Primary production and thermoclines

Primary production is the term used to describe the amount of biomass produced through photosynthesis and is therefore closely associated with phytoplankton growth. Primary productivity is governed by the availability of light and nutrients, the latter being the most limiting. Other factors such as wind strength/ direction/ frequency, precipitation and the regional climate also effect the production or growth of phytoplankton because they all influence the upper water column.
Coastal seas are more productive than open oceans for three reasons:
1. The influx of nutrients (nitrates and phosphates) from the land, rivers and tributries
2. The depth of the water means that there is no critical depth, ie the phytoplankton can't sink into darkness because the water is shallow
3. Shallow waters rarely have a thermocline present, which means that nutrients aren't locked in bottom water

Thermocline development

A thermocline is a zone of rapid temperature change between warm surface water and cooler deep water. Thermoclines are usually found between 10- 500 metres below the surface.

The factors that initiate phytoplankton growth in the spring (bloom) are vertical mixing and stratification of the water column, along with the length of photoperiod. During the winter months, phytoplankton growth is inhibited. In this period, the nitrogen, phosphorus, silicate and ammonia nutrients increase in concentrations, as little or no primary production is taking place to utilise them. When the water becomes stratified in the spring, there follows a bloom of diatoms. When silicate becomes limited (it is essential for diatom growth, being incorporated into their 'test'), other groups, such as flagellates, bloom, followed later by the dinoflagellates. This secondary part of the bloom is limited by the amount of phosphorus and nitrogen left after the initial diatom bloom. As the spring progresses to summer, surface waters warm and a more permanent thermocline develops. Colder, nutrient waters sink away from the photic zone; primary production slows.  The resulting phytoplankton community is one that can cope with reduced nutrient levels. With the onset of autumn, the sea becomes mixed once again, and at this time a secondary bloom of dinoflagellates may occur. As the light levels diminish in the latter part of the year, primary production decreases. The water then becomes mixed and this aids the distribution of nutrients throughout the water column (Nybakken, J., 2001 Marine Biology: an ecological approach. 5th Ed).