Photic Zone

Primary production in the photic zone is influenced by three major factors—nutrients and light, which are essential for photosynthesis, and grazing pressure, the rate at which the plants are eaten by herbivores. Nutrients, especially phosphate and nitrate, are often scarce in the photic zone because they are used up quickly by plants during photosynthesis. External inputs of nutrients are received through rainfall, riverflow, the weathering of rocks and soil and from human activities, such as sewage dumping. Nutrient enrichments also occur through internal physical processes such as mixing and upwelling that resuspend nutrients from deeper volumes of the water.

As plants in the photic zone grow and reproduce, they are consumed by herbivores, which excrete their wastes into the water column. These wastes and other organic particles then rain down into the lower volumes and eventually settle into the sediment. During periods of resuspension, such as remixing and upwelling, some of these nutrient-rich wastes are brought back up to the photic zone. Remixing refers to processes whereby the water of a lake is thoroughly mixed from top to bottom, usually by the force of wind.

Upwellings can sometimes occur in cool lakes with warm underground springs, but they are much more important in oceans. An upwelling is an area in the ocean where the deeper, nutrient-rich waters are brought to the surface. Oceanic upwellings can be caused when the wind tends to blow in a consistent direction across the surface of the ocean. This causes the water to pile up at the lee end of the wind's reach and, through the sheer weight of the accumulation, pushes down on the deeper volumes of water at the thick end. This pushing causes the deeper, nutrient-rich water to rise to the surface back at the region where the winds began.

Upwellings can also be caused by deep ocean currents that are driven upwards because of differences in water temperatures. Such upwellings tend to be very extensive. Upwellings can also occur on a short-term basis when underwater uplands and sea mounts force deep currents to the surface. Regardless of the origin of the resuspension event, these cooler, nutrient-rich waters stimulate the productivity of phytoplankton in the photic zone. Photic zones that are replenished with nutrients by either upwellings and or remixing events tend have very high primary production.

Light is essential to photosynthesis. The depth to which light penetrates a water column can vary substantially in space and time. The depth of the photic zone can vary from a few centimeters to several hundred meters. Sunlight is scattered and absorbed by particles and dissolved organic matter in the water column, and its intensity in water decreases with depth. In some cases, when nutrient concentrations are high, the photic zone becomes shallower. This is because the nutrients stimulate the growth of phytoplankton, and these cells then absorb more of the sunlight entering the water column and shade the layers below. Other areas may have very deep photic zones because the nutrient concentration is very small and therefore, the growth of primary producers is limited.

The ideal convergence of sufficient nutrients and sunlight occurs in relatively few areas of our oceans and lakes. These areas are, however, extremely productive. For example, areas off the coasts of Peru, northern Chile, eastern Canada, and Antarctica are responsible for much of the fish production of the world.