Ponds are a wonderful addition to any garden. They are beautiful, encourage wildlife and improve your microclimate. Pond habitats are also a source of nesting materials for birdlife, seeds as food and leaf stems for some frogs to lay their eggs.

Construction

If you have the room, bathtubs and IBCs cut in half make excellent ponds.  They provide adequate water volume for a stable system. They are also simple to drain should the need arise. Ponds that are too small generally lack the buffering capacity to adsorb environmental changes such as fluctuations in temperature.

Figure 1 From left: bathtub submerged in mulch, IBC cut in half to make a pond.

As with any home aquarium, adding a layer of pebbles or gravel (not sand) to the bottom of your pond is also important. A layer at least 5cm deep but up to 15cm deep in an IBC for example will allow pond detritus to settle to the bottom and not be resuspended in the water column. A biofilm will form on this gravel over time and help to remove nutrients from the water.

Water

Water quality is very important for an aquatic environment. It is ideal to use rainwater to fill your pond however, if using tap water be mindful that it is likely to require pH adjustment and chlorine removal. Chlorine can most easily be removed by allowing water to sit in the open (i.e. in a bucket) for at least 24 hours which will allow the chlorine to dissipate.

Aim to refresh 10% of the pond volume at a time, refreshing the entire volume gradually over a period of 1 month. This is important to prevent stagnation and the excessive build-up of algae. Changing the water gradually each month prevents shocking the system and causing problems.

Plants

Plants are beautiful, encourage wildlife, improve your microclimate but most of all they are an excellent way to very simply filter and clean your pond water.

Create ponds with a combination of floating plants (i.e. Azolla), submerged plants (i.e. hornwort, Ceratophyllum demersum) and emergent plants such as native reeds (Juncus spp.).

Submerged

Ceratophyllum demersum (Hornwort). Hornwort has dark green brushy & brittle foliage. Floats just beneath the water surface or sinks to the bottom of ponds and is a very useful plant for combating algae in ponds and providing shelter for fish eggs and young.

Floating

Azolla pinnata (Ferny Azolla) Light green but sometimes reddish-brown foliage with flat and triangular foliage.

Emergent

Baumea juncea (Bare Twig Rush) Grey-green circular culm-like stems. Deep reddish brown spikelets sparsely produced during the warmer months.

In a healthy and well-maintained system approximately 1/3 of the water volume should be occupied by submerged plants. Similarly, 1/3 of the water surface should have floating plants and a third of the pond should have emergent plants. A guide would be 3 large reed like plants in a pond the size of a bathtub. Juncus sp. are a great choice.

Full sun is required for plants to grow properly and filter the water,

Emergent plants (reeds and the like) will grow best when partially immersed. This can be achieved by stacking bricks in the bottom of your container and placing the potted plant on top. Adding gravel to the soil surface will prevent soil becoming dispersed in the water column should the pot become fully submerged (due to rain for example).

A combination of emergent (i.e. reeds and sedges), surface (i.e. native azolla or duck weed) and submerged (hornwort) plants will help you achieve a balanced system. If you have fish, ensure that you have at least 1/3 of the water surface free to allow adequate oxygen transfer. Ponds will thrive in full sun so long as they are regularly topped up.

Mosquitos and Fish

Ponds should be located in full sun and incorporate native fish to eradicate mosquito wrigglers.

Figure 2 Western carp gudgeon (Hypseleotris klunzingeri).

Introducing one or two small native fish in a pond the size of a bathtub is all that is required to control mosquito larvae. It is important not to overstock your pond as this leads to poor water quality. Glass fish, soft finned rainbows, pacific blue eyes and honey blue eyes are all great native fish choices, as are gudgeons (Hypseleotris spp.) which are commonly sold cheaply as ‘feeder fish’ at aquarium shops.

Frogs

QLD Frogs Society has great information on frog-friendly gardens & recommended lists of native plants for frog ponds. http://www.qldfrogs.asn.au/frog-friendly-gardens/

Figure 3 Stripped Marsh Frog (Lymnodynastes peronii) and Wavy marshwort (Nymphoides crenata).

With frogs it really is a matter of build it and they will come but there are two key pond management considerations for successful frog breeding environments. These are toad management and tadpole predation.

Toad management is a matter of removing adults but more importantly discarding toad eggs and leaving frog spawn in place.

Figure 4 From left: Stripped marsh frog spawn (fluffy floating ball) and toad eggs (back strings that sink).

To minimise frog tadpole predation, introduce only pacific blue eye fish into your pond system. Pacific blue eye fish are ideal for frog ponds as this native fish is one of the few that won’t eat frog tadpoles with will eat mosquito wrigglers.

Maintenance

A successful pond ecosystem requires regular maintenance. Harvesting of plants, topping up with fresh water and removal of toad eggs should all be done regularly. Ponds require far more maintenance during hot weather and relatively little during winter.

Floating plants should cover no more than a 1/3 of your water surface. Skim off excess floating plants like azolla and use them to mulch the garden (just be sure to keep them away from local waterways as, even though they are native, they can still cause problems such as clogging waterways with excessive growth).

If the water is green, it is likely that there are inadequate emergent plants to remove nutrients from the water or the water may be overstocked with fish.

For the plants to grow properly and filter the water, a full sun location is best.

You will also need to exchange the water body periodically by topping up about 10% of the water volume at a time.

Figure 5 Pond with excessive floating vegetation, in this case azolla which is preventing the transfer of oxygen to the water column and right, pond with excessive algal growth due to insufficient vegetation and overstocking with fish.