Strategies for Managing the Lakes of the Rotorua District New Zealand

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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.

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Strategies for Managing the Lakes of the Rotorua District New Zealand

  1. 1. 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 61
  2. 2. Burns, McIntosh and Scholes Figure 1-A map of the Rotorua District, New Zealand showing the twelve lakes under management in this district. 62
  3. 3. 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- 63
  4. 4. 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
  5. 5. 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
  6. 6. 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
  7. 7. 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
  8. 8. 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
  9. 9. 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
  10. 10. 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
  11. 11. Strategies for Managing the Lakes of the Rotorua District, New Zealand References attempt to determine desired target TLI or TSI values for individual lakes because increased societal pressures on lakes Burns N.M., J.C. Rutherford and J.S. Clayton. 1999. A monitoring and rivers is causing the TLI of most lakes to increase. and classification system for New Zealand lakes and reservoirs. Lake Reservoir Manag. 15:255-271. No single environmental index can summarize the ecological Burns N.M., G. Bryers and E. Bowman. 2000. Protocols for condition of a lake. The TLI is a good indicator of the water monitoring trophic levels of New Zealand lakes and reservoirs. quality of a lake but does not give information on the growth Published by NZ Ministry for the Environment, Wellington, of macrophytes. Sometimes the TLI of a water body improves NZ. 138 p. (http://www.mfe.govt.nz/publications/water/ because the macrophyte population is growing profusely, monitoring-trophic-status-of-nz-lakes01.html) absorbing much of the available nutrient from the overlying Burns N.M. 1999. Lake Rotorua and its Inputs in the 1990s. Lakes water. In fact, the TLI is best used in conjunction with a Consulting Report 99/3, Lakes Consulting, 42 Seabreeze Rd., Devonport, New Zealand. 53 p. submerged plant index such as the LakeSPI (Clayton et al. Burns N.M. and J.C. Rutherford. 1998. Results of monitoring 2002), which has been used on some of the Rotorua District New Zealand Lakes, 1992-1996. Report to the Ministry for lakes. It does not appear that any of these lakes are undergoing the Environment. NIWA Client Report: MFE80216. NIWA, rampant macrophyte growth, and the TLI is proving to be an Hamilton, New Zealand. acceptable management tool in this situation. Burns N.M. and E. Bowman 2000. Manual on Monitoring Trophic Levels of Lakes and Reservoirs. Lakes Consulting, Auckland, Warming trends have been observed in the Rotorua District New Zealand. 135 p. lakes. A warming trend has little effect on the majority of Burns, N.M. 2001. Trophic level trends in 12 Rotorua District lakes: deep lakes but increases water column stability in intermit- 1990 to 2000. Proceedings and Report–Rotorua Lakes 2000–A tently stratified lakes, causing more frequent occurrence of symposium on Research Needs in the Rotorua Lakes. Lakes sediment/water interface anoxia and subsequent nutrient Water Quality Society, Rotorua, New Zealand. release. Also, in stratified lakes that tend to just reach anoxic Carlson, R.E. and J. Simpson. 1996. A Coordinators Guide to Volunteer Lake Monitoring Methods. North American Lake conditions at the end of their stratified periods, such as Lake Management Society. 96 p. Okareka, any lengthening of their stratified periods can cause Clayton, J., T. Edwards and V. Froude. 2002. LakeSPI–a method a relatively large increase in anoxically regenerated nutrients. for monitoring ecological condition in New Zealand lakes. This means that should global warming continue, actions Technical Report. NIWA Client Report HAM2002-011, PO to diminish the external loading of nutrients to susceptible Box 11115, Hamilton, New Zealand. 