Hidden Depths of Zealandia Te Māra a Tāne lakes

Written by Freya Bacon-Bootham

At Zealandia Te Māra a Tāne we want to fix entire ecosystems, not just bring individual species back. No species or ecosystem exists in isolation, they are connected, the health of one impacting others. Ecological restoration is helping an entire ecosystem that has been degraded, damaged, or destroyed to recover. Considering the whole environment and the landscape in recovery is more effective as native species can reintroduce themselves, and the improved health of one ecosystem impacts the neighbouring ones. You can see this in the spillover of native manu/birds into Te Whanganui-a-Tara/Wellington from Zealandia Te Māra a Tāne. In Aotearoa, restoring whole ecosystems is important due to our numerous migratory species, such as tuna/eels, who need access to the ocean and back from their freshwater homes.   

Image credit: Janice McKenna

Aotearoa has a wealth of freshwater throughout the country. The problem is that many of these freshwater sources are unhealthy or degraded. Much of the damage has occurred due to agricultural run-off, wastewater, invasive species, and habitat destruction. Lakes make up a large proportion of our freshwater and hold significant intrinsic value, providing a stillness to the landscape and holding a place in personal and national identities. Zealandia Te Māra a Tāne has two man-made lakes, Roto Māhanga, the upper lake and Roto Kawau, the lower lake. The health of each of these lakes is very different, as Roto Kawau is less healthy and often has an algal bloom in summer.  

The Kaiwharawhara whaitua/catchment runs through Te Whanganui-a-Tara, with its headwaters beginning at Zealandia Te Māra a Tāne and ending at an open estuary into the harbour, the only coming from Wellington City. Thirteen species of native fish call the waters of the Kaiwharawhara home. Zealandia Te Māra a Tāne being the headwaters means that the water quality in our lakes and waterways impact the water and species living throughout the rest of the whaitua. This whaitua is an excellent local example of why whole ecosystem restoration is so important and the benefits of projects like Sanctuary to Sea. 

Image credit: Leon Berard

In the depths of Roto Māhanga lies one of the secrets to its health. Kākahi/freshwater mussels were translocated here in 2018 and are ecological engineers. They change their environment by filtering water and improving the water quality. Kākahi filter the water by feeding on various particles, such as phytoplankton, bacteria, and organic matter. They can filter up to 1 litre an hour, meaning if the population is big enough, kākahi can filter the volume of the entire lake in a few days. 

Unlike its ocean counterparts, this bivalve can’t stick to rocks, lacking the ‘beard’ to do so; instead, kākahi bury themselves in sediment. Their preferred environment is slow-moving water with a muddy substrate. Kākahi have a long history of being used as an indicator of water quality and abundance of other aquatic species like tuna/eels. Alongside being an indicator, kākahi were also a mahinga kai for iwi, and their shells were used to cut hair.  

Freshwater mussels are one of the world’s most threatened species, and the ones in Aotearoa are no different. Kākahi are threatened by unhealthy water overloading and blocking their gills, invasive plants destroying their habitat and introduced fish eating or outcompeting the native fish. Kākahi rely on native fish for their reproduction. The male kākahi releases sperm into the water, which the female then takes into her brood pouch to fertilise the eggs. Once the larvae has grown to the size of a grain sand, it leaves the mother and attaches itself to a native fish, such as toitoi/common bully, using a small tooth on its shell edge. The larvae kākahi stays attached to the host fish for about three weeks as it develops into a juvenile. It then drops off and buries itself in the sand for five years. The native fish playing this key role in the kākahi development are an example of the importance of whole ecosystem restoration.

Image credit: Scott Langdale

Banded kōkopu also call Roto Māhanga home and are an example of why connectivity between waterways is important. Banded kōkopu travel down to the mouth of the stream to breed, and the young then use their ability to climb and swim upstream to return to Roto Māhanga. 

Another important ecological engineer is kōura/freshwater crayfish, who can be found along the Te Māhanga stream and into Roto Kawau. As scavengers, kōura improve water quality by eating organic matter. They are mostly active at night and prefer areas with slow-moving water and shelter from predators. Your best chance of spotting them in the waterways at Zealandia Te Māra a Tāne is on a night tour. Introduced fish threaten kōura outside of Zealandia Te Māra a Tāne; perch in particular enjoy eating them. They were an important mahinga kai and were sometimes used in trades by iwi.  

Freshwater is a precious ecosystem filled with native biodiversity that needs our help. Protect the freshwaters near you and get involved in restoring them to a healthy ecosystem by joining community projects replanting the edges of streams or clearing the rubbish that falls into waterways. Find out more here.

You can get involved in restoring the Kaiwharawhara whaitua as part of the sanctuary to sea project.  Find out more here.