strategies for managing the lakes of the rotorua district new zealand

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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 BurnsLakes Consulting 42 Seabreeze Rd.

Devonport, New Zealand 1309

John McIntosh and Paul ScholesEnvironment Bay of Plenty

5 Quay St. Whakatane, New Zealand

AbstractBurns 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.

IntroductionThe Rotorua District lies in the center of the North Island of New Zealand. It is in an elevated area (300m above sea level) and is volcanic in origin. The terrain is hilly, with the steeper parts covered in native or pine forest and the less steep parts developed into sheep, beef or dairy pastures. Twelve lakes (Fig. 1), each of widely differing character, lie in this region. These lakes need careful management because the Rotorua District, with a world-renowned trout fishery and a number of dramatic thermal areas, is one of the most important tourist areas in New Zealand. This paper describes the development and implementation of data analyses used to: (1) identify changes in lake water quality more precisely; (2) establish

management strategies to maintain the water quality of the good lakes and improve the damaged lakes, and; (3) improve communication of monitoring results and management ac-tions to the Rotorua District community.

New Zealand is divided into 14 environmental regions on a discrete watershed basis, with few river watersheds belong-ing to two different regions. Each region has a Regional Council that has full custodial responsibility for protecting and managing all the natural resources in its region. Lake Rotorua has been monitored on an intermittent basis since mid-1960s, while the other lakes have been sampled periodi-cally. Many of the lakes have been researched at times by scientists from different organizations within New Zealand

Burns, McIntosh and Scholes

62

Figure 1-A map of the Rotorua District, New Zealand showing the twelve lakes under management in this district.


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(Jolly 1968, McColl 1972, Fish 1975, Rutherford 1984, Vin-cent et al. 1984). By the 1970s Lake Rotorua had degraded significantly. By 1990, it became apparent that many of the other lakes were deteriorating as well. Accordingly, EBOP upgraded its program of monitoring the lakes to determine where management was required and to gauge the effects of the management actions.

The soils of the lake watersheds tend to be phosphorus-ab-sorbing due to their allophane clay content, but the waters coming from cold springs in the watersheds tend to be high in phosphorus (Timperly 1983) due to minerals dissolved from the underlying geology. The pumice soils of this region are naturally low in nitrogen (Vincent 1982) and, in the for-est situation, the nitrogen is recycled with a relatively small amount lost to leaching. These factors result in good water quality for lakes in an unmodified condition, but with phyto-plankton growth limited by nitrogen availability (N-limited) rather than phosphorus availability (P-limited). The pasture soils have increased levels of nitrogen due to fertilizer, nitrogen-fixing clover and grazing animals. The livestock enhance the turnover of the nitrogen causing these soils to leach nitrogen, with some soluble nitrogen entering nearby streams and some entering deep ground water.

In the early 1900s most of the lakes were N-limited, and a number of them have remained that way. However, due to the increased supply of soluble nitrogen from land based activities, a number of the lakes have become P-limited, while some are now co-limited. Farming and urbanization have led to eutrophication of these lakes, while the tourist industry, in particular, requires that the lakes remain suitable for recreation. These two opposing human activities require the EBOP to manage the Rotorua lakes and their watersheds with great care. The methods used to reduce nutrient inputs to the lakes are: conversion of pasture back to forest; alum dosing; creation of riparian strips along streambanks; devel-oping wetlands; replacement of septic tanks with reticulated wastewater systems, and; diversion of wastewater inputs from a lake into nearby forests.

Methods of Managing the Rotorua LakesDecreasing Inputs to the Lakes

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 the lake since measurements by Fish (1975). Increases in discharge of treated wastewater to the lake from the city of Rotorua were strongly correlated with the increased TP and TN levels, and as a result, a plan to decrease loading to the lake by spraying treated Rotorua City wastewater into a nearby forest was implemented in 1991. The majority of

the nutrients from the wastewater that percolated through the forest soils were absorbed and filtered, then entered the Waipa stream and subsequently Lake Rotorua.

In the late 1970s, the Upper Kaituna Catchment Control Scheme was implemented to slow the deterioration of water quality in the two largest Rotorua district lakes, Lakes Ro-torua and Rotoiti. The lake management techniques employed to help control the problem included tree planting on erosion-prone soils, retirement and planting of riparian zones, and preservation and restoration of wetlands and lake margins. The original control scheme ended in the 1980s and its suc-cess evaluated in a study based on the Ngongotaha stream watershed by comparing stream nutrient loads before and after watershed modifications (Williamson et al. 1996). The study measured nitrogen and phosphorus in streams and rain-water runoff from different land use types. Land management techniques as implemented in the Upper Kaituna Catchment Control Scheme have been, and are being applied to all lake watersheds to sustain or improve water quality.

