Nitrate in freshwater and marine water

​​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals’ of the ANZECC & ARMCANZ (2000) guidelines.

The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.

Description of chemical

Nitrate is essential for growth of aquatic plants. The main issue with elevated levels of nitrate is its potential to stimulate algal growth and hence to be a factor in nuisance algal blooms and eutrophication of waterways — usually from human wastes or fertilisers. At high enough levels, nitrate can be toxic to aquatic life. Toxicity data were reviewed for both potassium nitrate (KNO3; CAS 7757-79-1) and sodium nitrate (NaNO3; CAS 7631-99-4).

Aquatic toxicology

Potassium nitrate was generally more toxic than sodium nitrate (many of the comparative tests were reported in the same publication). Figures are given as mg NO3/L.

Freshwater fish: (48 to 96-hour LC50): six species, 99 to 10,000 mg/L (x 1000 µg/L). Chronic 9-day NOEC of 14 mg/L to Australian Mogurnda adspersa.

Freshwater crustaceans: 48 to 96-hour LC50 to Daphnia magna, 23 to 4206 mg/L.

Freshwater molluscs: Lymnaea sp. 96-hour LC50, 664 mg/L.

Freshwater insects: two species, 72 to 96-hour LC50, 430 to 930 mg/L.

Freshwater hydra: Hydra viridissima 6 day chronic NOEC (population growth) of 9 mg/L (Australian).

Marine fish: six species, 96-hour LC50, 2536 to 13,280 mg/L.

Marine mollusc: 1 species, 96-hour LC50, 11,510 to 27,580 mg/L.

Australian and New Zealand data

The only chronic data were for potassium nitrate on Australian purple-spotted gudgeon Mogurnda mogurnda and hydra, Hydra viridissima. There were no overseas chronic data for comparison. Tests with the marine prawn Penaeus monodon (Muir et al. 1991), indicated that nitrate had a significant effect on survival of larvae at 1000 µg/L but no dose-response figures were given.

Guideline

As nitrates are a known stimulant for algal growth at low concentrations, it was considered acceptable to derive trigger values on an adequate number of data without algae. Separate marine figures were derived because of the apparent differences in sensitivity on the limited marine data.

A freshwater moderate reliability trigger value for nitrate toxicity as NO3 (nitrate) of 700 µg/L was calculated using the statistical distribution method 95% protection and the default acute-to-chronic ratio (ACR). Although a marine low reliability figure of 13,000 µg/L (13 mg/L) could be calculated using an assessment factor (AF) of 200 (limited data but a lesser factor due to essentiality), it is preferable to adopt the freshwater figure of 700 µg/L for nitrate toxicity as NO3 (nitrate) as a marine low reliability trigger value.

References

ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.

Muir PR, Sutton DC & Owens L 1991. Nitrate toxicity to Penaeus monodon protozoea. Marine Biology 108, 67-71.