Should States Augment Numeric Nutrient Criteria for Rivers & Streams with Macroinvertebrate Assessments?

The 2001 National Academy of Sciences review of the TMDL program recommended that water quality criteria be positioned as closely as possible to the biological (or human health) response in the stressor-response causal chain. For nutrient criteria, this means that a measure of algal density (e.g., chlorophyll a), submerged aquatic vegetation, and/or macroinvertebrate indices might serve to augment or replace phosphorus and nitrogen criteria. A number of recent high-profile US EPA and other regulatory agency efforts to develop water quality criteria using possible stressor-response relationships observed in field data highlight the importance of this as part of the nutrient criteria development process; the recent assessment of Florida nutrient criteria provides a contentious example. As two recent EPA Science Advisory Board reviews have made clear, the presence of an underlying cause and effect in the stressor-response relationship is critical to the effectiveness of such water quality criteria. Unfortunately there is little EPA guidance on how to develop sufficient evidence to support cause-effect conclusions. This lack of guidance increases the likelihood that water quality criteria, lacking a firm basis for establishing cause and effect, will be proposed or established in regulations, resulting in ineffective and inefficient criteria.

We know that phosphorus and nitrogen are essential nutrients for plant and animal life. Likewise, we know that the range of levels of phosphorus and nitrogen found in surface water bodies do affect aquatic biota such as algae and aquatic vegetation. Indeed, there is a well-established scientific basis linking nutrient concentrations in lakes/estuaries to chlorophyll a; this relationship has been observed in both cross-sectional and time series analyses of data.  However, while phosphorus and nitrogen are obviously essential for macroinvertebrate life, it has not been demonstrated that the range of phosphorus and nitrogen levels found in rivers and streams is a strong determinant of macroinvertebrate indicators of aquatic ecosystem health in these waterbodies. Observational data analyses suggest that other determinants of benthic macroinvertebrate indices (BMIs), such as variations in streamflow and temperature, sediments, and toxic substances may dominate cause-effect relationships. Thus one or more of these stressors may be the primary cause of observed changes in a river or stream benthic macroinvertebrate index that could falsely be attributed to nutrient levels.

When a state agency sets or modifies a water quality criterion, it is reasonable to expect that the state agency believes that its action will improve the probability of correct decisions on use impairment for a waterbody. In the specific situation of BMIs augmenting nutrient criteria, data analyses and rigorous causal analyses do not currently support that belief. Methods such as counterfactual analysis, Bayesian networks, and/or weight of evidence are needed to justify the causal relationship between nutrients and benthic macroinvertebrates in streams. Otherwise, we risk costly nutrient control measures that do not yield the expected benefits. This not only is a waste of critical resources, it undermines public confidence in legitimate efforts at environmental protection.