AUCKLAND’S LAKES

Take an interactive journey into the ecological history of Auckland’s lakes and discover how these lakes have changed over the last 1000 years.

AUCKLAND’S LAKES

The Auckland region is home to a diverse range of lakes with approximately 30 lakes greater than one hectare in size. Valued by many for their recreational amenities, these lakes also have a high cultural significance and provide essential habitat for our freshwater species.

The seven largest lakes (Kereta, Kuwakatai, Rototoa, Spectacle, Tomarata, Wainamu and Pupuke) have been regularly monitored since 1988. These lakes are classed as dune lakes except for Lake Pupuke which is a volcanic lake.

The health of these lakes and the organisms that live in them vary and is directly linked the historical changes that have occurred over the last 1000 years.

Let’s explore how these lakes have changed.

THE PROBLEM

Over the 800 years since human arrival, the ecosystem surrounding Lake Rototoa has been changed by human activity. These impacts have become more prominent in recent history, and the lake is now showing signs of stress, with a decline in water clarity and the health of native animals and plants. Monitoring between 2003 and 2011 has shown a concerning decline in all native species. Dwarf inanga are now virtually extinct in the lake after a decline of more than 99%. Kōura numbers have been reduced by 90%, and toitoi (common bully) by 80%.

Like many other lakes around New Zealand, Lake Rototoa’s ecological health is affected by a range of factors.

Rudd are a pest fish that were illegally released into the lake in the 1970s. Feeding on native aquatic plants, Rudd likely contributed to the reduction in water quality. Thirty years on, Perch (another pest fish) were illegally released into the lake in an effort to control the Rudd population. Perch change the food webs in lakes, effecting native fish. Pest fish often dominate ecosystems were they are introduced.

“Competition between native species is a fine balance in lake ecosystems. Introduced exotic fish are particularly harsh on native species as they are highly effective predators and rapid reproducers. Thus, their rate of predation of native species is far greater”.

Dr David Kelly – Freshwater ecologist. Cawthron Institute.

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PEST INVASION

Rising air temperatures due to climate change can reduce the mixing of lake water, which is part of the lake’s natural cycle and helps to keep the lake healthy. This infographic shows why the mixing of lake waters is essential for lake health.

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CLIMATE CHANGE

Modern-day agriculture methods rely on fertiliser such as nitrogen to aid pasture growth. The nutrients bind to soil particles and are transported into the lake following rainfall through run-off and groundwater. An oversupply of nutrients in the lake has negative consequences for water quality.

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AGRICULTURE

Forest vegetation acts as a filter reducing the amount of sediment and nutrients that enter waterways. Reducing the native forest of the native forest around the lake has increased levels of nutrients in the lake. These nutrients are a food source that encourage aquatic plant and algal growth and have a negative impact on lake health.

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FOREST REDUCTION

Nutrients provide a food source for aquatic plants and algae. However, when nutrients increase beyond natural levels, this has negative consequences for water quality. The increased nutrients favour certain species of algae and bacteria, causing them to multiply and change the dynamics of the lake ecosystem. The increase in nutrients entering the lake is mostly caused by changes in land use around the lake.

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NUTRIENTS

When pine trees are clear-felled at harvest time, there are significantly fewer roots holding the soil in place, bare earth is exposed and can erode into the lake. This soil reduces water clarity and can lead to an increase in nutrients.

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PINE PLANTATIONS

LAND USE CHANGE

Land use change over the last 50 years is evident from aerial images. Sand dunes once dominated the area to the west of Lake Rototoa, beyond the strip of native forest that lines the western shores today. In the 1970s pine trees were planted to stabilise the moving sand dunes. Pine forests typically managed on a 20-25 year rotation from planting to harvest. After the trees are felled the soil lays bare for months, if not years, and is susceptible to erosion by rainfall for up to eight years.

Native coastal forest once surrounded the rest of the lake, where farmland is present today.

THE SCIENCE

To understand what is causing the decline in Lake Rototoa’s health, we need to explore how the lake has changed since human arrival about 800 years ago.

Lake sediments are natural archives that continuously record environmental history, providing measures of current and historical lake condition equivalent to many centuries of environmental monitoring.

Everything that happens in or around the lake leaves a trace in the sediment, and this helps to build a picture of how lake ecosystems have changed over the last 1000 years—identifying whether current changes to lake health are part of the lake’s natural cycle.

