Oxygen-Depleting Aquatic Plants

While aquatic plants are known to produce oxygen through photosynthesis during the day, some can actually reduce dissolved oxygen (DO) levels under certain conditions. This typically happens at night (when plants respire and consume oxygen) or when plant matter decays.

The most problematic species are often fast-growing, surface‑covering, or invasive plants:

Duckweed (Lemna spp.) – Forms dense mats that block sunlight, suppressing photosynthesis of submerged plants and algae. At night, the mat itself respires heavily, lowering DO.

Water hyacinth (Eichhornia crassipes) – A notorious invasive plant. Its thick floating leaves shade the water column, while its massive root masses and decaying tissues create high biological oxygen demand (BOD).

Watermeal (Wolffia spp.) – Tiny floating plants that can cover entire pond surfaces, leading to oxygen depletion, especially in stagnant water.

Dense submerged plants (e.g., Elodea, Hydrilla) – During daytime they produce oxygen, but if overgrown and then die back suddenly (e.g., after herbicide treatment), their decomposition consumes large amounts of oxygen, causing fish kills.

Key takeaway: Any aquatic plant can lower DO if it dies and decays. But floating invasive plants that blanket the surface, plus overgrown submerged vegetation that collapses, are the main culprits for oxygen depletion in real‑world situations.