Lake Monitoring

A program of routine monitoring of the twelve lakes com-menced in 1990 and is ongoing. Initially all lakes were monitored bimonthly; later, the less stressed lakes were placed on a biannual monitoring schedule. Each lake has one sampling station where temperature and dissolved oxygen profiles are taken, secchi depths are measured, and epilim-nion and hypolimnion samples are collected. All samples are analyzed for dissolved and total nutrients, pH, conductivity and turbidity. Epilimnion samples are also analyzed for chlorophyll and phytoplankton species. All results are stored on the EBOP database. As is the case with many databases, the data storage is secure and available, but not conveniently accessible for quick use or reference. From 1995-1998, a number of reports were issued (e.g., EBOP 1997, Burns and Rutherford 1998) giving summary values of the different variables. The condition of each lake was also described in 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 be much more precisely determined while the degradation of some of the lakes was still minimal. Also, the need for better information was driven by the rising expectation of the lakeside communities that monitoring results would be available on an annual basis.

Lake Assessment

In 1999, the decision was taken to improve the then-current lakes monitoring program and its reporting capability. First, steps were taken to ensure that all data obtained would be high quality. All EBOP field procedures for the collection of samples were inspected by an outside agency and all labora-


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tory analytical procedures were re-examined. This was done in conjunction with a careful inspection of all data to identify possible outlier values and remove them from the database. Efforts were then made to find a data interpretation procedure that was quantitative rather than qualitative. Previously, the data analysis system only enabled the lakes to be classified in the normal descriptive terms of oligo-, meso- and eutrophic. These non-quantitative terms, together with the lack of trend assessment in the lakes, left room for considerable debate as to whether certain lakes were degrading to the extent of requiring costly remedial work.

Results of research that classified trophic levels in lakes quan-titatively based on chlorophyll a (Chla), Secchi depth (SD), total phosphorus (TP) and total nitrogen (TN) average annual values were published in 1999 (Burns et al. 1999). This pub-lication also described methods of determining trends with time in the deseasonalised values from the lakes, as well as recommending lake sampling procedures. In 2000, the New Zealand Ministry for the Environment (MFE) endorsed the procedures described in Burns et al. (1999), including them in its publication, ‘Protocols for Monitoring Trophic Levels of New Zealand Lakes and Reservoirs’ (Burns et al. 2000). EBOP adopted these MFE Protocols to assess monitoring results from the Rotorua District lakes.

In following the MFE Protocol, EBOP now first calculates a trophic level for each annual average value of the four key variables, Chla, SD, TP and TN, for each year for each lake (i.e., the TLx values where x = Chla, SD, TP and TN, respectively) using the equations shown below (Burns et al. 1999, 2000):

TLc = 2.22 + 2.54 log(Chla)

TLs = 5.10 + 2.60* log(1/SD - 1/40) *coefficient revised in 2000.

TLp = 0.218 + 2.92 log(TP)

TLn = -3.61 + 3.01 log(TN)

These equations normalize the annual average values, so that for the average New Zealand lake, TLx values were the

same, namely 3.7. By doing this, individual TLx values from a lake can be compared, allowing those that deviate most to be identified. For example, a TLn value significantly lower than the TLp value indicates the lake is N-limited. Similar TLn and TLp values indicate lakes that are co-limited. The Trophic Level Index value (TLI) and its standard error is calculated for each lake and year from:

TLI = 1/4 ( TLc + TLs + TLp + TLn).

A lake classification scheme was developed from the TLx and TLI values (Table 1; Burns et al. 1999, 2000).

