04 · The Pond Ecosystem
Understanding the biology is what separates a pond that runs itself from one that fights you forever. This is the "why" behind every technique later.
The nitrogen cycle
Fish waste, uneaten food, and rotting leaves release ammonia (NH₃), which is toxic. Nature converts it through two bacterial steps:
- Ammonia → Nitrite (NO₂⁻) by Nitrosomonas bacteria. Nitrite is also toxic.
- Nitrite → Nitrate (NO₃⁻) by Nitrobacter bacteria. Nitrate is far less toxic.
- Nitrate is absorbed by plants as fertilizer, closing the loop.
Key takeaway
Beneficial bacteria live on every surface — rocks, gravel, liner, filter media. Maximize surface area and you maximize biological filtration. This is why "pond gravel" and bio-media matter.
The nutrient competition model
Algae and plants want the same things: light, nitrogen, phosphorus, CO₂. Whoever gets there first wins.
- Dense planted margins and floating leaves shade the water, denying algae light.
- Fast-growing oxygenators and marginal reeds out-absorb nutrients before algae can use them.
- Result: a pond where green water and string algae struggle to gain a foothold.
Oxygen: the invisible requirement
Every process — bacteria, fish respiration, decomposition — consumes oxygen. Warm water holds less. A pond needs continuous surface exchange (waterfall, stream, fountain) or it goes anaerobic and smells like rotten eggs.
O₂ demand ≠ O₂ supply ⇒ anaerobic ⇒ odor + fish loss
Keep circulation 24/7 in warm months.
Trophic layers
| Layer | Role |
|---|---|
| Producers (plants, algae) | Capture light/CO₂, release O₂, consume nitrate |
| Decomposers (bacteria, fungi) | Break down waste into plant food |
| Primary consumers (snails, daphnia, tadpoles) | Graze algae and detritus |
| Secondary consumers (fish, dragonfly nymphs) | Control populations, add life |
Balance, not sterility
A healthy pond is not crystal-clear water. A slight green tint means phytoplankton are producing oxygen. Aim for balanced ecology, not a swimming pool.
The 60/30/10 rule
60% of the water surface should be covered by floating or emergent plants by midsummer. 30% should remain open water. 10% can be submerged oxygenators. This ratio controls algae naturally.
Feed the biology, not the fish
Overfeeding fish is the #1 cause of cloudy water and algae blooms. Feed only what fish consume in 2–3 minutes, once daily in summer, and stop entirely when water drops below 8°C.