Lake and Reservoir Management 21(1):61-72, 2005
© Copyright by the North American Lake Management Society 2005
Strategies for Managing the Lakes of the Rotorua
District, New Zealand
Noel Burns
Lakes Consulting
42 Seabreeze Rd.
Devonport, New Zealand 1309
John McIntosh and Paul Scholes
Environment Bay of Plenty
5 Quay St.
Whakatane, New Zealand
Abstract
Burns N., J. McIntosh and P. Scholes. 2005. Strategies for Managing the Lakes of the Rotorua District, New Zealand. Lake and
Reserv. Manage. Vol. 21(1):61-72.
The Rotorua district in New Zealand contains 12 nationally important lakes. Environment Bay of Plenty (EBOP), which has the
responsibility of managing the quality of these lakes, set a routine monitoring program for these lakes and adopted the method of
Burns et al. (1999, 2000) to analyse the data and calculate a numeric Trophic Level Index (TLI) value for each. In 1994, the district
community indicated a goal to maintain the present condition for most of the lakes and to improve the remainder. As a result, nu-
meric baseline TLI values were written into the Proposed Regional Water and Land Plan as the Rotorua District lake-water quality
objectives. This plan also required formation of a community action plan for the remediation of any lake that exceeded its baseline
TLI, a criterion that targeted five lakes. Deterioration in the water quality of these lakes is linked to urban expansion and gradual
conversion of forested land to pasture over the past 100 years. Draft action plans identifying causes of lake deterioration, together
with possible means of solving the problems, have been published for four lakes. Annual reports on the state of each lake have
been published since 2000. This lake management system has resulted in valuable communication between EBOP, the Rotorua
District Council and the communities living around the lakes, and has been instrumental in obtaining a cooperative approach to
solving the identified problems. Methods to remediate these lakes include: converting pasture back to forest; alum dosing; creating
riparian strips along streambanks; developing wetlands; installing reticulated sewage systems, and; diverting wastewater inputs
from a lake into nearby forests.
Key Words: lake monitoring; trophic levels; baseline trophic conditions; action plans; groundwater nitrate; water quality trends,
global warming, New Zealand lakes.
Introduction management strategies to maintain the water quality of the
good lakes and improve the damaged lakes, and; (3) improve
The Rotorua District lies in the center of the North Island of communication of monitoring results and management ac-
New Zealand. It is in an elevated area (300m above sea level) tions to the Rotorua District community.
and is volcanic in origin. The terrain is hilly, with the steeper
New Zealand is divided into 14 environmental regions on a
parts covered in native or pine forest and the less steep parts
discrete watershed basis, with few river watersheds belong-
developed into sheep, beef or dairy pastures. Twelve lakes
ing to two different regions. Each region has a Regional
(Fig. 1), each of widely differing character, lie in this region.
Council that has full custodial responsibility for protecting
These lakes need careful management because the Rotorua
and managing all the natural resources in its region. Lake
District, with a world-renowned trout fishery and a number
Rotorua has been monitored on an intermittent basis since
of dramatic thermal areas, is one of the most important tourist
mid-1960s, while the other lakes have been sampled periodi-
areas in New Zealand. This paper describes the development
cally. Many of the lakes have been researched at times by
and implementation of data analyses used to: (1) identify
scientists from different organizations within New Zealand
changes in lake water quality more precisely; (2) establish
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Burns, McIntosh and Scholes
Figure 1-A map of the Rotorua District, New Zealand showing the twelve lakes under management in this district.
62

Strategies for Managing the Lakes of the Rotorua District, New Zealand
(Jolly 1968, McColl 1972, Fish 1975, Rutherford 1984, Vin- the nutrients from the wastewater that percolated through
cent et al. 1984). By the 1970s Lake Rotorua had degraded the forest soils were absorbed and filtered, then entered the
significantly. By 1990, it became apparent that many of the Waipa stream and subsequently Lake Rotorua.
other lakes were deteriorating as well. Accordingly, EBOP
In the late 1970s, the Upper Kaituna Catchment Control
upgraded its program of monitoring the lakes to determine
Scheme was implemented to slow the deterioration of water
where management was required and to gauge the effects of
quality in the two largest Rotorua district lakes, Lakes Ro-
the management actions.
torua and Rotoiti. The lake management techniques employed
The soils of the lake watersheds tend to be phosphorus-ab- to help control the problem included tree planting on erosion-
sorbing due to their allophane clay content, but the waters prone soils, retirement and planting of riparian zones, and
coming from cold springs in the watersheds tend to be high preservation and restoration of wetlands and lake margins.