A procedure for deseasonalising the data on variables, described in Burns et al. (1999, 2000), is used by EBOP to determine significant changes in a variable with time for each of the four key variables in a lake. A significant change in a variable with time is evaluated in terms of its percent annual change (PAC value). The similarity or difference in the PAC values for the four key variables is statistically evaluated based on the premise that most or all of the key variables in a lake that has become more eutrophic should indicate a similar degree of change toward a more eutrophic status (i.e., have similar PAC values). This procedure is used to distinguish between a change in a lake caused by a change in a single 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 key variables in the lake resulting from an increase in nutri-ent loading. The greater the similarity in the change in the key variables, the greater the probability (see Table 2) that a trend will be designated as a “definite” change of trophic level: less similarity generates a lower probability, designat-ing either a “probable” or “possible” change. A lake will be designated as undergoing no change if there is no similarity in the PAC trend values of the different key variables (Burns et al. 1999, 2000) or if the PAC trends of the variables are not statistically significant (p>0.05).

EBOP realized that a benefit of adopting the MFE Protocol as their lake assessment method was that it could provide a numerical TLI value for each lake, and that this value could then remain as a reference value for comparison with any future TLI value. In 1994, the Rotorua District community

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 - 34Microtrophic 1.0 to 2.0 0.33 - 0.82 24 - 15 1.8 - 4.1 34 - 73Oligotrophic 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 - 337Eutrophic 4.0 to 5.0 5.0 - 12 3.6 - 1.6 20 - 43 337 - 725Supertrophic 5.0 to 6.0 12 - 31.0 1.6 - 0.7 43 - 96 725 -1558Hypertrophic 6.0 to 7.0 >31 <0.7 >96 >1558


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Tabl

e 2.

-PA

C a

nd T

LI R

epor

t for

Lak

e R

otoi

ti 19

92-2

003.

Lake

Rot

oiti

1992

to 2

003

site

s 1,

2 &

3 (1

Jul

199

2 -

30 J

un 2

003)

Per

cent

Ann

ual C

hang

e (P

AC

)

C

hla

SD

TP

TN

H

VO

D

Avg

PA

C

Std

Err

P

-Val

ue

Lake

(m

g/m

3 ) (m

) (m

gP/m

3 ) (m

g/m

3 ) (m

g/m

3 /day

)

Cha

nge

- U

nits

Per

Yea

r 0.

75

-0.0

6 0.

84

2.91

Ave

rage

Ove

r Pe

riod

7.

62

5.17

22

.27

271.

00P

ercen

t A

nn

ual

Ch

an

ge (

%/Y

ear)

9.8

4

1.1

6

3.7

7

1.0

7

0.0

0

3.9

6

2.0

6

0.1

5

B

urns

Tro

phic

Lev

el In

dex

Val

ues

and

Tren

ds

C

hla

SD

TP

TN

TL

c TL

s TL

p TL

n TL

I S

td. E

rr.

TLI T

rend

S

td. E

rr.

P-V

alue

P

erio

d (m

g/m

3 ) (m

) (m

gP/m

3 ) (m

g/m

3 )

A

vera

ge

TL a

v un

its/y

r TL

I tre

nd

Jul 1

992

- Ju

n 19

93

7.58

5.

32

20.4

3 26

7.10

4.

45

3.51

4.

04

3.69

3.

93

0.21

Ju

l 199

3 -

Jun

1994

4.

15

4.92

21

.33

273.

70

3.79

3.

61

4.10

3.

73

3.81

0.

10

Jul 1

994

- Ju

n 19

95

4.48

5.

37

19.9

7 25

3.27

3.

87

3.50

4.

02

3.62

3.

75

0.12

Ju

l 199

5 -

Jun

1996

5.

28

5.55

23

.40

265.

23

4.06

3.

46

4.22

3.

69

3.85

0.

17

Jul 1

996

- Ju

n 19

97

5.13

6.

06

17.7

0 24

4.67

4.

02

3.34

3.

86

3.58

3.

70

0.15

Ju

l 199

7 -

Jun

1998

6.

09

5.53

22

.28

288.

33

4.21

3.

46

4.15

3.

79

3.91

0.

17

Jul 1

998

- Ju

n 19

99

6.49

4.

40

27.0

0 28

5.07

4.

28

3.75

4.

40

3.78

4.

05

0.17

Ju

l 199

9 -

Jun

2000

8.

01

5.27

22

.56

252.

56

4.52

3.

52

4.17

3.

62

3.96

0.

23

Jul 2

000

- Ju

n 20

01Ju

l 200

1 -

Jun

2002

7.

30

5.44

23

.06

249.

15

4.41

3.

48

4.20

3.

60

3.92

0.

23

Jul 2

002

- Ju

n 20

03

18.4

6 3.

95

30.9

1 33

9.86

5.

44

3.89

4.

