Organic matter in seawater is a complex mixture of carbon-based compounds derived from both marine and terrestrial sources. Ultimately, it originates from living organisms and their byproducts, as well as land-based runoff.
Sources of Organic Matter
Seawater's organic matter comes from a variety of sources, which can be broadly categorized as:
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Marine Primary Production: This is the most significant source. Phytoplankton (microscopic marine plants) utilize photosynthesis to convert carbon dioxide and sunlight into organic compounds. When phytoplankton die, or are consumed by zooplankton, their organic matter is released into the water.
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River Runoff: Rivers carry dissolved and particulate organic matter from land, including decaying plant material, soil, and pollutants. This terrestrial organic matter is then introduced into the coastal ocean.
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Atmospheric Deposition: Airborne materials such as dust, pollen, and aerosols containing organic compounds are deposited onto the ocean surface.
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Marine Animal Waste and Decay: Waste products (feces, urine) from marine animals and the decomposition of dead organisms (fish, marine mammals, etc.) contribute significantly to the organic matter pool.
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Hydrothermal Vents: While less significant globally compared to photosynthesis, hydrothermal vents release dissolved organic compounds from the Earth's interior.
Types of Organic Matter
Organic matter in seawater can be further classified into:
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Dissolved Organic Matter (DOM): This is the fraction of organic matter that passes through a 0.2 or 0.7 micrometer filter. It includes a wide range of compounds, such as carbohydrates, amino acids, lipids, and humic substances. DOM is further categorized into:
- Labile DOM: Easily broken down by microorganisms.
- Refractory DOM: Resistant to microbial degradation and can persist in the ocean for long periods.
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Particulate Organic Matter (POM): This is the fraction of organic matter retained by a 0.2 or 0.7 micrometer filter. It includes living organisms (plankton, bacteria), dead cells, fecal pellets, and larger detritus particles.
Significance of Organic Matter
Organic matter plays a crucial role in marine ecosystems:
- Food Source: DOM and POM serve as a food source for marine bacteria and other heterotrophic organisms, forming the base of the marine food web.
- Carbon Cycling: Organic matter plays a vital role in the global carbon cycle, influencing the distribution and storage of carbon in the ocean.
- Nutrient Cycling: The decomposition of organic matter releases nutrients (nitrogen, phosphorus) that are essential for phytoplankton growth.
- Light Attenuation: Organic matter can absorb and scatter light, affecting the penetration of sunlight into the water column and thus influencing photosynthesis.
- Sequestration: Refractory DOM can be sequestered for long periods, potentially impacting climate change.
In summary, organic matter in seawater is a diverse and dynamic mixture of compounds originating from both marine and terrestrial sources, playing a critical role in marine ecosystems and the global carbon cycle.
