The presence of frogs, damselflies, and diving beetles in a garden pond is often taken as a sign that something has gone right. These animals are sensitive to dissolved oxygen levels and water chemistry, and their appearance in a pond typically indicates that the biological cycle has reached a functional state. Understanding what drives that cycle is useful before introducing any organisms, and essential if problems arise later.
The Nitrogen Cycle in a Pond
Fish waste, decomposing plant material, and uneaten food are all sources of ammonia (NH₃) in pond water. In high concentrations, ammonia is toxic to fish and most invertebrates. In a biologically active pond, two groups of bacteria — Nitrosomonas and Nitrobacter — convert ammonia first to nitrite (NO₂⁻) and then to nitrate (NO₃⁻). This two-step process is called nitrification.
Nitrate at moderate levels is not directly harmful to most pond inhabitants, but it is a nutrient that drives algae and aquatic plant growth. Submerged oxygenating plants, marginal plants, and floating-leaved species all consume nitrate as part of normal growth. In a well-planted pond, this creates a competitive pressure that limits free-floating algae.
Key principle: Mechanical filtration (pump and filter box) accelerates the nitrogen cycle by increasing surface area for bacteria, but it does not replace the role of plants. A heavily stocked pond with a filter but no plants still tends toward algae problems in summer.
The nitrogen cycle takes time to establish. In a newly filled pond, the bacterial populations are absent or very low, which is why new ponds often go through a "green water" phase in the first four to eight weeks. Introducing a small amount of substrate from an established pond — a handful of gravel or a mature plant with roots and attached sediment — can seed the process.
Dissolved Oxygen
Oxygen enters pond water at the surface through diffusion and through photosynthesis by submerged plants during daylight hours. It is consumed by aerobic bacteria, by fish respiration, and by the decomposition of organic matter.
Water holds less dissolved oxygen as temperature rises. This is particularly relevant in Poland in July and August, when surface water in a shallow, south-facing pond may reach 24–27 °C. At these temperatures, even modest fish loads can create oxygen deficits overnight, when plant photosynthesis stops but bacterial and fish respiration continues.
Signs of oxygen deficiency include fish gasping at the surface in the early morning, and a strong smell of hydrogen sulphide (rotten egg) from disturbed sediment. Adding a simple fountain or waterfall — even a small submersible pump — significantly increases surface agitation and oxygen exchange. Keeping sediment accumulation low through partial water changes and removal of dying vegetation also reduces the bacterial oxygen demand.
pH and Its Management
The pH of a healthy pond typically sits between 7.0 and 8.5. During active photosynthesis in the daytime, submerged plants remove CO₂ from the water, which causes pH to rise. At night, respiration releases CO₂, and pH falls. In a heavily planted pond, daily pH swings of one to two units are normal and not harmful.
Problems arise when pH rises above 9.5 (typically in a heavily algae-infested pond in hot, still weather) or falls below 6.5 (usually from excessive organic decomposition or in regions with naturally acidic soil and rainfall). Water from a typical Polish municipal supply has a pH between 7.0 and 7.8, which provides a reasonable starting point.
| pH range | Condition | Likely cause |
|---|---|---|
| Below 6.5 | Too acidic | Excessive leaf decomposition, acidic rainfall, acidic soil leaching |
| 7.0 – 8.5 | Normal range | Balanced planting and moderate organic load |
| 8.5 – 9.5 | High (manageable) | Active photosynthesis, high plant density, summer heat |
| Above 9.5 | Problematic | Dense blanket weed or algae bloom, low organic buffering |
Adding barley straw (loosely bundled and floated or partially submerged) is a traditional, evidence-supported method of inhibiting algae growth in ornamental ponds. As barley straw decomposes slowly, it releases compounds that inhibit the growth of algae without significantly affecting higher plants or invertebrates. This method takes several weeks to become effective and works best as a preventive measure rather than a treatment for an established bloom.
The Role of Plants in Natural Filtration
A commonly used guideline suggests that floating-leaved plants should cover roughly one-third to one-half of the pond surface in summer. This shading reduces the light available to free-floating algae while keeping enough open water for gas exchange and wildlife access.
Submerged oxygenators — species such as Myriophyllum spicatum (Eurasian watermilfoil, native to Poland), Ceratophyllum demersum (rigid hornwort), and Ranunculus aquatilis (water crowfoot) — absorb nutrients directly through their leaves and provide a biological alternative to algae for the available nitrogen and phosphorus. One bunch of submerged plant per 0.1–0.2 m² of pond surface is a reasonable starting density.
Algae Types and Responses
Not all algae are the same problem. Single-celled, free-floating algae (green water) typically peaks in spring and early summer in new ponds before the biological cycle establishes. It is unsightly but not directly harmful to fish. Blanket weed — filamentous green algae such as Spirogyra and Cladophora — grows in dense mats, particularly in hard water with high phosphate. Blanket weed is more persistent and can physically tangle around plants and small animals.
- Green water (suspended algae): Usually self-resolves within one to two months as plants establish. Patience and adding oxygenating plants are the primary response.
- Blanket weed: Requires physical removal (wind around a stick weekly) combined with addressing the nutrient source. Barley straw extract can help suppress regrowth.
- Blue-green algae (cyanobacteria): A different organism, not true algae. Can produce toxins. If it appears as a thick, scum-like layer with a blue-green or grey-green colour and an earthy smell, partial water changes and increased oxygenation are appropriate responses. Prolonged blooms in a garden pond indicate excessive nutrient load — typically from fish waste or lawn fertiliser runoff.
Seasonal Cycle in Polish Conditions
In Poland, the pond year follows a reasonably predictable pattern:
- March–April: Ice melts, fish become active, first algae growth begins before plants are active. Avoid feeding fish until water temperature reaches 8–10 °C consistently.
- May–June: Rapid plant growth. Biological balance typically improves significantly from mid-May as oxygenators and marginals establish leaf area. Water clarity usually improves.
- July–August: Peak thermal stress. Monitor oxygen levels, remove heavy algae manually, reduce fish feeding if temperatures exceed 24 °C.
- September–October: Net the pond before major leaf fall. Remove dying vegetation promptly. Reduce fish feeding as temperatures drop below 10 °C.
- November–February: Minimal intervention needed. Keep a section of ice-free surface in severe winters (float a ball or run a small heater for fish ponds). Do not break ice forcefully as the shock wave can stun or kill fish.
External reference: The European Fish Association and the UK's Pond Conservation organisation publish freely available information on pond ecology applicable to central European conditions.
Image credits: Wikimedia Commons. Frog on lily — CC BY-SA 3.0. European common frog — CC BY 2.0.