. Spring is a special time, as increasing light levels will suddenly result in a "bloom", dominated by diatom species, that rapidly draws down the supply of nutrients at the surface. Depending on weather conditions, this bloom, marked by very high phytoplankton concentrations, can suddenly appear in as little as a few days, followed by a "crash" once near-surface nutrients are exhausted.
In the CitSci program, nutrients are measured at the surface, and at 20m.