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Nitrate in waterways | The Rotorua Lakes | Pest fish removal | Pest fish in the Waikato | Denitrification beds | Useful websites |

Water resources are under increasing pressure from increased nutrient inputs, heavy metal and organic contaminants, and increased draw-offs. In Lake Rotorua, increased nutrient loads have led to frequent blue-green algal blooms, which are both a health hazard and unpleasant to look at.


Nitrate in waterways

In chemical terms, nitrate is an ionic compound that is made up of one atom of nitrogen and three atoms of oxygen. The molecule carries an overall negative charge: NO3-.

structure of a nitrate ion

Nitrate is highly soluble and leaches readily from the soil. Because it is a limiting factor for plant growth, when nitrate is readily available in waterways it can contribute to harmful algal blooms. These blooms, or population explosions of blue-green algae (Cyanobacteria) are typical of waterways that are strongly eutrophied. While eutrophication is a natural process, it is enhanced and accelerated by increased nutrient run-off into waterways as a result of human agricultural practices (including animal stocking rates and fertiliser application). Excessive nutrient loads cause rapid growth of algal populations, and we say there is a 'bloom' when you can easily see patches of algae in the water. (Remember that individual algal cells are microscopic, so that they have to be present in huge numbers before you can see them.)

algal bloom in lake rotorua
Cyanobacteria bloom from Lake Rotorua. Image courtesy of David Burger.

anabaena (cyanobacterium) showing heterocyst
Chain of cells from the cyanobacterium Anabaena, showing a heterocyte.
Image courtesy of Wendy Paul.

Why is this a problem? Apart from the toxins produced by many blue-green algae, when these masses of cells die their decay can remove oxygen from the water, particularly the deeper waters of a lake. This frees nutrients from the sediments at the bottom of the lake, which in turn can trigger further algal blooms when the deep and surface waters mix. The result is a form of positive feedback, where algal blooms die off and deep waters become anoxic, nutrient levels rise, and this in turn stimulates still more algal growth.

Animals can also be harmed by high levels of dissolved nitrate. Aquatic invertebrates and fish exposed to nitrate may be smaller, reach maturity later, or be less successful in reproduction. They may die at extremely high exposure levels. Early life stages of aquatic animals are more sensitive to nitrate than juvenile and adult animals, and amphibian tadpoles are particularly sensitive. In tadpoles, nitrate exposure can reduce the size and weight they have reached at metamorphosis. This may mean that they are less able to escape from predators, or to find food or mates.

Frogs and pollutants

Frogs are actually very sensitive indicators of environmental pollution. Occasionally you’ll see news stories about deformed frogs that have developed extra legs. Scientists have wondered whether these deformities are caused by environmental factors such as pollution, or perhaps parasitism. This idea has been tested by scientists in the US, who examined the effects of exposure to pesticides and parasites (they used a trematode worm related to liver flukes).

They found that tadpoles infected with trematodes developed extra legs, while tadpoles without parasites were normal. The next step was to expose tadpoles to pesticides (a control group was not exposed) and then take blood samples to look at the effect of pesticides on the tadpoles’ immune system before exposing the same tadpoles to trematodes. What was the result? Tadpoles exposed to pesticides had immune systems that didn’t work as well as normal tadpoles – and they also had a much higher rate of parasite infection.

Other studies have found that direct exposure to agricultural and industrial chemicals can also cause limb deformities in frogs. Frogs are sensitive indicators of pollution because they can easily absorb pollutants through their thin, sensitive skins.

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The Rotorua Lakes

The Rotorua Lakes are of considerable local, regional, and national importance. However, the majority of them are threatened - to a greater or lesser degree - by the environmental effects of increased nutrient levels. These nutrient loads come from human land use practices, including wastewater and sewage from urban areas and agricultural runoff. A major research and restoration effort is now under way in an attempt to maintain and improve the lake environments.

Many of the lakes, such as Okaro and Rotorua, are subject to algal blooms. These blooms are both unsightly and also a potential threat to health - and they signal that the lakes are in poor ecological condition.

How does this work? 

During the warmer months, a lake like Okaro stratifies into a warmer, less dense, upper layer (epilimnion) overlying a cooler, denser, deep layer (hypolimnion). During this period, there is little mixing between the two layers, and the deep layer becomes anoxic (lacking in oxygen). Anoxia in the bottom waters is produced through lack of resupply of oxygen from the surface and bacteria consuming the oxygen through respiration. Bacterial breakdown of dead plant and animal material produces ammonium as a by-product. Due to limited mixing, ammonium concentrations increase and become quite high in the bottom waters.  Similarly, phosphorus is locked into the sediments when the lake water is oxygenated, but when the bottom water becomes anoxic, phosphorus in its dissolved form is released into the water column. As the air temperature cools, the lake's surface temperature drops and the lake waters mix, bringing the nutrient-rich water to the surface and triggering algal growth.

phosphorus in lake rotorua sediments
Image courtesy of David Hamilton

The algal blooms that often follow mixing of the surface and bottom waters can be quite unsightly (image courtesy of Environment Bay of Plenty):

an algal bloom at the edge of lake rotorua

Scientists need ongoing data on the lakes' water quality in order to predict blooms and, in the long term, to help manage the environment so that blooms can be prevented or minimised.

