Inside Dairy | Why ponds need wetlands

Myth

A duck pond is just as effective as a constructed wetland for reducing nitrogen loss.

Busted

Ponds can be a great addition to a dairy farm. They create habitat, trap sediment and phosphorus, and often attract ducks, making them a valued place to spend time with mates during open season. They also look good and may remove small amounts of nitrogen.

But if your goal is reducing nitrogen loss, a traditional open-water pond isn’t specifically designed to deliver that outcome.

Why ponds don’t work on their own

Nitrogen removal in wetlands happens through a natural process called denitrification (see graphic). This relies on bacteria that need specific conditions: shallow water, wetland plants, organic matter and slow water movement.

Deep, open-water ponds don’t provide those conditions. Water tends to move through too quickly and lacks the dense vegetation needed for effective nitrogen processing. Constructed wetlands are designed specifically to reduce nitrogen and typically need:

70% emergent vegetation to support biological activity.
Shallow water (300–400 mm) for plants to grow and thrive.
A shape that allows water to move around the wetland, not through it (a width three to 10 times the length, with ‘fingers’ or peninsulas).
The right size: 1–5% of the contributing catchment area.

Together, these features increase water residence time, which is critical for nitrogen removal.

How a pond fits into a wetland

Ponds are built differently and their deeper open water is ideal for trapping sediment and phosphorus, creating bird and fish habitat and increasing biodiversity.

While ponds remove relatively little nitrogen on their own, they can improve the performance and life span of a constructed wetland when placed at the start of the system.

Start with a goal, then build a solution

If you’re considering a pond, wetland or a combination of both, start by asking yourself: ‘What am I trying to achieve?’. There’s no wrong answer, but having clarity helps.

Options might include:

Nitrogen reduction
Capture sediment and phosphorus
Improve biodiversity
Duck habitat and recreation
A mix of all of the above

Key design considerations

A pond needs to be large and deep enough to allow sediment and phosphorus to settle.
Good flow control so water moves slowly through the pond and into the wetland.
Planting native wetland species in the main wetland area is critical for denitrification and further nutrient removal.
The system works best when the wetland is at least 1-5% of the catchment area.

For more practical design tips, check out DairyNZ's constructed wetland guidelines.

Wetland development case study

Careful design and ecological planting turned this unproductive gully into a high-value terraced wetland.

Environmental DNA sampling from the main drain at the base of the gully identified more than 800 unique species living in and around the water, including an endangered native fish.

The digger operator worked carefully around the existing natural plants, lifting and replanting them on the wetland floor. By using what was already growing, this helped to keep costs down and supported native regeneration.

Read more about this project and others at dairynz.co.nz/wetland-development