Chronic toxicity of ammonia to New Zealand freshwater invertebrates: A mesocosm study

Freshwater macroinvertebrate communities were established within 12 artific ial streams or "toroidal" mesocosms and exposed to three replicated concent rations of ammonia for 29 days at constant temperature (16 degrees C) and p H (median 8.4. The criterion units (CU = measured [ammonia]/US EPA 1985 chr onic criterion value of total ammonia in the LOW, MED, and HIGH treatments were 2.0, 4.8, and 13 CUs respectively, and 1.9, 5.8, and 12 CUs for the un ionized ammonia. Macroinvertebrates were tolerant of the ammonia exposures with no significant (p > 0.1) effect on taxa richness; number of taxa in th e orders Ephemeroptera, Plecoptera, and Trichoptera (EPT); or the quantitat ive macroinvertebrate community index (QMCT), a biotic index proposed for a ssessing effects of organic enrichment in New Zealand streams. Significant differences (p < 0.05) occurred for the mean abundance and the numbers of E PT individuals (QEPT), with the HIGH treatment significantly lower (-41%) t han the control for both abundance and QEPT. Of the major species, only the mayflies Deleatidium sp. (Ephemeroptera: Leptophlebiidae) and Coloburiscus humeralis (Ephemeroptera: Oligoneuriidae) showed significant reductions in abundance, with only the caddisflies Beraeoptera roria (Trichoptera: Conoe sucidae) and Confluens sp. (Trichoptera: Conoesucidae) showing significant increases in abundance. The abundance of juvenile Deleatidium sp. had a neg ative concentration-response relationship that resulted in an 82% decrease in abundance in the HIGH treatment. Drift of invertebrates showed no respon se to ammonia treatments. The 29-day EC50 values for Deleatidium sp, for to tal and unionized ammonia were 2.15 mg (N)/L (pH 8.4) and 0.145 mg (NH3-N)/ L. No observed effect concentration ((NOEC) values were 0.95 mg (N)/L and 0 .066 mg (NH3-N)/L, and the threshold effect concentration (TEC) was 1.49 mg (N)/L and 0.102 mg (NH3-N)/L. Comparison of the Deleatidium sp. chronic am monia sensitivity data with the US EPA 1985 chronic criterion value (CCC = 0.45 mg [N]/L, pH 8.4, 16 degrees C) showed the TEC value for total ammonia was 3.3 x CCC, and 2.2X higher than the updated US EPA 1998 criteria. The endings suggest that use of the US EPA criteria would provide minimal prote ction for Deleatidium for chronic ammonia exposure, and that development of site-specific criteria, covering a wide range of environmental conditions, may be required to adequately protect this species.