57

4.01

4.

48

0.35

A

verages

7.3

0

5.1

8

22.8

6

271.8

9

4.3

1

3.5

5

4.1

7

3.7

1

3.9

4

0.0

7

0.0

4

0.0

2

0.0

333

Th

e gu

ide

used

in th

e P

AC

ave

rage

S

umm

ary

P-V

alue

eva

luat

ion

is:

PAC

= 3

.96

± 2.

06 %

per

yea

r P

-Valu

e R

an

ge

In

terp

reta

tion

P-V

alue

= 0

.15

P

< 0.

1 D

efini

te C

hang

e

0.1

< P

< 0.

2 Pr

obab

le C

hang

eT

LI

Val

ue =

3.9

4 ±

0.07

TL

I un

its

0.2

< P

< 0.

3 Po

ssib

le C

hang

eT

LI

Tre

nd =

0.0

4 ±

0.02

TL

I un

its p

er y

ear

0.3

< P

N

o C

hang

eP-

Val

ue =

0.0

333

Ass

essm

ent

Mesotr

op

hic

Prob

ab

le D

egred

ati

on


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indicated a wish for most of the lakes to maintain their pres-ent condition, with the balance of the lakes to be improved. As a result, numeric baseline TLI values were written into the Proposed Regional Water and Land Plan (EBOP 2002) as lake water quality objectives for the Rotorua lakes. The Proposed Regional Water and Land Plan also required re-medial action plans to be formulated for any lake with a 3-year moving average TLI exceeding its baseline TLI values. Some of the baseline values are lower than the current TLI values. Action Plans are based on lake modelling results to determine the nutrient input reductions required for damaged lakes to return to their baseline TLI values. Maintenance of the baseline quality from that existing in 1994 has been accepted as a policy of the Regional Water and Land Plan (EBOP 2004).

In order to implement these policies, EBOP now publishes an annual report on the status and recent changes (including TLI, TLx and time trend results) in each lake to promote Rotorua District community acceptance of the requirements and costs of proper lake management. These assessments and the timely annual publication of these values and results are made considerably easier for EBOP by the computer pro-gram LakeWatch (Lakes Consulting 2000), which employs the equations and concepts outlined in Burns et al. (1999, 2000). The program facilitates the determination of trends in all variables and calculates TLI values used in the EBOP an-nual lake status reports (Table 2; Scholes 2004). LakeWatch also creates its own database of all the lake monitoring data, enabling quick and easy access to any data and computed result by EBOP staff.

Another EBOP initiative is the endowment of a chair for lake research, namely the B.O.P. Chair in Lake Management and Restoration at the University of Waikato, Hamilton, New 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).

Results of Monitoring and Management ActionsDecrease in Inputs to Lake Rotorua

The diversion of Rotorua treated wastewater into the Waipa forest reduced the nutrient inputs to Lake Rotorua by 33 x 103 kg yr-1 of TP and 121 x 103 kg yr-1 of TN. These reduc-tions were significant in terms of the external loads of 43 x 103 kg yr-1 of TP and 536 x 103 kg yr-1 of TN to the lake in 1998 (Burns 1999).

Improvements to the Ngongotaha watershed were followed by decreases of 27% for particulate P, 26% for soluble P and 40% for particulate N, but an increase of 26% for soluble N in the stream water (Williamson et al. 1996). The new veg-

etation growing along the fenced-off stream banks (riparian strips) had retained particles washing from the watershed and had absorbed some soluble P.

The explanation for the increased content of nitrate in the district streams relates to the effect of nitrogen leaching from the urine spots of grazing animals. The resulting nitrate has entered the groundwater and is now increasingly entering the streams. Some of this nitrate is over 50 years old (Tay-lor 1977), indicating that the ground water in many cases is deep. Even if the entry of nitrate into the groundwater were to cease today, the problem could take up to 50 years to disappear. Fencing of streams and lake margins to exclude grazing animals has been progressing around the Rotorua lakes since the 1970s, a method that has proved effective for the management of surface runoff, but not for nitrate in sub-surface runoff.