One way of obtaining the necessary information is to use automated buoys that make regular data recordings and transmit these to a land station. A prototype buoy was placed in Lake Rotorua in July 2007 by a research team led by David Hamilton (University of Waikato).

prototype buoy for measuring water quality indicators
Prototype buoy for measuring weather conditions and water quality, Lake Rotorua
Image courtesy of Warwick Powrie.

Every 15 minutes the buoy records data on weather (wind speed & direction, air temperature, relative humidity, barometric pressure, rainfall) and water quality (surface and bottom dissolved oxygen, chlorophyll fluorescence, water temperature every 2m). Future buoys will also record pH, light absorption, redox potential and nitrate. Live data for the Rotorua station can be viewed here, using 'guest' as the user name and 'guest' as the password. 

Data from the buoy clearly show changes in surface (blue) and bottom waters (green) - the waters are stratified when there is a marked difference in their temperatures.

dissolved oxygen at surface & depth, lake rotorua

And sampling data clearly show the anoxic conditions that prevail in the deeper waters of many of the region's lakes:
dissolved oxygen in rotorua lakes & taupo
Decline of O2 with increasing depth, Rotorua Lakes & Lake Taupo.
Figure courtesy of David Hamilton & Chris McBride.
View a larger version of this image.

A comparison of modern and historical data (McColl, 1973) from the Rotorua lakes clearly shows how water quality has declined over the past 50 years. This is particularly severe in Lake Okaro, where the bottom waters have been anoxic for over 40 years. This means that mixing of the surface and deep waters regularly brings large amounts of dissolved nutrients to the surface, triggering significant algal blooms.

Image courtesy of David Hamilton

Concern over the health of the lakes has led to several recent initiatives to improve their water quality. Lake Okaro has been the subject of a combination of chemical treatment of the lake water - most recently an application of zeolite - and changes in land management practices (construction of a wetland at the lake's margin, reducing nutrient runoff from adjacent farms). These have seen a marked reduction in the amount of dissolved phosphorus in the water. (Download a figure showing the results of these treatments - image courtesy of David Hamilton.)

Application of zeolite to Lake Okaro. Image courtesy of Deniz Ozkundakci.

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Pest fish removal and ecosystem health

Pest fish, such as perch and koi carp, can also have a significant effect on water quality.  One example is the Karori Reservoir in Wellington, which has been subject to regular algal blooms. In natural waterways the algae are kept under control by crustaceans that graze on them, but the pest fish predate so heavily on the crustacea that this control of algal growth is lost.

Algal bloom in Karori Reservoir. Image courtesy of Susie Wood.

An on-going research project by a team of researchers from the University of Waikato has seen large numbers of perch removed from the reservoir. There have been two large-scale removals so far, through a combination of netting and electrofishing (using a specially equipped boat). Both events were followed by marked increases in the number of crustaceans and a decline in concentration of algae (phytoplankton) in the water (view the results here).

Electrofishing in Karori Reservoir. Image courtesy of Brendan Hicks.

Karori fish removal team: Brendan Hicks, Nick Ling, Dudley Bell,
Jeroen Brijs and Warwick Powrie. Image courtesy of Brendan Hicks.

Visit the LERNZ website for more information about these and other research projects. LERNZ stands for Lake Ecosystem Restoration NZ.

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Pest fish in the Waikato

In 2005 scientists from the University of Waikato carried out a survery of the lower Waikato River. Using the University's electrofishing boat, they fished 5.63km of waterways from 27 sites on the river and its tributaries - an estimated total area of 2.52 ha. The survey yielded a total of 2,915 fish.

That total represented 7 species of introduced fish and 6 native species. The most common species was common smelt, then koi carp and feral goldfish.  There were relatively few longfin eels, trout, bullies and torrentfish.

While the number of carp was less than that of smelt, they made up almost 70% of the total fish biomass caught in the river: 285kg of the total 410kg. The researchers commented that "biomass is a more accurate reflection of the potential ecological impact of koi carp than their density" (Hicks et al. 2005).

B.J.Hicks, N.Ling, M.W.Osborne, D.G.Bell & C.A.Ring (2005) Boat electrofishing survey of the lower Waikato River and its tributaries. CBER Contract Report 39
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Denitrification beds - dealing to nutrient problems

Nitrogen inputs into the New Zealand environment have increased dramatically in the last few decades. Too much nitrogen in the environment can lead to adverse effects including pollution of streams and lakes, production of greenhouse gases, and changes in biodiversity.