in phosphorus (Timperly 1983) due to minerals dissolved The original control scheme ended in the 1980s and its suc-
from the underlying geology. The pumice soils of this region cess evaluated in a study based on the Ngongotaha stream
are naturally low in nitrogen (Vincent 1982) and, in the for- watershed by comparing stream nutrient loads before and
est situation, the nitrogen is recycled with a relatively small after watershed modifications (Williamson et al. 1996). The
amount lost to leaching. These factors result in good water study measured nitrogen and phosphorus in streams and rain-
quality for lakes in an unmodified condition, but with phyto- water runoff from different land use types. Land management
plankton growth limited by nitrogen availability (N-limited) techniques as implemented in the Upper Kaituna Catchment
rather than phosphorus availability (P-limited). The pasture Control Scheme have been, and are being applied to all lake
soils have increased levels of nitrogen due to fertilizer, watersheds to sustain or improve water quality.
nitrogen-fixing clover and grazing animals. The livestock
enhance the turnover of the nitrogen causing these soils to
Lake Monitoring
leach nitrogen, with some soluble nitrogen entering nearby
streams and some entering deep ground water. A program of routine monitoring of the twelve lakes com-
menced in 1990 and is ongoing. Initially all lakes were
In the early 1900s most of the lakes were N-limited, and a
monitored bimonthly; later, the less stressed lakes were
number of them have remained that way. However, due to
placed on a biannual monitoring schedule. Each lake has one
the increased supply of soluble nitrogen from land based
sampling station where temperature and dissolved oxygen
activities, a number of the lakes have become P-limited,
profiles are taken, secchi depths are measured, and epilim-
while some are now co-limited. Farming and urbanization
nion and hypolimnion samples are collected. All samples are
have led to eutrophication of these lakes, while the tourist
analyzed for dissolved and total nutrients, pH, conductivity
industry, in particular, requires that the lakes remain suitable
and turbidity. Epilimnion samples are also analyzed for
for recreation. These two opposing human activities require
chlorophyll and phytoplankton species. All results are stored
the EBOP to manage the Rotorua lakes and their watersheds
on the EBOP database. As is the case with many databases,
with great care. The methods used to reduce nutrient inputs
the data storage is secure and available, but not conveniently
to the lakes are: conversion of pasture back to forest; alum
accessible for quick use or reference. From 1995-1998, a
dosing; creation of riparian strips along streambanks; devel-
number of reports were issued (e.g., EBOP 1997, Burns and
oping wetlands; replacement of septic tanks with reticulated
Rutherford 1998) giving summary values of the different
wastewater systems, and; diversion of wastewater inputs from
variables. The condition of each lake was also described in
a lake into nearby forests.
general terms. No trend analyses were carried out. A review
of the monitoring program concluded that the assessment of
lake state, and the change of each lake with time, needed to
Methods of Managing the Rotorua be much more precisely determined while the degradation
Lakes of some of the lakes was still minimal. Also, the need for
better information was driven by the rising expectation of
Decreasing Inputs to the Lakes the lakeside communities that monitoring results would be
available on an annual basis.
Rutherford (1984) examined available data on Lake Rotorua
and determined that there had been a considerable increase
in total nitrogen (TN) and total phosphorus (TP) levels in
Lake Assessment
the lake since measurements by Fish (1975). Increases in
discharge of treated wastewater to the lake from the city In 1999, the decision was taken to improve the then-current
of Rotorua were strongly correlated with the increased TP lakes monitoring program and its reporting capability. First,
and TN levels, and as a result, a plan to decrease loading to steps were taken to ensure that all data obtained would be
the lake by spraying treated Rotorua City wastewater into high quality. All EBOP field procedures for the collection of
a nearby forest was implemented in 1991. The majority of samples were inspected by an outside agency and all labora-
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Burns, McIntosh and Scholes
Table 1-Lake types, trophic levels and values of the four key variables that define the different lake types.
Chla Secchi Depth TP TN
Lake Type Trophic Level (mg m-3) (m) (mg P m-3) (mg N m-3)
Ultra-microtrophic 0.0 to 1.0 0.13 - 0.33 31 - 24 0.84 - 1.8 16 - 34
Microtrophic 1.0 to 2.0 0.33 - 0.82 24 - 15 1.8 - 4.1 34 - 73
Oligotrophic 2.0 to 3.0 0.82 - 2.0 15 - 7.8 4.1 - 9.0 73 -157
Mesotrophic 3.0 to 4.0 2.0 - 5.0 7.8 - 3.6 9.0 - 20 157 - 337
Eutrophic 4.0 to 5.0 5.0 - 12 3.6 - 1.6 20 - 43 337 - 725
Supertrophic 5.0 to 6.0 12 - 31.0 1.6 - 0.7 43 - 96 725 -1558
Hypertrophic 6.0 to 7.0 >31 <0.7 >96 >1558
tory analytical procedures were re-examined. This was done same, namely 3.7. By doing this, individual TLx values from
in conjunction with a careful inspection of all data to identify a lake can be compared, allowing those that deviate most to
possible outlier values and remove them from the database. be identified. For example, a TLn value significantly lower
Efforts were then made to find a data interpretation procedure than the TLp value indicates the lake is N-limited. Similar
that was quantitative rather than qualitative. Previously, the TLn and TLp values indicate lakes that are co-limited. The
data analysis system only enabled the lakes to be classified in Trophic Level Index value (TLI) and its standard error is
the normal descriptive terms of oligo-, meso- and eutrophic. calculated for each lake and year from:
These non-quantitative terms, together with the lack of trend
TLI = 1/4 ( TLc + TLs + TLp + TLn).
assessment in the lakes, left room for considerable debate
as to whether certain lakes were degrading to the extent of
A lake classification scheme was developed from the TLx
requiring costly remedial work.
and TLI values (Table 1; Burns et al. 1999, 2000).
