Autochthonous carbon to freshwater food webs primarily comes from which source?

• A. Terrestrial leaf litter input.
• B. Atmospheric carbon deposition.
• C. Terrestrial organic matter import from watershed.
• D. In-stream algae and aquatic macrophytes.

Answer: (D)

Autochthonous carbon is carbon fixed within the ecosystem by photosynthesis. In freshwater systems, the primary source is in-stream algae and aquatic macrophytes, which produce organic matter right in the water column or on substrates. This in-situ production fuels the food web, providing a readily available energy base for herbivores and higher trophic levels. Terrestrial inputs like leaf litter and watershed-derived organic matter are allochthonous—they come from outside the system and generally feed detrital pathways rather than the autochthonous production within the water. Atmospheric CO2 can contribute to the dissolved inorganic carbon pool, but the dominant autochthonous carbon source is the in-stream photosynthetic organisms themselves.