Managing the lakes of the Rotorua District, New Zealand

Figures & data

Figure 1 A map of the Rotorua District, New Zealand showing the twelve lakes under management in this district. The enlarged area is shown as Figure 2.

Table 1 Long-term trends and three yearly average TLI values for the Rotorua District lakes in comparison to the TLI values set in the Regional Water and Land Plan, together with LakeSPI condition.

Lake	3 yearly average TLI to 2007 TLI units	Regional Water & Land Plan TLI units	Long Term Trend** Based on analysis over many years data (>13)	Lake Type Based on Trophic Level	LakeSPI Condition
Okaro	5.5*	5.0	Definite improvement	Supertrophic	Poor
Rotorua	4.9*	4.2	Degraded, no change	Eutrophic	Poor
Rotoehu	4.6*	3.9	Degraded, no change	Eutrophic	Average (Poor)
Rotoiti	4.1*	3.5	Definite degradation	Eutrophic	Poor
Rotomahana	3.9	3.9	No change	Mesotrophic	Good
Rerewhakaaitu	3.5	3.6	No change	Mesotrophic	Average
Okareka	3.3*	3.0	No change	Mesotrophic	Average
Tikitapu	3.0*	2.7	Possible degradation	Oligotrophic	Good
Okataina	2.8*	2.6	No change	Oligotrophic	Good
Tarawera	2.8*	2.6	No change	Oligotrophic	Average
Rotoma	2.5*	2.3	Definite degradation	Oligotrophic	Good
Rotokakahi	—	3.1 ^1	—	Mesotrophic	Average

Figure 2 Map of Lakes Rotorua and Rotoiti showing the Ohau Channel and the Kaituna River.

Figure 3 Average total phosphorus and annualized residual concentrations for Lake Okaro.

Table 2 TLI Report for Lake Rotoma.

Lake Rotoma: 1994 to 2007 (1 Jul 1994–30 Jun 2007)
Percent Annual Change (PAC)
Lake		Chla (mg/m3)	SD (m)		TP (mgP/m3)	TN (mg/m3)		HVOD (mg/m3/day)		Avg PAC	Std Err		p-Value
Change–Units Per Year		0.03	(0.00)		0.18	2.88		(0.36)
Average Over Period		1.3	(12.25)		4.35	148.96		(19.03)
Percent Annual Change (%/Year)		2.31	0		4.14	1.93		0		1.68	0.78		0.1
Burns Trophic Level Index Values and Trends
Period	Chla (mg/m3)	SD (m)	TP (mgP/m3)	TN (mg/m3)	TLc	TLs	TLp	TLn	TLI Average	Std. Err. TL av	TLI Trend units/yr	Std. Err. TLI trend	p-Value
Jul 1994–Jun 1995	0.93	12.71	3.44	139.67	2.14	2.26	1.79	2.85	2.26	0.22
Jul 1995–Jun 1996	0.85	15.61	2.83	139.83	2.04	1.9	1.54	2.85	2.08	0.28
Jul 1996–Jun 1997		14				2.09			2.09	0
Jul 2000–Jun 2001	1.96	9.81	3.27	137.91	2.96	2.66	1.72	2.83	2.54	0.28
Jul 2002–Jun 2003	1.47	9.36	4	129.84	2.65	2.73	1.98	2.75	2.53	0.19
Jul 2003–Jun 2004	1.37	14.16	4.91	147.07	2.56	2.07	2.24	2.91	2.45	0.19
Jul 2004–Jun 2005	1.33	13.01	4.85	150.24	2.54	2.22	2.22	2.94	2.48	0.17
Jul 2005–Jun 2006	1.27	14.3	6.33	164.55	2.48	2.06	2.56	3.06	2.54	0.21
Jul 2006–Jun 2007	1.29	11.23	4.29	176.41	2.5	2.46	2.07	3.15	2.60	0.23
Averages	1.31	12.69	4.24	148.19	2.48	2.27	2.01	2.92	2.42	0.07	0.04	0.02	0.0542
SUMMARY:
PAC = 1.68 ± 0.78% per year			The guide used in the PAC average
p-Value = 0.01			p-Value evaluation is:
			p-Value Range				Interpretation
TLI Value = 2.42 ± 0.07 TLI units				P	< 0.1		Definite Change
TLI Trend = 0.04 ± 0.02 TLI units per year			0.1 <	P	< 0.2		Probable Change
p-Value = 0.0542			0.2 <	P	< 0.3		Possible Change
			0.3 <	P			No Change
ASSESSMENT:
Oligotrophic
Definite Degredation

Figure 4 Current and planned future nutrient loads to lake Rotoma.