Results of research that classified trophic levels in lakes quan-
A procedure for deseasonalising the data on variables,
titatively based on chlorophyll a (Chla), Secchi depth (SD),
described in Burns et al. (1999, 2000), is used by EBOP to
total phosphorus (TP) and total nitrogen (TN) average annual
determine significant changes in a variable with time for each
values were published in 1999 (Burns et al. 1999). This pub-
of the four key variables in a lake. A significant change in a
lication also described methods of determining trends with
variable with time is evaluated in terms of its percent annual
time in the deseasonalised values from the lakes, as well as
change (PAC value). The similarity or difference in the PAC
recommending lake sampling procedures. In 2000, the New
values for the four key variables is statistically evaluated
Zealand Ministry for the Environment (MFE) endorsed the
based on the premise that most or all of the key variables in a
procedures described in Burns et al. (1999), including them
lake that has become more eutrophic should indicate a similar
in its publication, ‘Protocols for Monitoring Trophic Levels
degree of change toward a more eutrophic status (i.e., have
of New Zealand Lakes and Reservoirs’ (Burns et al. 2000).
similar PAC values). This procedure is used to distinguish
EBOP adopted these MFE Protocols to assess monitoring
between a change in a lake caused by a change in a single
results from the Rotorua District lakes.
key variable by, for example, a flood loading silt into a lake
and decreasing the SD, versus a change in most or all of the
In following the MFE Protocol, EBOP now first calculates
key variables in the lake resulting from an increase in nutri-
a trophic level for each annual average value of the four
ent loading. The greater the similarity in the change in the
key variables, Chla, SD, TP and TN, for each year for each
key variables, the greater the probability (see Table 2) that
lake (i.e., the TLx values where x = Chla, SD, TP and TN,
a trend will be designated as a “definite” change of trophic
respectively) using the equations shown below (Burns et al.
level: less similarity generates a lower probability, designat-
1999, 2000):
ing either a “probable” or “possible” change. A lake will be
TLc = 2.22 + 2.54 log(Chla) designated as undergoing no change if there is no similarity
in the PAC trend values of the different key variables (Burns
TLs = 5.10 + 2.60* log(1/SD - 1/40) et al. 1999, 2000) or if the PAC trends of the variables are
*coefficient revised in 2000. not statistically significant (p>0.05).
TLp = 0.218 + 2.92 log(TP) EBOP realized that a benefit of adopting the MFE Protocol
as their lake assessment method was that it could provide a
TLn = -3.61 + 3.01 log(TN)
numerical TLI value for each lake, and that this value could
then remain as a reference value for comparison with any
These equations normalize the annual average values, so
future TLI value. In 1994, the Rotorua District community
that for the average New Zealand lake, TLx values were the
64

Table 2.-PAC and TLI Report for Lake Rotoiti 1992-2003.
Lake Rotoiti 1992 to 2003 sites 1, 2 & 3 (1 Jul 1992 - 30 Jun 2003)
Percent Annual Change (PAC)
Chla SD TP TN HVOD Avg PAC Std Err P-Value
Lake (mg/m3) (m) (mgP/m3) (mg/m3) (mg/m3/day)
Change - Units Per Year 0.75 -0.06 0.84 2.91
Average Over Period 7.62 5.17 22.27 271.00
Percent Annual Change (%/Year) 9.84 1.16 3.77 1.07 0.00 3.96 2.06 0.15
Burns Trophic Level Index Values and Trends
Chla SD TP TN TLc TLs TLp TLn TLI Std. Err. TLI Trend Std. Err. P-Value
Period (mg/m3) (m) (mgP/m3) (mg/m3) Average TL av units/yr TLI trend
Jul 1992 - Jun 1993 7.58 5.32 20.43 267.10 4.45 3.51 4.04 3.69 3.93 0.21
Jul 1993 - Jun 1994 4.15 4.92 21.33 273.70 3.79 3.61 4.10 3.73 3.81 0.10
Jul 1994 - Jun 1995 4.48 5.37 19.97 253.27 3.87 3.50 4.02 3.62 3.75 0.12
Jul 1995 - Jun 1996 5.28 5.55 23.40 265.23 4.06 3.46 4.22 3.69 3.85 0.17
Jul 1996 - Jun 1997 5.13 6.06 17.70 244.67 4.02 3.34 3.86 3.58 3.70 0.15
Jul 1997 - Jun 1998 6.09 5.53 22.28 288.33 4.21 3.46 4.15 3.79 3.91 0.17
Jul 1998 - Jun 1999 6.49 4.40 27.00 285.07 4.28 3.75 4.40 3.78 4.05 0.17
Jul 1999 - Jun 2000 8.01 5.27 22.56 252.56 4.52 3.52 4.17 3.62 3.96 0.23
Jul 2000 - Jun 2001
Jul 2001 - Jun 2002 7.30 5.44 23.06 249.15 4.41 3.48 4.20 3.60 3.92 0.23
Jul 2002 - Jun 2003 18.46 3.95 30.91 339.86 5.44 3.89 4.57 4.01 4.48 0.35
