What is the largest source of dissolved organic material (DOM) in the epipelagic zone? A) Viruses B) Bacteria C) Phytoplankton D) Zooplankton

What the question is really asking

You are being asked which group contributes the most dissolved organic material (DOM) to surface ocean waters, where photosynthesis is active.

Why phytoplankton dominate DOM production in the epipelagic

In the epipelagic zone, phytoplankton fix carbon via photosynthesis and a portion of that newly made organic matter is released directly into the water as DOM (often called extracellular release or exudation). Phytoplankton cells also break apart (lysis) from stress and grazing, which further adds dissolved organic compounds.

Why the other choices are not the “largest source”

• Viruses: They can increase DOM indirectly by causing cell lysis (the “viral shunt”), but viruses are not typically considered the largest direct source.
• Bacteria: Bacteria are major consumers of DOM (they take it up for energy and growth) and also produce some DOM, but they are not usually the dominant source in the epipelagic.
• Zooplankton: They contribute to DOM through sloppy feeding and excretion, yet their contribution is generally smaller than phytoplankton release at the base of the food web.

Quick check

Because the epipelagic is the sunlit, high-primary-production layer, the biggest DOM input tends to come from the organisms doing most of the primary production: phytoplankton.