Chlorophyll-a Determination in Accordance with Chinese National Standards: Sampling, Testing Methods

Chlorophyll-a determination in accordance with Chinese national and industry standards, such as HJ 897-2017 and SL 88-2012, is the core analytical procedure used to quantify phytoplankton biomass and evaluate eutrophication in lakes, reservoirs, rivers, and coastal waters. This article explains the applicable standard framework, accepted sample types, spectrophotometric and fluorometric testing methods, quality control requirements, and typical applications, providing environmental professionals and accredited third-party laboratories with a practical technical reference.

Technical Overview

Chlorophyll-a (Chl-a) is the principal photosynthetic pigment found in nearly all photoautotrophic phytoplankton, and its concentration in water is universally regarded as a reliable proxy for algal biomass and an early indicator of eutrophication. In China, the determination of chlorophyll-a is governed by a well-established framework of national environmental protection standards and industry standards, which specify sampling, extraction, instrumental measurement, and calculation procedures.

• HJ 897-2017, Water Quality - Determination of Chlorophyll-a - Spectrophotometric Method: the primary standard method for surface water such as rivers, lakes, and reservoirs, based on 90% acetone extraction with acidification correction.
• SL 88-2012, Determination of Chlorophyll in Water - Spectrophotometric Method: a water-resources industry standard based on hot ethanol extraction, widely used in hydrological and reservoir monitoring networks.
• GB 17378.7-2007, The Specification for Marine Monitoring - Part 7: specifies spectrophotometric and fluorometric procedures for chlorophyll-a in seawater and offshore environmental surveys.
• HJ/T 91-2002 and HJ 493-2009: technical specifications for surface water monitoring and for the preservation and management of water samples, which define sampling representativeness and holding conditions.
• ISO 10260:1992, Water Quality - Measurement of Biochemical Parameters - Spectrometric Determination of the Chlorophyll-a Concentration: the international reference method for comparison and data harmonization.

Because chlorophyll-a serves as the benchmark parameter in comprehensive trophic state index (TLI) evaluations of lakes and reservoirs, accurate standard-compliant determination is essential for environmental authorities, water utilities, research institutions, and accredited third-party laboratories operating under national laboratory accreditation schemes.

Testing Samples

Lakes and Reservoirs

Lake and reservoir water is the most common sample matrix. Samples are typically collected at routine monitoring stations as depth-integrated or surface samples, and the filtration volume is selected according to the expected algal biomass, generally between 0.5 L and 5 L. Chlorophyll-a results from these samples directly support trophic state classification and algal bloom early warning.

Rivers and Estuarine Water

Flowing water bodies are sampled at defined cross-sections or confluence points to track phytoplankton transport and the influence of nutrient inputs. In estuaries where salinity varies, extraction efficiency and turbidity interference must be evaluated as part of method validation.

Coastal and Marine Waters

Seawater samples collected during offshore and coastal surveys are analyzed according to the marine monitoring specification GB 17378.7, using either spectrophotometry or the more sensitive fluorometric technique, particularly for oligotrophic offshore stations where chlorophyll-a concentrations are very low.

Drinking Water Sources and Treatment Plants

Source water intakes and process water at treatment plants are monitored for chlorophyll-a to anticipate taste-and-odor events, filter clogging, and disinfection by-product formation associated with algal blooms, enabling proactive adjustment of coagulation and pre-oxidation dosing.

Aquaculture Water and Effluents

ponds, recirculating aquaculture systems, and WWTP polishing units where algae grow may be tested to assess ecological condition and compliance with discharge-related ecological objectives.

Sample handling follows strict preservation rules: water is collected in amber glass containers, protected from light, kept at approximately 4 degrees Celsius, and filtered as soon as possible, ideally within a few hours and no later than 24 hours. Filters retaining the phytoplankton are folded, wrapped in aluminum foil, and frozen in the dark until extraction, in line with HJ/T 91-2002 and HJ 493-2009.

Testing Items

The core deliverable of the test is the chlorophyll-a concentration in the original water sample, expressed in micrograms per liter, supported by correction parameters and method performance data. Typical reporting items include:

• Chlorophyll-a concentration (µg/L) in the water sample, calculated from the measured pigment mass and the filtered volume.
• Phaeophytin-a concentration and acidification-corrected chlorophyll-a, which distinguish living algal pigment from degradation products of zooplankton grazing and cellular senescence.
• Chlorophyll-b and chlorophyll-c concentrations, where trichromatic spectrophotometric equations are applied for green-algae and diatom dominated communities.
• Accessory pigment profiles and pigment ratios, where HPLC-based analysis is requested for community composition or bloom senescence studies.
• Quality control data, including field and laboratory blanks, duplicate precision, matrix spike recovery, calibration verification, and the reported method detection limit.

Testing Methods

Acetone Extraction Spectrophotometry (HJ 897-2017)

The national standard method for surface water involves the following sequence:

• A measured volume of water is filtered under low vacuum onto a glass-fiber filter, and the filtered volume is recorded.
• The filter is folded, protected from light, and stored frozen to disrupt cells and preserve pigments.
• The retained phytoplankton is extracted with 90% acetone in the dark at low temperature for at least 12 hours, typically with homogenization or freeze-thaw assistance to improve recovery.
• The extract is clarified by centrifugation, and absorbance is measured at 664 nm with a 750 nm turbidity correction.
• The extract is acidified with dilute hydrochloric acid to convert chlorophyll-a to phaeophytin-a, and absorbance is measured again, allowing the monochromatic acidification correction calculation.
• Chlorophyll-a concentration is calculated from the absorbance difference before and after acidification, corrected for extraction volume, sample volume, and cuvette path length.

