What role do dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) play in freshwater light regimes and microbial processes?

What role do dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) play in freshwater light regimes and microbial processes?

• A. They absorb light, reducing PAR and inhibiting primary production.
• B. They reflect light and enhance photosynthesis.
• C. They have no impact on microbial processes.
• D. They remove dissolved oxygen directly

Answer: (A)

Dissolved organic carbon and colored dissolved organic matter control how far light can penetrate in freshwater by absorbing light, especially in the blue part of the spectrum. That absorption makes water appear tea-colored and reduces the amount of photosynthetically active radiation (PAR) that reaches phytoplankton. With less PAR, primary production declines because photosynthesis is light-limited at greater depths or in stained waters. At the same time, DOC serves as a carbon source for heterotrophic microbes, so microbial communities can keep processing carbon and respiring even when light is limited for phytoplankton. Photochemical reactions driven by light on DOC can also create reactive species that influence microbial activity and DOC chemistry. So the key idea is that absorption by DOC/CDOM attenuates light, lowering PAR and primary production, while still supporting microbial processes that rely on DOC.

Dissolved organic carbon and colored dissolved organic matter control how far light can penetrate in freshwater by absorbing light, especially in the blue part of the spectrum. That absorption makes water appear tea-colored and reduces the amount of photosynthetically active radiation (PAR) that reaches phytoplankton. With less PAR, primary production declines because photosynthesis is light-limited at greater depths or in stained waters. At the same time, DOC serves as a carbon source for heterotrophic microbes, so microbial communities can keep processing carbon and respiring even when light is limited for phytoplankton. Photochemical reactions driven by light on DOC can also create reactive species that influence microbial activity and DOC chemistry. So the key idea is that absorption by DOC/CDOM attenuates light, lowering PAR and primary production, while still supporting microbial processes that rely on DOC.