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Geosciences LibreTexts

11.1: Primary Productivity

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  • The oceans make up the largest habitat on the planet, stretching over approximately 71% of the Earth’s surface and it contains 97% of the Earth’s water. Subsequently, the oceans are home to a great diversity of living organisms that can be separated into two groups: autotrophs and heterotrophs. Autotrophs are organisms that use inorganic compounds (e.g. carbon dioxide) to produce organic products, (e.g. sugars and proteins). Heterotrophs consume the organic products produced by autotrophs and release inorganic compounds as a by-product. Phytoplankton are an example of marine photoautotrophs or autotrophs that obtain energy via photosynthesis. Whales are examples of heterotrophs. They consume plankton and releases carbon dioxide as a by-product of cellular respiration. This latter example illustrates the clear connection between the biological processes of autotrophs and heterotrophs through food webs.

    Primary productivity is the process where inorganic substances are synthesized by organisms to produce simple organic materials. Primary producers, or autotrophs, are responsible for this phenomenon. Common examples of primary producers include diatoms, dinoflagellates, and coccolithophores. Primary producers can either be photoautotrophs, organisms that synthesize organic compounds using the sun as a source of energy, or chemoautotrophs, organisms that synthesize organic compounds from inorganic molecules found in the environment. Both photosynthesis and chemosynthesis contribute to the oceans’ primary productivity, but photosynthesis is the dominant process with respect to the amount of carbon fixed and energy stored in organic compounds. Photosynthesis is used by autotrophs at the sea surface and high in the water column where light is abundant. Contrastly, chemosynthesis usually occurs in deeper water where little to no light is present.

    There is a multitude of factors that determine the effectiveness of primary productivity. While the amount of water, carbon dioxide, inorganic nutrients, and sunlight all play a major role in how productivity, not all of these components act as limiting factors. Neither water or carbon dioxide act as limiting factors in the ocean, as they are abundantly available in the environment. For photoautotrophs, one of the greatest limiting factors is sunlight and light penetration into water columns. Consequently, most photoautotrophs are found near the ocean’s surface (a zone aptly named “the photic zone”) and few are found in the mixing zones at lower depths. Additionally, nutrients such as inorganic nitrogen, phosphorus, iron and/or silica are limiting on living organisms due to their scarcity in the ocean.  

    Primary production is the most basic building block for energy and the basis for food webs in all environments and ecosystems. In the ocean, autotrophs which are responsible for primary production consist of phytoplankton, marine plants, and macroalgae since they all perform photosynthesis. All photoautotrophs capture solar energy by utilizing the pigmentation such as Chlorophyll A, a pigment that is especially effective in capturing light energy in the blue and red wavelengths of light. There are various pigments used by the many different photoautotrophs in the ocean.      

    Primary production is often referred to in two ways: gross and net primary production. The entirety of the organic compounds produced by the primary producers is referred to as gross primary production. As with everything it is impossible to have 100% efficiency, the autotrophs require a part of the organic compounds that they produce for their own respiration processes. Net primary production is the measure of the organic matter that is produced by the autotrophs and available for consumption of the heterotrophs.