Averages 7.30 5.18 22.86 271.89 4.31 3.55 4.17 3.71 3.94 0.07 0.04 0.02 0.0333
The guide used in the PAC average
Summary P-Value evaluation is:
PAC = 3.96 ± 2.06 % per year P-Value Range Interpretation
P-Value = 0.15 P < 0.1 Definite Change
0.1 < P < 0.2 Probable Change
Strategies for Managing the Lakes of the Rotorua District, New Zealand
TLI Value = 3.94 ± 0.07 TLI units 0.2 < P < 0.3 Possible Change
TLI Trend = 0.04 ± 0.02 TLI units per year 0.3 < P No Change
P-Value = 0.0333
Assessment
Mesotrophic
Probable Degredation
65

Burns, McIntosh and Scholes
indicated a wish for most of the lakes to maintain their pres- etation growing along the fenced-off stream banks (riparian
ent condition, with the balance of the lakes to be improved. strips) had retained particles washing from the watershed
As a result, numeric baseline TLI values were written into and had absorbed some soluble P.
the Proposed Regional Water and Land Plan (EBOP 2002)
The explanation for the increased content of nitrate in the
as lake water quality objectives for the Rotorua lakes. The
district streams relates to the effect of nitrogen leaching from
Proposed Regional Water and Land Plan also required re-
the urine spots of grazing animals. The resulting nitrate has
medial action plans to be formulated for any lake with a 3-
entered the groundwater and is now increasingly entering
year moving average TLI exceeding its baseline TLI values.
the streams. Some of this nitrate is over 50 years old (Tay-
Some of the baseline values are lower than the current TLI
lor 1977), indicating that the ground water in many cases is
values. Action Plans are based on lake modelling results to
deep. Even if the entry of nitrate into the groundwater were
determine the nutrient input reductions required for damaged
to cease today, the problem could take up to 50 years to
lakes to return to their baseline TLI values. Maintenance
disappear. Fencing of streams and lake margins to exclude
of the baseline quality from that existing in 1994 has been
grazing animals has been progressing around the Rotorua
accepted as a policy of the Regional Water and Land Plan
lakes since the 1970s, a method that has proved effective
(EBOP 2004).
for the management of surface runoff, but not for nitrate in
In order to implement these policies, EBOP now publishes sub-surface runoff.
an annual report on the status and recent changes (including
TLI, TLx and time trend results) in each lake to promote
Utilizing the Lake Assessment Results
Rotorua District community acceptance of the requirements
and costs of proper lake management. These assessments and The first results using the TLI system were made public by
the timely annual publication of these values and results are oral presentation at the 2001 Rotorua Lakes Symposium
made considerably easier for EBOP by the computer pro- (McIntosh 2001, Burns 2001), a public symposium held
gram LakeWatch (Lakes Consulting 2000), which employs biannually by the Lakes Water Quality Society in Rotorua.
the equations and concepts outlined in Burns et al. (1999, At this symposium, Burns (2001) explained the TLI system
2000). The program facilitates the determination of trends in and presented TLI information on each lake. McIntosh (2001)
all variables and calculates TLI values used in the EBOP an- presented information (Table 3) showing the 1994 baseline
nual lake status reports (Table 2; Scholes 2004). LakeWatch TLI and then-current TLI values for each lake, thereby iden-
also creates its own database of all the lake monitoring data, tifying the lakes requiring remedial action. Much discussion
enabling quick and easy access to any data and computed ensued, and some initial resistance to the concept of using TLI
result by EBOP staff. values as the basis of management decisions arose because
it was a new, untried approach to lake management in New
Another EBOP initiative is the endowment of a chair for lake
Zealand. Some also doubted that reliance on TLI values
research, namely the B.O.P. Chair in Lake Management and
would reveal ecological problems, such as a prevalence of
Restoration at the University of Waikato, Hamilton, New
cyano-bacteria or macrophytes in a lake.
Zealand, the closest university to the Rotorua lakes, enabling
an active program of research on processes in these lakes
(http://cber.bio.waikato.ac.nz/hamilton.shtml).
Table 3.-The Baseline TLI values determined for the 12
Rotorua District lakes.
