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Ecology
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.
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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-.
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.)
Cyanobacteria bloom from Lake Rotorua. Image courtesy of David Burger.
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
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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.
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):
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 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.
And sampling data clearly show the anoxic conditions that prevail in the deeper waters of many of the region's lakes:
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.
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.)
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Pest fish removal and ecosystem health
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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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Denitrification beds - dealing to nutrient problems
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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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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
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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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