Utilizing the Lake Assessment Results

The first results using the TLI system were made public by oral presentation at the 2001 Rotorua Lakes Symposium (McIntosh 2001, Burns 2001), a public symposium held biannually by the Lakes Water Quality Society in Rotorua. At this symposium, Burns (2001) explained the TLI system and presented TLI information on each lake. McIntosh (2001) presented information (Table 3) showing the 1994 baseline TLI and then-current TLI values for each lake, thereby iden-tifying the lakes requiring remedial action. Much discussion ensued, and some initial resistance to the concept of using TLI values as the basis of management decisions arose because it was a new, untried approach to lake management in New Zealand. Some also doubted that reliance on TLI values would reveal ecological problems, such as a prevalence of cyano-bacteria or macrophytes in a lake.

Table 3.-The Baseline TLI values determined for the 12 Rotorua District lakes.

Draft Regional Water and Land 3-yr average Lake Plan Baseline TLI TLI to 2000

Rotoma 2.3 2.3Okataina 2.6 2.6Tarawera 2.6 2.6Tikitapu 2.7 2.7Okareka 3.0 3.4*Rotokakahi 3.1 3.2Rotoiti 3.5 3.9*Rerewhakaaitu 3.6 3.6Rotomahana 3.9 3.8Rotoehu 3.9 4.7*Rotorua 4.2 4.6*Okaro 5.0 5.7*

* Lakes exceeding their designated baseline TLI.


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After the 2001 symposium, dialogue continued between EBOP and the Rotorua District community on the use of TLI values for guidance about future remedial work. EBOP explained that the TLI was primarily a measure of trophic level of a lake; the index was not an index of infectious bacteria, specific phytoplankton-type presence or extent of macrophyte invasion. Lakes had to be sampled for specific problems as well as for trophic level indicators, and their specific problems considered as separate issues along with changes in trophic level. Further, the use of four variables in a summation-type numerical index, when compared with earlier baseline values of the index, provided the possibility of early detection of increasing trophic level. This dialogue was assisted by the release of the Annual Reports on the water quality of the Rotorua Lakes (Scholes 2004). At the 2003 Rotorua Lakes Symposium, the use of the TLI system to determine which lakes required attention was generally accepted. The presentations and discussions focused on the means to remediate watersheds requiring improvement.

Management Options and Actions on 12

Rotorua District Lakes

Each of the twelve lakes is discussed below on the basis of the information supplied in Table 4 and other information on each lake.

Lake Okaro

Lake Okaro is a small, supertrophic lake that has degraded, largely because it is situated in fertile country that has been mostly converted to pasture. Table 4 shows that the Lake Okaro TLI exceeded its designated Baseline TLI by 0.5 tli units in 2003, and a draft action plan (McIntosh 2003a) has been developed to obtain community involvement in improv-ing its water quality. Lake Okaro is N-limited, with TLn less than TLp (TLp-TLn = 0.5). Modeling of the lake and its watershed shows that the lake requires a reduction of 400 kg yr-1 soluble phosphorus (SP) and 3,320 kg yr-1 total inorganic nitrogen (TIN) inputs. Internal loading from the sediments of the lake has been calculated at 380 kg yr-1 TP and 2,400 kg yr-1 TN; thus, the external load reduction needs to be 20 kg yr-1 TP and 920 kg yr-1 TN. Options being considered to achieve this reduction are: conversion of 2.0 km2 of the 3.37 km2 pasture area in the watershed to forestry (because of the 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 stream flows into the lake (McIntosh 2003a). While these options are being considered, a trial alum dose of 0.6 g/m3 as aluminum was applied to the lake in December 2003 to speed its recovery.

Lake Rotorua

Lake Rotorua is a big, relatively shallow eutrophic lake occupying a volcanic crater, which has been largely filled in with sediment over time. The largest town in the region, Rotorua, is situated on its shores (Fig. 1) and has damaged the lake by discharge of treated wastewater into the lake. The lake has a TLI of 4.9 and a baseline TLI of 4.2 (Table 4). It is N-limited (TLp-TLn = 0.7). The model predicted that the decrease of treated wastewater input to the lake following its diversion to the nearby Waipa forest would lead to slow-but-steady improvement in the condition of the lake (Rutherford 1996), but this has not happened. The PAC values of the lake in 2003 were Chla = 9.5% yr-1; SD non-significant trend; TP = −2.4 % yr-1; TN = 1.3% yr-1, indicating that algal and TN concentrations have increased, while TP has decreased. These changes are likely due to climatic warming (1992-2002) and increasing loads of nitrate to the lake.