Results of Monitoring and Draft Regional
Management Actions Water and Land 3-yr average
Lake Plan Baseline TLI TLI to 2000
Decrease in Inputs to Lake Rotorua Rotoma 2.3 2.3
Okataina 2.6 2.6
The diversion of Rotorua treated wastewater into the Waipa
Tarawera 2.6 2.6
forest reduced the nutrient inputs to Lake Rotorua by 33 x
Tikitapu 2.7 2.7
103 kg yr-1 of TP and 121 x 103 kg yr-1 of TN. These reduc-
Okareka 3.0 3.4*
tions were significant in terms of the external loads of 43 x Rotokakahi 3.1 3.2
103 kg yr-1 of TP and 536 x 103 kg yr-1 of TN to the lake in Rotoiti 3.5 3.9*
1998 (Burns 1999). Rerewhakaaitu 3.6 3.6
Rotomahana 3.9 3.8
Improvements to the Ngongotaha watershed were followed Rotoehu 3.9 4.7*
by decreases of 27% for particulate P, 26% for soluble P and Rotorua 4.2 4.6*
40% for particulate N, but an increase of 26% for soluble N Okaro 5.0 5.7*
in the stream water (Williamson et al. 1996). The new veg-
* Lakes exceeding their designated baseline TLI.
66

Strategies for Managing the Lakes of the Rotorua District, New Zealand
Lake Rotorua
After the 2001 symposium, dialogue continued between
EBOP and the Rotorua District community on the use of
Lake Rotorua is a big, relatively shallow eutrophic lake
TLI values for guidance about future remedial work. EBOP
occupying a volcanic crater, which has been largely filled
explained that the TLI was primarily a measure of trophic
in with sediment over time. The largest town in the region,
level of a lake; the index was not an index of infectious
Rotorua, is situated on its shores (Fig. 1) and has damaged
bacteria, specific phytoplankton-type presence or extent of
the lake by discharge of treated wastewater into the lake. The
macrophyte invasion. Lakes had to be sampled for specific
lake has a TLI of 4.9 and a baseline TLI of 4.2 (Table 4). It
problems as well as for trophic level indicators, and their
is N-limited (TLp-TLn = 0.7). The model predicted that the
specific problems considered as separate issues along with
decrease of treated wastewater input to the lake following its
changes in trophic level. Further, the use of four variables
diversion to the nearby Waipa forest would lead to slow-but-
in a summation-type numerical index, when compared with
steady improvement in the condition of the lake (Rutherford
earlier baseline values of the index, provided the possibility
1996), but this has not happened. The PAC values of the lake
of early detection of increasing trophic level. This dialogue
in 2003 were Chla = 9.5% yr-1; SD non-significant trend; TP
was assisted by the release of the Annual Reports on the
= −2.4 % yr-1; TN = 1.3% yr-1, indicating that algal and TN
water quality of the Rotorua Lakes (Scholes 2004). At the
concentrations have increased, while TP has decreased. These
2003 Rotorua Lakes Symposium, the use of the TLI system
changes are likely due to climatic warming (1992-2002) and
to determine which lakes required attention was generally
increasing loads of nitrate to the lake.
accepted. The presentations and discussions focused on the
means to remediate watersheds requiring improvement. From 1992-2000, the lake experienced a warming trend
of 0.19°C yr-1, with long warm, calm periods and the de-
velopment of sediment-water interface anoxia. The anoxia
Management Options and Actions on 12
resulted in large releases of soluble P and N to the overlying
Rotorua District Lakes water from the sediments, with single regeneration episodes
producing 178% and 84% of the annual loads of dissolved
Each of the twelve lakes is discussed below on the basis of
reactive phosphorus (DRP) and TP respectively, and 66%
the information supplied in Table 4 and other information
and 32% of the annual loads of total inorganic nitrogen
on each lake.
(TIN) and TN respectively (Burns 1999). Another factor
affecting Lake Rotorua, and in fact all the Rotorua district
Lake Okaro lakes, is the increasing nitrate content of most streams in
the area (Williamson et al. 1996, Rutherford 2003). This is
Lake Okaro is a small, supertrophic lake that has degraded,
a particularly important issue because Rotorua is N-limited,
largely because it is situated in fertile country that has been
and the TN entering the lake via the inflowing streams has
mostly converted to pasture. Table 4 shows that the Lake
been estimated to have increased by 245 x 103 kg yr-1 from
Okaro TLI exceeded its designated Baseline TLI by 0.5 tli
1984-2002. This compares with a reduction of about 128
units in 2003, and a draft action plan (McIntosh 2003a) has
x 103 kg yr-1 in the TN loading from the sewage diversion,
been developed to obtain community involvement in improv-
giving an overall increase of 117 x 103 kg yr-1 in loading to
ing its water quality. Lake Okaro is N-limited, with TLn less
2002 (Rutherford 2003). One indicator of positive response
than TLp (TLp-TLn = 0.5). Modeling of the lake and its
is that when the lake had a period without regeneration
watershed shows that the lake requires a reduction of 400 kg
episodes from July 1998-June 1999, the TLI was 4.3, down
yr-1 soluble phosphorus (SP) and 3,320 kg yr-1 total inorganic
from a TLI of 4.8 the previous year and close to its baseline
nitrogen (TIN) inputs. Internal loading from the sediments
value of 4.2 (Scholes 2004). Because the water quality of
of the lake has been calculated at 380 kg yr-1 TP and 2,400
Lake Rotoiti depends largely on the quality of the water it
kg yr-1 TN; thus, the external load reduction needs to be 20
receives from Lake Rotorua (Fig. 2), a draft action plan is
kg yr-1 TP and 920 kg yr-1 TN. Options being considered to
in preparation that combines the two lakes. Research into
achieve this reduction are: conversion of 2.0 km2 of the 3.37
nutrient regeneration processes was carried out by Waikato
km2 pasture area in the watershed to forestry (because of the
University in 2003 and 2004.
