Excess nutrients and associated nuisance algae are among the top 5 causes of impairment of Montana’s wadeable streams. In 2008, the state of Montana developed ecoregion-specific numeric criteria for nutrients and nuisance algae, based on stressor-response studies and reference stream data. Harm to beneficial uses was associated with nutrient levels from about the 73rd to 99th percentiles of reference (mean: 86th), hence numeric criteria were set at the 75th & 90th percentiles for the plains and mountains respectively. Since 2008, more Montana reference streams have been characterized, and more stressor-response studies have been conducted (both in and out of the region).
Results/Conclusions
These studies suggest that some of the ecoregion-specific criteria developed in 2008 were overly restrictive (e.g., in the Canadian Rockies) while other criteria were insufficiently protective (e.g., in the Middle Rockies). Updated criteria take newer studies into account and focus on total nitrogen and total phosphorus, and on their ratio in comparison to Redfield. Water quality standards based on control of a single nutrient (i.e., P) could result in unwanted ecological consequences in Montana’s rivers and streams. Background nutrient levels in our western reference streams are usually quite low and have TN:TP ratios close to the Redfield ratio (7:1 by mass). The nuisance diatom Didymosphenia geminata has, in recent years, produced nuisance growths in low P streams, thriving in waters where N:P ratios are high (34:1 on average). In contrast, nuisance levels of the green alga Cladophora glomerata are often found in high P streams with low N:P ratios. Updated ecoregion-specific criteria for the summer growing season for TN and TP and recommendations for N:P ratios are presented for 18 ecoregions (6 level IIIs and 12 level IVs)in Montana. As an example, criteria for various subregions of the Middle Rockies ecoregion range from 30 to 105 ppb TP and from 250 to 300 ppb TN.