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 resulted in large releases of soluble P and N to the overlying water from the sediments, with single regeneration episodes producing 178% and 84% of the annual loads of dissolved reactive phosphorus (DRP) and TP respectively, and 66% and 32% of the annual loads of total inorganic nitrogen (TIN) and TN respectively (Burns 1999). Another factor affecting Lake Rotorua, and in fact all the Rotorua district lakes, is the increasing nitrate content of most streams in the area (Williamson et al. 1996, Rutherford 2003). This is a particularly important issue because Rotorua is N-limited, and the TN entering the lake via the inflowing streams has been estimated to have increased by 245 x 103 kg yr-1 from 1984-2002. This compares with a reduction of about 128 x 103 kg yr-1 in the TN loading from the sewage diversion, giving an overall increase of 117 x 103 kg yr-1 in loading to 2002 (Rutherford 2003). One indicator of positive response is that when the lake had a period without regeneration episodes from July 1998-June 1999, the TLI was 4.3, down from a TLI of 4.8 the previous year and close to its baseline value of 4.2 (Scholes 2004). Because the water quality of Lake Rotoiti depends largely on the quality of the water it receives from Lake Rotorua (Fig. 2), a draft action plan is in preparation that combines the two lakes. Research into nutrient regeneration processes was carried out by Waikato University in 2003 and 2004.

Lake Rotoehu

Lake Rotoehu is a moderately sized, relatively shallow lake that is intermittently stratified. The lake experienced a long period of stratification in 1993. Prior to 1993, the lake had a 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


68

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 geothermal sources. The water quality of the lake is good 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 of geothermal inputs.

Lake Rerewhakaaitu

Lake Rerewhakaaitu is strongly phosphorus limited (TLp-

TLn = -1.4; Table 4) and went through a major period of 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-leased phosphorus from the re-watered sediments. In 2003 the TLI was 3.2 compared to its baseline value of 3.6. The

Table 4.-Basic data on the 12 Rotorua District Lakes and their watersheds.

Lakes Okaro Rotorua Rotoehu Rotomahana Rerewhakaaitu Rotoiti

Lake Area (km2) 0.32 80.8 8 9 5.8 34.6Max. Depth (m) 18 45 13.5 125 15.8 110Av. Depth (m) 12.1 11 8.2 60 7 31.5Av. Annual Chla (mg m-3) 33 14.8 12 5.1 5.3 7.3Av. Annual SD (m) 1.6 2.5 2.3 4.2 5 5Av. Annual TP (mgPm-3) 122 44 36 25 7.4 23.2Av. Annual TN (mgNm-3) 1250 426 456 222 380 277Av. TLc (TLI units) 5.9 5 4.8 4 3.8 4.3Av. TLs (TLI units) 5 4.5 4.5 3.8 3.7 3.7Av.TLp (TLI units) 6.3 5 4.8 4.1 2.7 4.2Av.TLn (TLI units) 5.7 4.3 4.4 3.4 4.1 3.7TLp - 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 38.2 118.6Pasture 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

Figure 2.-Map of Lakes Rotorua and Rotoiti showing the Ohau

channel and the Kaituna River.


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lake is hydraulically perched, so some of the groundwater in its watershed does not enter the lake. An in-depth discus-sion document on the management of this lake is available McIntosh et al. (2001).

Lake Rotoiti

Lake Rotoiti is a deep lake that remains stratified for up to 8 months of the year. It receives approximately 70% of its nutrient load from Lake Rotorua via the Ohau Channel, which connects the two lakes (Fig 2). Lake Rotoiti is N-limited (TLp -TLn = 0.5), similar to Lake Rotorua (Table 4). In 1957 the Lake Rotoiti hypolimnion still had 29% DO saturation in April (Jolly 1968), a condition now normally found in Janu-ary of each year. As the water quality of Lake Rotorua has deteriorated, so has the water quality of Lake Rotoiti (Vincent et al. 1984). The lake is now anoxic for up to three months per year and annually experiences large scale nutrient regenera-tion. The lake baseline TLI of 3.5 existed before large-scale nutrient regeneration occurred. This baseline is exceeded by the 2003 3-yr average TLI of 4.3 (Table 4). A joint action plan for Lakes Rotorua and Rotoiti is in preparation. Serious consideration is being given to deflecting the Lake Rotorua inflow from the Ohau Channel to the nearby Kaituna River, which is the outflow from the lake (see Fig.2).