lower nutrient exports from forestry); fencing off 5- to 10-m
wide ungrazed riparian strips along the banks of the inflow-
ing stream, and; increasing the size of the wetland where the Lake Rotoehu
stream flows into the lake (McIntosh 2003a). While these
Lake Rotoehu is a moderately sized, relatively shallow lake
options are being considered, a trial alum dose of 0.6 g/m3
that is intermittently stratified. The lake experienced a long
as aluminum was applied to the lake in December 2003 to
period of stratification in 1993. Prior to 1993, the lake had a
speed its recovery.
TLI of 3.8, just below its baseline value of 3.9 (Table 4), and
since 1993, the TLI has remained at 4.7, with occurrences
67

Burns, McIntosh and Scholes
Table 4.-Basic data on the 12 Rotorua District Lakes and their watersheds.
Lakes Okaro Rotorua Rotoehu Rotomahana Rerewhakaaitu Rotoiti
Lake Area (km ) 0.32 80.8 8 9 5.8 34.6
2
Max. Depth (m) 18 45 13.5 125 15.8 110
Av. Depth (m) 12.1 11 8.2 60 7 31.5
Av. Annual Chla (mg m-3) 33 14.8 12 5.1 5.3 7.3
Av. Annual SD (m) 1.6 2.5 2.3 4.2 5 5
Av. Annual TP (mgPm-3) 122 44 36 25 7.4 23.2
Av. Annual TN (mgNm-3) 1250 426 456 222 380 277
Av. TLc (TLI units) 5.9 5 4.8 4 3.8 4.3
Av. TLs (TLI units) 5 4.5 4.5 3.8 3.7 3.7
Av.TLp (TLI units) 6.3 5 4.8 4.1 2.7 4.2
Av.TLn (TLI units) 5.7 4.3 4.4 3.4 4.1 3.7
TLp - TLn 0.5 0.7 0.4 0.7 -1.4 0.5
Baseline TLI 5.0 4.2 3.9 3.9 3.6 3.5
3-yr. Average TLI to 2003 5.5* 4.9 4.7 3.7* 3.3* 4.3*
Watershed Area (km2) 4.07 507.8 56.7 79.9 38.2 118.6
Pasture 95.7% 51.8% 40.00% 41.40% 76.70% 23.90%
Forest/Scrub 4.3% 39.4% 58.70% 56.80% 20.90% 73.10%
Urban 0.0% 8.1% 0.00% 0.00% 0.00% 1.10%
Wetlands 0.0% 0.2% 0.40% 0.40% 2.40% 0.20%
*2 yr. Average
of sediment nutrient release almost every year. The lake
receives a fairly large loading of DRP from a spring and is
N-limited as a result (TLp-TLn = 0.4). It now experiences
large blooms of cyanobacteria each summer. Modeling of
the lake shows a reduction of 11.6 x 103 kg yr-1 and 100 kg
yr-1 t/yr of TIN and SP respectively is needed to achieve its
baseline TLI (McIntosh 2003b). A number of options are
being considered to reduce the annual loads, including: the
conversion of 3.85 km2 of pasture into forest, the installation
of fences 5-10 m from stream banks to create nutrient absorb-
ing riparian strips; the construction of additional wetlands,
with each 0.1 km2 of wetland predicted to remove about
2000 and 30 kg yr-1 of N and P respectively; the installation
of a trench filled with sawdust across one of the groundwater
inflows to denitrify groundwater that flows through it, and;
the injection of alum into a tributary stream as it flows over
a weir to remove phosphorus.
Lake Rotomahana
Lake Rotomahana is unusual, being strongly influenced by
Figure 2.-Map of Lakes Rotorua and Rotoiti showing the Ohau
geothermal sources. The water quality of the lake is good
channel and the Kaituna River.
and has been stable over the period of monitoring. The 2003
3-yr average TLI is 3.7, while the baseline TLI value for the
lake is 3.9 (Table 4). The TLI of the lake is showing signs of
steady improvement, possibly due to changes in the fluxes TLn = -1.4; Table 4) and went through a major period of
of geothermal inputs. deterioration from 1995-1997 when its TLI reached 4.2.
This period coincided with the lake returning to its ‘usual’
level after an earlier low-water period, which may have re-
Lake Rerewhakaaitu
leased phosphorus from the re-watered sediments. In 2003
Lake Rerewhakaaitu is strongly phosphorus limited (TLp- the TLI was 3.2 compared to its baseline value of 3.6. The
68

Strategies for Managing the Lakes of the Rotorua District, New Zealand
Table 4.-(Continued).
