Analytical methods for physicochemical and microbiological water quality
Lake Wairarapa monitoring sites are accessed by boat and the Lake Onoke monitoring sites (including the two upstream monitoring sites) are accessed by wading from the lake or river edge. Water samples are collected in accordance with the sub-surface grab method for sampling isothermal lakes described in Smith et al. (1989) and in the case of Lake Onoke, a ‘grab pole’ is used to collect water samples in an effort to minimise the potential effects of re-suspension of lake-bed sediments (caused by wading) on the samples. Note that the sub-surface grab method differs from protocols outlined in Burns et al. (2000) for the sampling of isothermal lakes.
Field measurements (conductivity, dissolved oxygen and temperature) are taken using a YSI DSS field meter which is calibrated on the day of sampling. Secchi disc measurement methodology is consistent with the procedure outlined in Burns et al. (2000) except that an underwater viewer is not used. Note that all field measurements collected from Lake Onoke (and upstream sites) are made from a ‘wading position’, although care is taken to minimise any disturbance of lakebed sediments.
Water samples requiring laboratory analysis are stored on ice upon collection and couriered overnight to RJ Hill Laboratories in Hamilton. The variables monitored and current analytical methods are summarised in Table A2.1. All lake water samples collected for dissolved nutrient analysis are filtered in the laboratory.
| Variable | Method | Detection limit |
|---|---|---|
| pH | pH meter. APHA 4500-H+ B 22nd ed. 2012 | 0.1 pH units |
| Turbidity | Analysis using a Hach 2100N, Turbidity meter. APHA 2130 B 22nd Ed. 2012 | 0.05 NTU |
| Total suspended solids | Filtration using Whatman 934 AH, Advantec GC-50 or 1-2 equivalent filters (nominal pore size 1.2 - 1.5μm), gravimetric determination. APHA 2540 D 22nd Ed. 2012 | 2 mg/L |
| Volatile suspended solids | Filtration (GF/C, 1.2 μm). Ashing 550°C, 30 min. Gravimetric. APHA 2540 E 22nd Ed. 2012 | 2 mg/L |
| Nitrate-N | Calculation: (Nitrate-N + Nitrite-N) - Nitrite-N | 0.001 mg/L |
| Nitrite-N | Automated Azo dye colorimetry, Flow injection analyser. APHA 4500-NO 3 - I (modified) 22nd Ed. 2012 | 0.001 mg/L |
| Nitrate-N + Nitrite-N (NNN) | Total oxidised nitrogen. Automated cadmium reduction, Flow injection analyser. APHA 4500-NO 3 - I (modified) 22nd Ed. 2012 | 0.001 mg/L |
| Ammoniacal nitrogen | Phenol/hypochlorite colorimetry. Flow injection analyers. (NH 4 -N = NH 4 +-N + NH 3 -N) APHA 4500-NH 3 F 22nd Ed. 2012 | 0.005 mg/L |
| Total Kjeldahl nitrogen | Kjeldahl digestion, phenol/hyperclorite colorimetry (Discrete Analysis). APHA 4500-N Org C. (modified) 4500- F (modified) 22nd Ed. 2012 | 0.1 mg/L |
| Total nitrogen | Calculation: TKN + Nitrate-N + Nitrite-N | 0.05 mg/L |
| Dissolved reactive phosphorus | Filtered sample. Molybdenum blue colorimetry. Flow injection analyser. APHA 4500-P G 22nd Ed. 2012 | 0.001 mg/L |
| Total phosphorus (mg/m3) | Total Phosphorus digestion, ascorbic acid colorimetry. Discrete Analyser. APHA 4500-P B & E (modified from manual analysis) 22nd Ed. 2012 | 0.004 mg/L |
| Chlorophyll a | Acetone extraction. Spectroscopy. APHA 10200 H (modified) 22nd Ed. 2012 | 0.003 mg/L |
| Pheophytin a (mg/m3) | Acetone extraction. Spectroscopy. APHA 10200 H (modified) 22nd Ed. 2012 | 0.003 mg/L |
| Absorbance at 340 nm | Filtered sample. Spectrophotometry, 1cm cell. APHA 5910 B 22nd Ed. 2012 | 0.002 AU/cm |
| Absorbance at 440 nm | Filtered sample. Spectrophotometry, 1cm cell. APHA 5910 B 22nd Ed. 2012 | 0.002 AU/cm |
| Absorbance at 780 nm | Filtered sample. Spectrophotometry, 1cm cell. APHA 5910 B 22nd Ed. 2012 | 0.002 AU/cm |