The method achieves detection limits on the order of 0.1 µg/L and is suitable for the full range of trophic conditions encountered in rivers, lakes, and reservoirs.

Ethanol Extraction Spectrophotometry (SL 88-2012)

This water-resources industry standard uses hot ethanol extraction of the filtered sample, followed by measurement of absorbance at characteristic wavelengths such as 665 nm and 649 nm. Ethanol is a safer solvent than acetone and provides efficient pigment recovery, and the method is widely applied within hydrological monitoring networks for reservoirs, irrigation sources, and ecological flow assessments. When data from acetone-based and ethanol-based procedures are compared, extraction efficiency differences must be documented during method validation.

Fluorometric Determination

Fluorometry measures chlorophyll-a via its natural fluorescence, typically with excitation near 430 nm and emission near 670 nm, combined with acidification to correct for phaeophytin interference. The technique offers sensitivity roughly an order of magnitude better than spectrophotometry and is preferred for oligotrophic lakes, deep offshore seawater, and vertical profiling. Modern in-situ fluorescence sensors and multi-wavelength algal probes extend this principle to continuous, real-time monitoring, although sensor readings are calibrated and periodically verified against laboratory standard method results.

HPLC Pigment Analysis

High-performance liquid chromatography separates chlorophyll-a from its degradation products and from accessory pigments such as fucoxanthin, zeaxanthin, and peridinin, enabling both precise quantification and chemotaxonomic inference of algal community composition. It is mainly used in research programs, remote sensing validation campaigns, and investigations where pigment integrity is critical.

Quality Assurance and Quality Control

Reliable results depend on rigorous quality control, including:

• Strict light protection throughout sampling, filtration, extraction, and measurement, since chlorophyll-a degrades rapidly under illumination.
• Field blanks, filter blanks, and solvent blanks to verify that no pigment contamination or carryover occurs.
• Parallel duplicate samples with acceptance criteria for relative deviation, typically within 20% at low concentrations.
• Matrix spike and recovery checks, generally targeting the 80% to 120% range.
• Wavelength and photometric accuracy verification of the spectrophotometer, matched cuvettes, and documented temperature control of the extraction step.
• Reporting of the method detection limit determined according to the laboratory's validated procedure and accreditation scope.

Applications

Chlorophyll-a data determined under national standards support a wide spectrum of environmental management activities:

• Lake and reservoir eutrophication assessment using the comprehensive trophic state index, where chlorophyll-a is the reference parameter; TLI values below 30 indicate oligotrophic conditions, 30 to 50 mesotrophic conditions, and values above 50 indicate eutrophic conditions graded from mild to severe.
• Algal bloom early warning and emergency response, where rising chlorophyll-a concentrations trigger enhanced monitoring frequency and source water protection measures.
• Drinking water safety management, including intake switching, pre-treatment optimization, and control of taste, odor, and algal toxin risks.
• Ecological and hydrological research, such as primary productivity estimation, food-web studies, and evaluation of restoration projects.
• Marine environmental monitoring and red tide surveillance under GB 17378.7 offshore survey programs.
• Ground-truth calibration and validation of satellite remote sensing products for water color and bloom mapping.
• Aquaculture pond management and wastewater treatment plant optimization where algal dynamics affect process performance.

FAQ

Which national standard applies to chlorophyll-a determination in freshwater?

For surface water such as rivers, lakes, and reservoirs, HJ 897-2017 is the applicable spectrophotometric standard method. For water-resources monitoring networks, SL 88-2012 with ethanol extraction is also accepted, and seawater is covered by GB 17378.7-2007. The choice of method should match the monitoring program requirements and the laboratory's accredited scope.

What is the difference between the acetone and ethanol extraction methods?

Both methods filter phytoplankton and extract pigments for spectrophotometric measurement. The acetone method specified in HJ 897-2017 extracts with cold 90% acetone in the dark, while the ethanol method in SL 88-2012 uses hot ethanol. Extraction efficiency, calculation equations, and wavelength settings differ, so results should be interpreted with the method clearly identified and method comparability documented when datasets are combined.

Why is acidification performed during measurement?

Acidification converts chlorophyll-a into phaeophytin-a, which has a different absorbance. By measuring the extract before and after acidification, the laboratory can distinguish active chlorophyll-a from degradation products, yielding a corrected value that better represents living algal biomass and reducing overestimation in grazed or senescent populations.

How should samples be collected and preserved?

Samples should be collected in amber glass bottles, shielded from light, cooled to approximately 4 degrees Celsius, and filtered promptly, ideally within a few hours. Retained filters are wrapped in foil and frozen in the dark until extraction. Chlorophyll-a is photosensitive, so any exposure to light or delay in filtration can cause significant negative bias.

What units and detection limits are typical?

Results are reported in micrograms per liter of chlorophyll-a in the original water sample. Spectrophotometric methods typically achieve detection limits around 0.1 µg/L, while fluorometric methods reach substantially lower levels, making them suitable for oligotrophic lakes and open ocean waters.

Can in-situ sensor readings replace laboratory testing?

In-situ fluorescence sensors provide valuable continuous, real-time trends for bloom surveillance, but their readings are affected by turbidity, temperature, pigment packaging effects, and algal community composition. Regulatory reporting and trophic state assessment therefore rely on laboratory determination using the national standard method, with sensors used as calibrated screening tools that are periodically verified against laboratory data.