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Primary Productivity

Patterns of Primary Productivity in the Ocean

To explain the patterns of primary productivity in the ocean you need to think in terms of limiting factors, that is the essential resource that is in least supply, that will run out first and limit production. The four essential “resources” required for photosynthesis are:

  • carbon dioxide
  • water
  • nutrients
  • light

Carbon dioxide is available abundantly and is not a limiting factor for production. The other three can be limiting in different areas and at different times. Keep in mind that the biology of the oceans is driven by the physics of the oceans!

Estimating primary productivity

Data collected by satellites make it easy to estimate primary productivity in the ocean by relating it to chlorophyll concentrations at the ocean's surface (which can be measure by instruments on satellites). Areas with high chlorophyll concentrations have high primary productivity; areas with low chlorophyll concentrations have low primary productivity.

Figure 1. Chlorophyll concentrations in the ocean during mid-summer in the Northern Hemisphere. Areas with low chlorphyll concentrations are indicated in purple; blue is slightly higher levels; yellow is even higher levels, and orange/red is the highest levels of chlorophyll.

Factors Affecting the Patterns of Primary Productivity

Primary productivity isn't even distributed in the surface waters of the oceans...look at Figure 1, some places have high chlorphyll levels (and therefore high primary productivity levels) whereas others have low levels. Why? These patterns are created mostly by physical factors. The availability of sunlight is a big one; it mostly affects the polar regions and is seasonal in nature (winter has low light levels and therefore low primary productivity; the opposite happens in summer).

Some other physical factors that are important in controlling primary productivity are upwelling, downwelling, and closeness to the continents. These three factors influence the amount of nutrients in the ocean's surface waters. Upwelling occurs when cold, nutrient-rich water comes up to the surface and is common along the equator and along some coastlines (where prevailing offshore winds force deep water to the surface). Downwelling is the opposite of upwelling; surface water (along with the nutrients it contains) is forced downward to depths without enough sunlight for photosynthesis. The last factor—proximity (closeness) to the continents—is an interesting one. River runoff brings nutrients to the coastal ocean and stimulates primary productivity (as long as there aren't any other limiting factors). The farther away from the continents, the less nutrients there are from river runoff.

Take a look at Figure 1 and answer questions 1–7 on your worksheets. When you have answered the questions, check your answers.

That's it!

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