LAKE ROTOTOA’S SEDIMENT CORE

Pollen grains are used to identify what types of vegetation lived around the lake. This, in turn, shows how the landscape has changed historically. An abundance of pollen from native trees suggests a largely unmodified landscape. Pollen from bracken fern is often an indication of land clearance. Grass pollen suggests pasture, while pine pollen shows up when pines have been planted in the region near the lake.

Pollen on lake surface.

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POLLEN

Charcoal is used to reconstruct the occurrence of fires. Throughout Māori and European eras fires were used as a tool for land clearance.

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CHARCOAL

Bacteria are sensitive to changes in the chemical makeup of the water and sediment. Certain species of bacteria thrive under specific conditions; for example, some flourish when dissolved oxygen is limited, while others prefer increased nutrients such as nitrogen.

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BACTERIA

Algae occur naturally in lakes, but when there are shifts in temperature, light or nutrients the amount and type of algae changes.

Some algae, known as cyanobacteria, can grow very rapidly, especially in summer when they bloom by ‘joining hands’ and turn the water green. When the levels of algae in the water column increase it reduces the light and oxygen levels which can affect the health of animals living in the lake. They also produce toxins that are harmful to humans and animals and make the water unsafe for drinking and recreational activities.

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ALGAE

LAKE ROTOTOA: 1000 YEARS OF ECOLOGICAL TRANSFORMATION

The results from the sediment core show the changes that have occurred in the catchment and lake over the last 1000 years. A catchment is an area of land including the hills, forests and pasture from which water drains into the lake. Changes in the catchment, such as land-use can directly or indirectly impact the health of the lake.

HOW HAVE THESE CHANGES IMPACTED THE LAKE?

Dr. Andrew Rees, an environmental scientist at Victoria University of Wellington explains some of the probable causes for the decline in Lake Rototoa’s ecological health.

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PEST FISH

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INCREASED NITROGEN

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EUTROPHICATION

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FOREST REDUCTION

CALL TO ACTION

Lake Rototoa is often referred to as a jewel in Auckland’s crown. However, results show the health of this lake is in decline. There have been dramatic changes in the water quailty and types of organisms living in this lake. Bacteria that thrive in high nitrogen and low oxygen environments are increasing.

Worryingly, these conditions often favour species that are harmful to lake ecosystems and humans. For example, there has been a marked change in the number of cyanobacteria (toxic algae). If this increase continues, this could affect native species like kākahi (freshwater mussels), kōura (freshwater crayfish), banded kokopu, toitoi (common bully) and impact our ability to use this lake for recreation activities.

However, this decline has only occurred in recent history, and it is not too late to help this lake. Urgent action needs to be taken to halt and reverse this decline by:

Controlling pest fish numbers.
Ensuring there is a wide strip of native vegetation planted around the entire lake.
Ensuring that best practice land management plans are in place in the broader catchment to minimise nutrients and sediment entering the lake.
More frequent monitoring to help increase the understanding of changes occurring in the lake.

Lakes380 Our Lakes’ Health: past, present, future

Lakes380—Our Lakes’ Health: past, present, future is a five-year research project that will enrich our understanding of the environmental, social and cultural histories of about 10% of New Zealand’s 3,800 lakes greater than 1 ha. This involves collecting and analysing lake sediments and water samples, as well as interviews. Currently, approximately 200 lakes are monitoring with data spanning 30 years or less. Capturing 1000 years of lake history will contribute new knowledge to assist in establishing informed and achievable restoration aspirations; now and for generations to come.

PRODUCER & DESIGNER

Mckayla Holloway (Cawthron Institute)

WEB DEVELOPER

Rowan Satherley

CONTRIBUTORS

Lizette Reyes, Marcus Vandergoes, Xun Li (GNS Science).
Jane Groom (Auckland Council). Rose Gregersen (Auckland University).
Andrew Rees, Jamie Howarth, Sarah-Jane O’Connor (Victoria University of Wellington).
Georgia Thomson-Laing, Heni Unwin, John Pearman, Jonathan Puddick, Lucy Thompson, Susie Wood (Cawthron Institute).
The Lakes380 team.

Crown copyright ©. Unless otherwise stated, this copyright work is licensed for re-use undera Creative Commons Attribution 4.0 International licence (CC BY-NC-SA 4.0). Except for any photographs, please seek permission by sending a request to the stated image owner.