Lake Okareka

Picturesque Lake Okareka is a borderline oligotrophic/meso-trophic lake. It is strongly P-limited (TLp-TLn = -1.0; Table 4), possibly because it receives a relatively large supply of

nitrogen from pastures and the septic tanks in its watershed. The lake is stratified and reaches very low hypolimnetic DO levels at the end of the stratified season. In the early 1990s the 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 (the lake has warmed at the rate of 0.04°C yr-1 since 1994), which results in slightly longer periods of stratification, that in turn lead to the development of hypolimnetic anoxia at the end of the stratified season. A draft action plan for the lake has been published, and modeling of Lake Okareka has shown that the load to the lake needs to be reduced by 2.320 and 70 kg yr-1 for N and P respectively (EBOP 2003). The options for obtaining such a decrease in load are: to convert 5.15 km2 from pasture into forest; to create 5- to 10-m wide fenced riparian strips along streams, and; to divert a stream so that it enters the lake through a wetland, removing a pre-dicted 300 kg yr-1 and 10 kg yr-1 from N and P loads to the lake respectively. The possibility of connecting the septic tanks of the homes in the watershed to a treatment plant also exists, which could reduce the N input to the lake by 1940 kg yr-1 and the P input by 10-20 kg yr-1.

Lake Tikitapu

Lake Tikitapu, is a P-limited (TLp-TLn = -1.4; Table 4), relatively small lake with good water quality that is frequently used for recreational water sports. This lake provides an example of the need for careful monitoring combined with a quantitative water quality assessment system. No noticeable deterioration of the water quality has been reported by the

Table 4.-(Continued).

Lakes Okareka Tikitapu Okataina Tarawera Rotoma Rotokakahi

Lake Area (km2) 3.3 1.46 11 41.7 11 4.52Max. Depth (m) 33.5 27.5 78.5 87.5 83 32Av. Depth (m) 20 18 39.4 50 36.9 17.5Av. Annual Chla (mg m-3) 4.5 2 2.1 1.6 1.5 3.89Av. 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 10.2Av. Annual TN (mgNm-3) 225 196 123 122 136 209Av. TLc (TLI units) 3.8 2.9 3 2.7 2.6 3.6Av. 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 3.1Av.TLn (TLI units) 3.5 3.3 2.7 2.6 2.8 3.4TLp - 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 18.7Pasture 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


70

users of the lake, but monitoring data show that anoxia is oc-curring in the bottom waters, possibly triggering phosphorus release in this phosphorus-limited lake (Scholes 2004). If the 2003/2004 monitoring data yield a 3-yr running average above 2.7, then this lake will require preparation of an action plan for its remediation.

Lake Okataina

Lake Okataina is a good quality oligotrophic lake with similar TLp and TLn values (TLp-TLn = -0.2; Table 4). Lake-level change can influence the quality of this lake, and some water quality indicators (Chla, SD) show signs of deterioration (Scholes 2004), creating a need for continued monitoring.

Lake Tarawera

Lake Tarawera is a balanced lake with similar TLp and TLn values (TLp-TLn = 0.1; Table 4). It is a good quality oligotrophic lake but may be in a declining state. The TLI exceeds the baseline TLI of Environment Bay of Plenty’s regional plan (Table 4).

Lake Rotoma

Lake Rotoma was not monitored from July 1996-June 2000. Before this non-sampling period, the TLI of the lake was below its baseline level of 2.3, but in the two years of monitoring from July 2000-June 2003, its TLI has averaged 2.5. It is strongly P-limited (TLp-TLn = -1.1; Table 4). The increased average TLI value indicates that some deteriora-tion in the water quality may be occurring. At this stage in the monitoring it is difficult to tell whether negative trends in water quality are cyclic and may recover, or are ongoing (Scholes 2004).

Lake Rotokakahi

Lake Rotokakahi is privately owned, and no recent monitor-ing has been carried out. Monitoring of the Te Wairoa Stream at the outlet of the lake shows nutrient levels to be similar to levels previously recorded on Lake Rotokakahi (Burns and Rutherford 1998). A TLI estimated using nutrient and Chla data from the Te Wairoa Stream shows a slightly elevated TLI to that calculated from Lake Rotokakahi water quality data (Scholes 2004).