Figure 5 Fluorescence and dissolved oxygen profiles on April 24, 2007 at Site 1, Lake Tikitapu (fluorescence units – μL as Chla equivalent).

Table 3 Comparison of 4 different sets of lake management criteria.

The Rotorua Lakes	Clear Lake (IDNR 2005)	Lake Herman (SDDENR 2005)	Lake Tegel and Schlachtensee (Schauser and Chorus 2007)
The numerical standard selected for these lakes is their W&LP TLIs and this is written into legislation.	The narrative standard of the prevention of nuisance algal blooms and the provision of water quality to support the lakes designated uses was translated into Carlson's (Carlson and Simpson 1996) Trophic State Index (TSI) values of TSP = 70; TSChla = 65; TSSD = 65 for a TSI = 66.7.	The narrative standard of providing water to satisfy the beneficial uses of contact recreation, stock watering, irrigation, and warm-water semi-permanent fish was translated into a TSP = 68; TSChla = 74.7; TSSD = 74.7 for a TSI = 73.9.	There was no narrative standard; a numerical target of achieving TP concentration of 30 ug P/L was chosen.

Table 4 Methods of determining nutrient loads to lakes.

The Rotorua Lakes	Clear Lake (IDNR 2005)	Lake Herman (SDDENR 2005)	Lake Tegel and Schlachtensee (Schauser and Chorus 2007)
Land loss coefficients (Menneer et al. 2004) were used to estimate nutrients draining from agricultural and forestry land. Total loads to some lakes can be estimated using eqn (1). Direct calculations are done for loads from geothermal sources, drains, treatment plants and internal loads.	The (Vollenweider 1982) combined OECD relationship empirical model was used to calculate the external load that would sustain the observed in-lake P concentration.	The Agricultural Non-point Source Model (Walker 1996) was used to estimate the load from a watershed that contained no significant point sources.	The loads for each lake were taken as those calculated earlier (Klein 1990 and Heinzmann et al. 1991).

Table 5 Lake loading models used to determine the nutrient loads that would achieve the designated values for the numerical standards.

The Rotorua Lakes	Clear Lake (IDNR 2005)	Lake Herman (SDDENR 2005)	Lake Tegel and Schlachtensee (Schauser and Chorus 2007)
The model developed by Hoare (1980) was used together with Nurnberg's (1984) method of estimating retention coefficients.	The Vollenweider (1982) combined OECD relationship empirical model was chosen (no reference given but the equations used are shown in the report).	BATHTUB model (Walker 1996) was used.	The Vollenweider (1976) and the modified One-Box models (Gachter and Imboden 1985, SAS 1989) were used.

Table 6 Different acceptable means to achieve load reduction targets.

The Rotorua Lakes	Clear Lake (IDNR 2005)	Lake Herman (SDDENR 2005)	Lake Tegel and Schlachtensee (Schauser and Chorus 2007)
Action Plans for the impaired lakes have been or are being developed and then implemented.	The TMDL approval application included an implementation Plan to achieve the required P load reduction.	The required reduction in the external load would be achieved by implementing a number of Best Management Practices in the watershed.	The plan was to put the entire water input to each lake through Phosphorus Elimination Plants.

Table 7 Results of actions taken to reduce loads.

The Rotorua Lakes	Clear Lake (IDNR 2005)	Lake Herman (SDDENR 2005)	Lake Tegel and Schlachtensee (Schauser and Chorus 2007)
Some planned actions have been implemented and some results have been obtained, as described above.	No published reports are known regarding the implementation and results of the load reduction plans.	No published reports are known regarding the implementation and results of the load reduction plans.	The Phosphorus Elimination Plants were built and Schlachtensee has reached the target of 30 ugP/L for some of the time with an accompanied large reduction in Chla concentrations. P and Chla concentrations have reduced considerably in Lake Tegel but the P target has not been reached because of continued internal loading.

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