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3. Photosynthesis
In general, growth of photosynthetic organisms in aquatic ecosystems is controlled by the amount of the limiting nutrient available, and by light levels (which correspond to water depth)
Growth of aquatic plants and other aquatic primary producers such as phytoplankton is often limited by the concentrations of some of the vital dissolved nutrients they require. In most natural freshwater ecosystems the limiting nutrient is phosphorus.
While sunlight can penetrate through water, it can only do so for a limited distance. In general marine photosynthetic organisms can only grow when the light they receive is more than 1% of the amount of light incident on the water surface. This means that most phytoplankton, the tiny microorganisms which are found in surface layers of natural waters such as lakes and the sea, grow in the upper tens of metres of the water column. The oceans are commonly more than 1,000 metres deep and can be more than 5,000 metres deep. Plant life does not penetrate these depths.
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What determines the location and the rate of growth of aquatic primary producers?