81 p. lakes will be needed to lower the possibility of the onset of Environment B.O.P. 1997. Rotorua Lakes Summary Report. EBOP anoxia. Environmental Report 97/21. Environment B.O.P., Whakatane, New Zealand. The Rotorua District lakes are a national resource of New Environment B.O.P. 2002. Proposed Regional Water and Land Plan. Zealand and need to be maintained in good condition for pres- Resource Planning Publication 2002/01. Environment B.O.P., ent and future generations. The use of Trophic Level Index Whakatane, New Zealand. Environment B.O.P. 2003. Lake Okareka Catchment Management values combined with careful monitoring has enabled EBOP Action Plan. EBOP Environmental Publication 2003/01, PO to determine when regulatory numerical baseline TLI values Box 364, Whakatane, New Zealand. 30 p. (http://www.envbop. are exceeded. The timely disclosure of easily interpretable govt.nz/publications/plans/strategies.asp) numerical results to the communities living around the lakes Environment B.O.P. 2004. Proposed Regional Water and Land in annual reports, facilitated by the use of the LakeWatch Plan as amended by Council decisions. Resource Planning program, has resulted in these communities being most Publication 2002/01. Environment B.O.P., Whakatane, New interested in implementing lake management strategies. Zealand. This interest has been focused into careful consideration of Fish, G.R. 1975. Lakes Rotorua and Rotoiti, North Island: their options by the provision of Action Plans that give quantita- trophic status and studies for a nutrient budget. Fisheries Research Bulletin No. 8. Ministry of Agriculture and Fisheries, tive options for the remediation of lakes whose TLI values Wellington, New Zealand. have exceeded their baseline values. Little discussion now Havens, K.E. 2004. Is there a Common Language regarding the occurs on whether the remedial work should be undertaken, Trophic State of lakes? Lakeline Summer 2004:33-36. but much discussion on how it should be undertaken. Jolly, V.H. 1968. The comparative limnology of some New Zealand lakes. N. Z. J.Mar. Freshw. Res. 2:214-259. Lakes Consulting. 2000. LakeWatch–a program for the evaluation Acknowledgments of lake and reservoir monitoring data. Lakes Consulting, 42 Seabreeze Rd., Devonport, New Zealand. (www.lakewatch. The authors wish to thank the reviewers of this article for net) their helpful assistance. McColl, R.H. 1972. Chemistry and Trophic Status of Seven New Zealand Lakes. N. Z. J. Mar. Freshw. Res. 6:399-447. 71
  12. 12. Burns, McIntosh and Scholes McIntosh, J., N. Ngapo, C.E. Stace, G. Ellery and J. Gibbons-Davies. Scholes, P. 2004: Rotorua Lakes Water Quality 2003. EBOP 2001. Lake Rerawhkaaitu Project. EBOP Environmental Environmental Publication 2004/02, PO Box 364, Whakatane, Publication 2001/15, PO Box 364, Whakatane, New Zealand. New Zealand. 75 p. 57 p. (http://www.envbop.govt.nz/publications/plans/ Taylor, C.B. 1977. Preliminary measurements of tritium, deuterium strategies.asp) and oxygen-18 in lakes and groundwater of Volcanic Rotorua McIntosh, J. 2001. Environment B.O.P.’s lake management plans, Region, New Zealand. DSIR Rep. INS-R-227. Institute of present and future. Proceedings and Report–Rotorua Lakes Nuclear Sciences, Wellington, New Zealand. 2000–A symposium on Research Needs in the Rotorua Lakes. Timperly, M.H. 1983. Phosphorus in spring waters of the Taupo Lakes Water Quality Society, Rotorua, New Zealand. Volcanic Zone, North Island, New Zealand. Chemical Geology McIntosh, J. 2003a. Lake Okaro Catchment Management Draft :287-306. Working Paper. EBOP Environmental Publication 2003/17, Vincent, W.F. 1982. Biological transformations of nitrogen in PO Box 364, Whakatane, New Zealand: 23pp. NZ freshwaters. In ‘Nitrogen Balances in New Zealand McIntosh, J. 2003b. Lake Rotehu Catchment Management Action Ecosystems.’ Conference Proceedings, DSIR, Wellington, Plan Draft Working Paper. EBOP Environmental Publication New Zealand. 2003/12, PO Box 364, Whakatane, New Zealand. 27 p. Vincent, W.F., M.M. Gibbs, S.J. Dryden. 1984. Accelerated Rutherford, J.C. 1984. Trends in Lake Rotorua water quality. N. Z. eutrophication in a New Zealand lake: Lake Rotoiti, New J. Mar. Freshw. Res. 18: 355-365. Zealand. N. Z. J. Mar. Freshw. Res. 18:431-440. Rutherford, J.C. 1996. Predictions of phosphorus in Lake Rotorua Williamson, R.B., C.M. Smith, A.B. Cooper. 1996. Watershed following load reductions. N. Z. J. Mar. Freshw. Res. 30:383- riparian management and its benefits to a eutrophic lake. J. 396. Water Resour. Plan. Manag. 24-32. Rutherford, J.C. 2003. Lake Rotorua nutrient targets. NIWA Client Report HAM2003-155. NIWA, Hamilton, New Zealand. 55 p. 72

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