Hidden Depths of ZEALANDIA’s Lakes

Every summer, some rivers and lakes around New Zealand turn soupy green, as algae (cyanobacteria) increases in abundance, blooming in the sun-warmed water. In normal numbers, the algae is an essential part of our freshwater ecosystems. When overabundant, some species that bloom also produce toxins which pose a health risk to humans and animals that come into contact with the water.  

While the risk to people and our pets is front of mind, these blooms often also pose issues for our native ecosystems.  

Even Wellington’s ZEALANDIA ecosanctuary is not immune. The sanctuary’s lower reservoir Roto Kawau is a man-made reservoir that was once part of Wellington’s water supply. Now it sits within 225 hectares of recovering native forest, behind the predator exclusion fence that keeps out rats, possums, mustelids and more, allowing endangered wildlife to thrive. 

As the weather warms during summer, algae begin to bloom in the reservoir. It’s unsightly, smelly, and makes the electric boat ride that ferries visitors from shore to shore much less pleasant. Worse, for many aquatic species, living in the lower reservoir is like living in the smoggy heart of Los Angeles, gasping for breath.  There are native fish species like banded kōkopu, koura/freshwater crayfish, and tuna/longfin eels living in the tributary streams that feed the reservoir, but they can’t thrive in the lake’s unhealthy environment. 

Why does the bloom occur? There are two main causes. The first is that the lake is very deep, which means that in summer it has a warm upper layer and a cold lower layer, where the sun doesn’t reach. This separation of the layers (stratification) combined with the buildup of nutrients at the bottom makes the upper layer nutrient poor and the lower layer nutrient rich. The algae at Zealandia can move up and down through the water, getting nutrients from lower in the lake in the evening and photosynthesising at the surface during day.  

The second reason is that ZEALANDIA’s lower reservoir has a very large population of introduced perch. The majority of these perch are tiny, and through sheer numbers have affected the delicate balance of the reservoir’s ecosystem. As Dr Susie Wood, Senior Scientist in Microalgae and Algal Biotechnology at the Cawthron Institute, says, “Fish such as perch eat small aquatic organisms that would usually feed on and reduce cyanobacteria.” With up to 22,000 perch in the lake, the tiny invertebrates like mayfly nymphs and water beetles who would normally graze on the algae, like sheep grazing on grass, are eaten and therefore cannot control its growth. 

All around the country this summer, lakes and rivers will be suffering the effects of algal blooms. In most places this is driven by a combination of factors: summer’s low water flows and warm temperatures, plus excess nutrients (especially nitrogen) from land run-off. 

In ZEALANDIA, by contrast, the protected sanctuary valley forms an ideal catchment for the small streams and seepages that flow through to the two reservoirs, then onwards into the Kaiwharawhara Stream and down to the harbour. The driver of the algal blooms in this case is not the quality of the catchment, but likely the perch living in the lake and the release of nutrients that results from temperature stratification. This gives ZEALANDIA the opportunity to tackle the problem in a way that is not possible in most river catchments. One of the key projects that ZEALANDIA is now exploring is the removal of perch so that the lake can be restored, becoming a safe habitat for endangered freshwater species in the same way that the valley itself is a sanctuary for birdlife, reptiles and invertebrates.