DiscussionThe major actions to date on nutrient input management have been the diversion of wastewater from Lake Rotorua, research on watershed management techniques (Williamson 1996), formulation of the Water and Soil Plan (EBOP 2002, 2004), evaluation of the groundwater nitrate problem (Ruth-

erford 2003) and planting of trees in the lake margins of Lake Rotoehu. A number of farmers have modified dairy-shed wastewater management to ensure that no wastes sprayed onto land subsequently enter streams. Plans to build small waste treatment plants to eliminate septic tank usage in the 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 in-depth discussion of possible options, between lakeside residents, farmers and the Regional and District Councils to decide on management actions for lake remediation.

In lakes that are strongly P- or N-limited, the 4-variable TLI does not always give a true reflection of changes in the productivity of a lake. The concentration of the limiting nutri-ent may be increasing while that of the non-limiting nutrient may be declining, causing the calculated TLI to not change much even while the productivity of the lake is increasing, as in the case of Lake Rotorua. Examining and sometimes taking guidance from the TLx values rather than the TLI is important. In the case of lakes strongly limited by the avail-ability of one nutrient only, it may be preferable to calculate the TLI from the TLc, TLs and the TLx of the limiting nutri-ent only and use this 3-variable TLI for guidance. Particular attention should be paid to the TLc because chlorophyll concentration is the most direct measure of trophic condition. However, monitoring the TLx of the non-limiting nutrient is always advisable, because it indicates the potential increase in the trophic level of a lake if more of the limiting nutrient becomes available.

Another well-known lake water quality index is Carlson’s Trophic State Index (TSI; Carlson and Simpson 1996). This index was developed using data from American lakes, while Burns’ TLI index (Burns et al. 1999, 2000) was developed using New Zealand lake data. Both systems use the same four key variables. The TLI system is used in New Zealand and has the feature that the boundary of each lake type is denoted by an integer value (Table 1). The two systems give similar results, with TLI values being equal to TSI values calculated from the same data set when divided by 11.0 (i.e., TLI = TSI/11.0; Burns and Bowman 2000). The TLx and TSx values are similar with TLc = 1.078(TSc/11.0), TLs = 0.986(TSs/11.0), TLp = 1.079(TSp/11.0), TLn = 0.868(TSn/11.0).

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 index value. Comparison is then possible with previous or future index values, the only way absolute change in lake-tro-phic level can be readily observed. As Havens (2004) points out, scientists have different concepts of what constitutes eutrophy, and these concepts can change with time, as in the case of Lake Okeechobee. If possible, lake managers should


71

attempt to determine desired target TLI or TSI values for individual lakes because increased societal pressures on lakes and rivers is causing the TLI of most lakes to increase.

No single environmental index can summarize the ecological condition of a lake. The TLI is a good indicator of the water quality of a lake but does not give information on the growth of macrophytes. Sometimes the TLI of a water body improves because the macrophyte population is growing profusely, absorbing much of the available nutrient from the overlying water. In fact, the TLI is best used in conjunction with a submerged plant index such as the LakeSPI (Clayton et al. 2002), which has been used on some of the Rotorua District lakes. It does not appear that any of these lakes are undergoing rampant macrophyte growth, and the TLI is proving to be an acceptable management tool in this situation.

Warming trends have been observed in the Rotorua District lakes. A warming trend has little effect on the majority of deep lakes but increases water column stability in intermit-tently stratified lakes, causing more frequent occurrence of sediment/water interface anoxia and subsequent nutrient release. Also, in stratified lakes that tend to just reach anoxic conditions at the end of their stratified periods, such as Lake Okareka, any lengthening of their stratified periods can cause a relatively large increase in anoxically regenerated nutrients. This means that should global warming continue, actions to diminish the external loading of nutrients to susceptible lakes will be needed to lower the possibility of the onset of anoxia.

The Rotorua District lakes are a national resource of New Zealand and need to be maintained in good condition for pres-ent and future generations. The use of Trophic Level Index values combined with careful monitoring has enabled EBOP to determine when regulatory numerical baseline TLI values are exceeded. The timely disclosure of easily interpretable numerical results to the communities living around the lakes in annual reports, facilitated by the use of the LakeWatch program, has resulted in these communities being most interested in implementing lake management strategies. This interest has been focused into careful consideration of options by the provision of Action Plans that give quantita-tive options for the remediation of lakes whose TLI values have exceeded their baseline values. Little discussion now occurs on whether the remedial work should be undertaken, but much discussion on how it should be undertaken.

AcknowledgmentsThe authors wish to thank the reviewers of this article for their helpful assistance.

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