Reference and current Trophic Level Index of New Zealand lakes: benchmarks to inform lake management and assessment

Figures & data

Figure 1. Summary of analysis steps to predict current and reference Trophic Level Index (TLI) values for 1031 lakes in New Zealand. Acronyms are defined in the text.

Table 1. Summary of NWQMN lake water quality data used to develop models to predict annual mean chlorophyll a (chl a) concentrations and annual mean Secchi depth (Secchi).

Dependent variable modelled	Variable	Units	Number of lakes	Number of measurements per lake (mean in parentheses)	Minimum of annual mean values	Maximum of annual mean values	Mean of annual mean values	Mean coefficient of variation (%)
Chl a	TN	mg m^−3	73	10–261 (66)	39	3266	567	45
	TP	mg m^−3	73	10–261 (68)	4.7	349.0	44.8	93
	Chl a	mg m^−3	73	10–262 (68)	0.41	184.59	15.37	100
Ref_ Chl a	TN	mg m^−3	66	10–261 (65)	39	1440	367	38
	TP	mg m^−3	66	10–261 (66)	4.7	145.9	22.9	38
	Chl a	mg m^−3	66	10–262 (66)	0.41	44.54	6.67	100
Secchi	Chl a	mg m^−3	43	10–45 (34)	0.41	184.59	12.22	95
	Secchi	m	43	10–261 (79)	0.11	15.28	4.83	36
Ref_Secchi	Chl a	mg m^−3	16	51–262 (129)	1.03	17.56	3.93	85
	Secchi	m	16	48–261 (149)	2.64	15.28	8.26	25

Note: ‘Ref_’ denotes reference values.

Figure 2. Locations and geomorphic type of the 1031 lakes for which current and reference Trophic Level Index values were estimated in this study. Adapted from Abell et al. (2019).

Table 2. Summary of models used to predict current and reference state values of chlorophyll a and Secchi depth in 1031 New Zealand lakes.

Dependent variable	Model	SEE	RMSE	R^2	Notes
log10chl a (mg m^−3)	$-1.80+0.70\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}T{N}_{lake}+0.55\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}T{P}_{lake}$	0.21	0.21	0.89	chl a, mean lake chlorophyll a concentration in mg m^−3 TNlake (or TPlake), mean in-lake surface TN (or TP) concentration in mg m^−3
log10Ref_Chl a (mg m^−3)	$-1.73+0.65\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}Ref\mathrm{\_}T{N}_{lake}+0.59\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}Ref\mathrm{\_}T{P}_{lake}$	0.20	0.21	0.84	Ref_, denotes reference state values
Secchi^0.5 (m)	$3.46-0.74\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}Chla-0.79\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}Chla\cdot d-0.35\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}\left({\displaystyle \frac{Fetch\cdot U^2}{z_{max}}}\right)\cdot \left(1-d\right)$	0.32	1.68	0.89	Secchi, mean lake Secchi depth in m, Fetch, maximum lake fetch in m, U, mean windspeed at lake (m s^−1), zmax, maximum lake depth in m, d, equal to 0 if zmax<20 m or equal to 1 if zmax ≥ 20 m
Ref_Secchi^0.5 (m)	$3.42-1.46\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}Ref\mathrm{\_}Chla$	0.35	2.48	0.70

Note: SEE: standard error of the estimate; RMSE: root mean square error of a 10-fold cross validation.

Table 3. Summary of estimated values of Trophic Level Index (TLI) variables for 1031 lakes.

Variable	Units	State	Percentiles					Mean
			10th	25th	50th	75th	90th
Chlorophyll a	mg m^−3	Reference	1.10	1.17	1.53	2.81	5.11	2.42
		Current	1.39	1.88	2.98	7.52	20.56	8.32
Secchi depth	m	Reference	6.01	8.01	10.35	11.44	11.71	9.62
		Current	2.12	3.59	5.88	9.37	10.90	6.48
TLI	TLI units	Reference	2.31	2.36	2.60	3.16	3.73	2.82
		Current	2.53	2.84	3.28	4.18	5.11	3.58

Figure 3. Predicted TLI for 1031 New Zealand lakes corresponding to current and reference states. Vertical lines denote 95% confidence intervals. Lakes are ordered by ascending current TLI values.

Figure 4. Estimated TLI for New Zealand lakes corresponding to reference and current conditions, differentiated by lake geomorphic type. Sample sizes are shown in parentheses. Only types with sample size ≥25 are presented and lakes with an undefined geomorphic type (n = 42) are not shown.

Figure 5. Frequency (counts) distribution of estimated departure from reference Trophic Level Index (TLI) for 1031 New Zealand lakes categorised based on geomorphic type.

Abell JM, Özkundakci D, Hamilton DP, van Dam-Bates P, McDowell RW. 2019. Quantifying the extent of anthropogenic eutrophication of lakes at a national scale in New Zealand. Environmental Science & Technology. 53:9439–9452. doi: 10.1021/acs.est.9b03120

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