Lakes Okareka Tikitapu Okataina Tarawera Rotoma Rotokakahi
Lake Area (km ) 3.3 1.46 11 41.7 11 4.52
2
Max. Depth (m) 33.5 27.5 78.5 87.5 83 32
Av. Depth (m) 20 18 39.4 50 36.9 17.5
Av. Annual Chla (mg m-3) 4.5 2 2.1 1.6 1.5 3.89
Av. Annual SD (m) 6.9 6 9.2 8 10.9
Av. Annual TP (mgPm-3) 6.1 3.8 6.2 7 3.3 10.2
Av. Annual TN (mgNm-3) 225 196 123 122 136 209
Av. TLc (TLI units) 3.8 2.9 3 2.7 2.6 3.6
Av. TLs (TLI units) 3.2 3.4 2.8 3 2.5
Av.TLp (TLI units) 2.5 1.9 2.5 2.7 1.7 3.1
Av.TLn (TLI units) 3.5 3.3 2.7 2.6 2.8 3.4
TLp - TLn -1.0 -1.4 -0.2 0.1 -1.1 -0.3
Baseline TLI 3.0 2.7 2.6 2.6 2.3 3.1
3-yr. Average TLI to 2003 3.2 3.1* 2.9* 2.9* 2.5*
Watershed Area (km2) 18.7 5.7 56.8 144.9 29.1 18.7
Pasture 55.80% 3.5% 90.4% 75.5% 22.8% 72.2%
Forest/Scrub 40.70% 96.5% 9.6% 21.1% 71.5% 27.8%
Urban 2.90% 0.0% 0.0% 0.7% 1.1% 0.0%
Wetlands 0.20% 0.0% 0.0% 0.0% 0.2% 0.0%
*2 yr. Average
lake is hydraulically perched, so some of the groundwater nitrogen from pastures and the septic tanks in its watershed.
in its watershed does not enter the lake. An in-depth discus- The lake is stratified and reaches very low hypolimnetic DO
sion document on the management of this lake is available levels at the end of the stratified season. In the early 1990s the
McIntosh et al. (2001). lake did not become anoxic, but it now does just at the end of
the season and experiences some nutrient regeneration. This
development could be partly due to short term climate change
Lake Rotoiti
(the lake has warmed at the rate of 0.04°C yr-1 since 1994),
Lake Rotoiti is a deep lake that remains stratified for up to which results in slightly longer periods of stratification, that
8 months of the year. It receives approximately 70% of its in turn lead to the development of hypolimnetic anoxia at
nutrient load from Lake Rotorua via the Ohau Channel, which the end of the stratified season. A draft action plan for the
connects the two lakes (Fig 2). Lake Rotoiti is N-limited (TLp lake has been published, and modeling of Lake Okareka has
-TLn = 0.5), similar to Lake Rotorua (Table 4). In 1957 the shown that the load to the lake needs to be reduced by 2.320
Lake Rotoiti hypolimnion still had 29% DO saturation in and 70 kg yr-1 for N and P respectively (EBOP 2003). The
April (Jolly 1968), a condition now normally found in Janu- options for obtaining such a decrease in load are: to convert
ary of each year. As the water quality of Lake Rotorua has 5.15 km2 from pasture into forest; to create 5- to 10-m wide
deteriorated, so has the water quality of Lake Rotoiti (Vincent fenced riparian strips along streams, and; to divert a stream
et al. 1984). The lake is now anoxic for up to three months per so that it enters the lake through a wetland, removing a pre-
year and annually experiences large scale nutrient regenera- dicted 300 kg yr-1 and 10 kg yr-1 from N and P loads to the
tion. The lake baseline TLI of 3.5 existed before large-scale lake respectively. The possibility of connecting the septic
nutrient regeneration occurred. This baseline is exceeded by tanks of the homes in the watershed to a treatment plant also
the 2003 3-yr average TLI of 4.3 (Table 4). A joint action exists, which could reduce the N input to the lake by 1940
plan for Lakes Rotorua and Rotoiti is in preparation. Serious kg yr-1 and the P input by 10-20 kg yr-1.
consideration is being given to deflecting the Lake Rotorua
inflow from the Ohau Channel to the nearby Kaituna River,
Lake Tikitapu
which is the outflow from the lake (see Fig.2).
