Expert Workshop: Nutrient Enrichment Indicators in Streams
For the past 15 years, the U.S. Environmental Protection Agency has encouraged states and Tribes to adopt numeric criteria into water quality standards to protect waters from the widespread and growing problem of nutrient pollution. Excess nutrients (nitrogen and phosphorus) cause algal growth that degrades aquatic communities and cause fish kills, degrades beaches and shorelines with nuisance algae, and adversely affect human health from algal toxins and trihalomethane formation in drinking water. State progress toward adopting numeric nutrient criteria has been limited in flowing waters in part because of the technical challenge of developing numeric nutrient criteria when multiple factors (e.g., light, flow) can influence responses (e.g., algal biomass) and confound nutrient response models. Such conditions can make it difficult to predict nitrogen and phosphorus concentrations that adversely affect aquatic life. One approach to overcome such challenges and to reduce uncertainty when implementing numeric criteria is to integrate biological response indicators with numeric nutrient criteria in a decisional framework.
This workshop proceedings document captures the insight of the technical experts. This information will be beneficial in efforts to provide technical support for states on the derivation and implementation of numeric nutrient criteria in flowing waters.
The content describes the primary workshop findings.
Which Indicators are Most Sensitive to Nutrient Pollution in Streams and most Predictive of Impacts to Higher Trophic Levels?
- Nutrients: TN and TP concentrations provide a direct measure of nutrient pollution. Thus, measured concentrations above thresholds known to adversely affect aquatic life should indicate impairment.
- Primary producers: Chl-a, percent visual coverage of algae and in-stream macrophytes, and measures of algal assemblage (e.g., diatoms and soft-bodied algae) are the most sensitive response indicators of nutrient pollution in streams. Algal assemblage indicators are widely recommended as sensitive nutrient response indicators in all waters.
- