What Festival Plant Waste Does to the Oxygen in a Pond

When a marigold garland sinks into a pond, bacteria begin breaking down the organic matter almost immediately. The decomposition of plant cellulose and pigment compounds is aerobic, it consumes dissolved oxygen. A single large garland introduces enough organic load to measurably depress oxygen levels in a small, shallow water body within 48 hours. Banana stems, which are dense with moisture and starch, decompose more slowly but release a larger total carbon load over days. Leaf cups and coconut husks follow the same curve. The problem is not any one offering. It is the volume arriving in a compressed window of time.

Freshwater fish require dissolved oxygen above roughly 5 milligrams per litre to feed, swim, and reproduce normally. Below 3 mg/L, most species enter physiological stress. Below 2 mg/L, fish begin surfacing and gulping air, the visible sign of a hypoxic event. The biological oxygen demand, or BOD, of decomposing plant matter spikes sharply in the first few days after immersion. In enclosed or slow-moving ponds with limited surface aeration, the water column cannot replenish oxygen fast enough. Bottom-dwelling fish and invertebrates, catfish, loaches, freshwater crabs, snails, are hit first because oxygen depletion begins at the sediment layer where decomposing material settles. Rohu, catla, and common carp, the species most common in urban Indian ponds, show measurable gill damage at sustained low-oxygen exposure even when they survive the event itself.

Decomposing organic matter releases phosphorus and nitrogen into the water column. These are the same nutrients that drive algal blooms. In warm water, and October in most of India is still warm, cyanobacteria, commonly called blue-green algae, can double their population in under 24 hours given sufficient nutrients and light. A dense bloom shades out submerged aquatic plants, cutting off their photosynthesis and removing a secondary oxygen source. When the bloom itself dies and decomposes, it triggers a second BOD spike. The result is a two-stage oxygen crash: one from the festival waste, one from the bloom it feeds. Cyanobacterial blooms also produce hepatotoxins and neurotoxins that accumulate in fish tissue and are lethal to waterfowl, dogs, and other animals that drink from or wade in the water.

The post-monsoon period, when most major festivals fall, is already the most biologically active time of year for Indian freshwater systems. Water temperatures are high, microbial activity is elevated, and dissolved oxygen is naturally lower in warm water than in cold. A pond in October is already running closer to its biological margin than the same pond in January. The mass arrival of organic material at this moment, rather than being spread across months, overwhelms the water body's buffering capacity. Smaller urban ponds with no inlet or outlet, which account for a large share of the water bodies that receive immersed offerings, have no mechanism to flush the dissolved nutrients or the deoxygenated water out.

Recovery depends almost entirely on the size of the water body and whether it has any flow. A large lake with active inflows may return to baseline dissolved oxygen within two to three weeks. A small urban pond with a heavy organic load and no circulation can remain hypoxic for months, with the sediment layer retaining elevated nutrient levels for longer still. Repeated annual loading without any organic matter removal progressively raises the pond's baseline BOD and reduces its resilience. Over years, this is one of the documented pathways by which a functioning freshwater ecosystem becomes a eutrophic, fish-depleted, algae-choked water body. The fish do not disappear in a single dramatic event. They thin out gradually, season by season, until the pond supports only the most hypoxia-tolerant species, and then none.