Lake Tikitapu, is a P-limited (TLp-TLn = -1.4; Table 4),
relatively small lake with good water quality that is frequently
Lake Okareka
used for recreational water sports. This lake provides an
Picturesque Lake Okareka is a borderline oligotrophic/meso- example of the need for careful monitoring combined with a
trophic lake. It is strongly P-limited (TLp-TLn = -1.0; Table quantitative water quality assessment system. No noticeable
4), possibly because it receives a relatively large supply of deterioration of the water quality has been reported by the
69

Burns, McIntosh and Scholes
users of the lake, but monitoring data show that anoxia is oc- erford 2003) and planting of trees in the lake margins of Lake
curring in the bottom waters, possibly triggering phosphorus Rotoehu. A number of farmers have modified dairy-shed
release in this phosphorus-limited lake (Scholes 2004). If wastewater management to ensure that no wastes sprayed
the 2003/2004 monitoring data yield a 3-yr running average onto land subsequently enter streams. Plans to build small
above 2.7, then this lake will require preparation of an action waste treatment plants to eliminate septic tank usage in the
plan for its remediation. Lake Okareka and parts of the Lake Rotoiti watersheds are
under consideration. Action Plans have been recently is-
sued (EBOP 2003 McIntosh 2003a, 2003b) with subsequent
Lake Okataina
in-depth discussion of possible options, between lakeside
Lake Okataina is a good quality oligotrophic lake with similar residents, farmers and the Regional and District Councils to
TLp and TLn values (TLp-TLn = -0.2; Table 4). Lake-level decide on management actions for lake remediation.
change can influence the quality of this lake, and some water
In lakes that are strongly P- or N-limited, the 4-variable
quality indicators (Chla, SD) show signs of deterioration
TLI does not always give a true reflection of changes in the
(Scholes 2004), creating a need for continued monitoring.
productivity of a lake. The concentration of the limiting nutri-
ent may be increasing while that of the non-limiting nutrient
Lake Tarawera may be declining, causing the calculated TLI to not change
much even while the productivity of the lake is increasing,
Lake Tarawera is a balanced lake with similar TLp and
as in the case of Lake Rotorua. Examining and sometimes
TLn values (TLp-TLn = 0.1; Table 4). It is a good quality
taking guidance from the TLx values rather than the TLI is
oligotrophic lake but may be in a declining state. The TLI
important. In the case of lakes strongly limited by the avail-
exceeds the baseline TLI of Environment Bay of Plenty’s
ability of one nutrient only, it may be preferable to calculate
regional plan (Table 4).
the TLI from the TLc, TLs and the TLx of the limiting nutri-
ent only and use this 3-variable TLI for guidance. Particular
Lake Rotoma attention should be paid to the TLc because chlorophyll
concentration is the most direct measure of trophic condition.
Lake Rotoma was not monitored from July 1996-June
However, monitoring the TLx of the non-limiting nutrient is
2000. Before this non-sampling period, the TLI of the lake
always advisable, because it indicates the potential increase
was below its baseline level of 2.3, but in the two years of
in the trophic level of a lake if more of the limiting nutrient
monitoring from July 2000-June 2003, its TLI has averaged
becomes available.
2.5. It is strongly P-limited (TLp-TLn = -1.1; Table 4). The
increased average TLI value indicates that some deteriora-
Another well-known lake water quality index is Carlson’s
tion in the water quality may be occurring. At this stage in
Trophic State Index (TSI; Carlson and Simpson 1996). This
the monitoring it is difficult to tell whether negative trends
index was developed using data from American lakes, while
in water quality are cyclic and may recover, or are ongoing
Burns’ TLI index (Burns et al. 1999, 2000) was developed
(Scholes 2004).
using New Zealand lake data. Both systems use the same
four key variables. The TLI system is used in New Zealand
Lake Rotokakahi and has the feature that the boundary of each lake type is
denoted by an integer value (Table 1). The two systems give
Lake Rotokakahi is privately owned, and no recent monitor-
similar results, with TLI values being equal to TSI values
ing has been carried out. Monitoring of the Te Wairoa Stream
calculated from the same data set when divided by 11.0
at the outlet of the lake shows nutrient levels to be similar to
(i.e., TLI = TSI/11.0; Burns and Bowman 2000). The TLx
levels previously recorded on Lake Rotokakahi (Burns and
and TSx values are similar with TLc = 1.078(TSc/11.0),
Rutherford 1998). A TLI estimated using nutrient and Chla
TLs = 0.986(TSs/11.0), TLp = 1.079(TSp/11.0), TLn =
data from the Te Wairoa Stream shows a slightly elevated
0.868(TSn/11.0).
TLI to that calculated from Lake Rotokakahi water quality
data (Scholes 2004). EBOP realized the importance for long term lake manage-
ment to use rationalized TLx or TSx values for the four key
variables so that they can be combined into a numerical lake
Discussion index value. Comparison is then possible with previous or
future index values, the only way absolute change in lake-tro-
The major actions to date on nutrient input management
phic level can be readily observed. As Havens (2004) points
have been the diversion of wastewater from Lake Rotorua,
out, scientists have different concepts of what constitutes
research on watershed management techniques (Williamson
eutrophy, and these concepts can change with time, as in the
1996), formulation of the Water and Soil Plan (EBOP 2002,
case of Lake Okeechobee. If possible, lake managers should
2004), evaluation of the groundwater nitrate problem (Ruth-
70

Strategies for Managing the Lakes of the Rotorua District, New Zealand
References
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72