One source of excess nitrogen is wastewater reaching surface waters. Treatment systems have been designed to remove nitrogen from wastewater, but removing the lat bit of nitrogen before discharge is always difficult.

Louis Schipper (Earth & Ocean Sciences, Waikato University) and Stewart Cameron (GNS Science, Taupo) are trialling a new approach to reducing losses of nitrogen (in the form of nitrate, NO3-) to surface waters from wastewater - denitrification beds. These beds are essentially very large containers filled with wood chips, through which wastewater passes. The wood chips support denitrification, in which microbes convert nitrate to harmless nitrogen gas that's released to the atmosphere. Over time a complex group of microbes degrade the wood chips and provide simpler carbon compounds to the denitrifiers that produce the nitrogen gas.

A large denitrification bed for treating water discharges from a glasshouse.
Image courtesy of Louis Schipper.

To date, results have been impressive - large amounts of nitrogen have been removed from a range of wastewaters derived from dairy farms, glasshouses, and small subdivision.

Nitrate removal from glasshouse discharge prior to land application.
Note that the system was not designed to achieve total nitrate removal.
Image courtesy of Louis Schipper.
A larger version of this image is available to download.

The researchers continuing to investigate how to optimise these systems across a wide range of locations and wastewater types. They also need to determine how long these systems will operate without maintenance.
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Useful websites

There are a number of websites that provide good-quality resources on issues associated with nitrate in waterways. These are listed and linked to below, with the name of the source organisation and a brief description of each site.

NIWA | Environment Waikato | Environment BoP | LERNZ

NIWA resources

Estimating nitrate levels in the sea
An update on NIWA's research into predicting oceanic nitrate levels.

Nitrogen cycling in a crab’s burrow
Mud crabs (
Helice crassa) build networks of interconnected passages in estuarine mud. These passages assist the distribution of materials, such as oxygen and nitrate, through the sediments. Included in this website are  the importance of nitrogen in regulating primary production in a mangrove ecosystem, and the nitrogen cycle processes that occur in the swamp.

Keeping track of agricultural nitrates
Describes the use of ROTAN, a GIS-based modelling system of land-use practices and their environmental impacts.

Managing dairy nitrate pollution
A brief outline of NIWA’s research into the use of constructed wetlands and wood-chip filters in removing pollutants from dairy farm drainage. 

Importance of nitrate to fish population
A summary of research into the relationship between oceanic nitrate and the western hoki population. The site includes seasonal effects of nutrient dispersion in ocean surface, relative effect of fishing and environmental change on hoki population, and provides a pdf of the original research paper.

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Environment Waikato resources

Nitrate contamination of groundwater, Nitrate in groundwater
An introduction to the issues associated with nitrate contamination of groundwater. Focuses on Hamilton and Pukekohe as regions with high nitrate levels in groundwater. Provides general pointers in reducing nitrate in groundwater and links to other useful sites on nitrates in groundwater in the Waikato region.

Map of nitrate concentration around Waikato 
Gives an estimate of nitrate concentration in towns such as Pukekohe, Meremere, Morrinsville, Hamilton, Matama, Cambridge,  Te Awamutu and Taupo.

Fertiliser use on farms
Provides data on fertiliser use and leaching on dairy and sheep/beef farms in the Waikato region.

How nitrate is monitored in groundwater 
Describes where and how nitrate in groundwater is monitored in the Waikato region. Also gives an overview of the Ministry of Health’s guidelines and standards for nitrate concentrations in groundwater.

Example of a nitrate mitigating project 
A 2006 technical report evaluating a strategy for using artifical drainage flows to reduce nitrates in Waikato. Includes comparative data from other regions in New Zealand.

Treating land containing nitrates from dairy effluent 
The case study describes how a Waikato dairy factory is using a land treatment system to avoid discharging effluent into a local stream.

Example of report on nitrification and nitrate loss
This Environment Waikato report is a review of national and international literature on the effects of nitrification and urease inhibitors on nitrate leaching, greenhouse gas emissions and release of  ammonia from some pastoral systems.
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Environment Bay of Plenty resources

Changes in nitrate levels as groundwater ages
A 2004 report by the Institute of Geological and Nuclear Science on groundwater age, time trends in water chemistry and future nutrient load in the lakes of Rotorua and Okareka area. Includes background information on declining water quality of and physical hydrogeology of Lakes Rotorua and Akareka.

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LERNZ resources

Ways of reducing external nutrient load 
A brief description of point and diffuse source pollution and methods to reduce external nutrient load at these sources. Mentioned at this site are the purposes, advantages and disadvantages of  artificial/enhancement wetlands, riparian plantings, silt trap and fencing to exclude stock.

Measuring nitrate using a buoy (Real-life buoy pictures from the LERNZ photo gallery) 
The site describes the design and construction of a buoy used to measure the water temperature florescence, dissolved oxygen, redox, pH and chloride and nitrate ions of a lake.

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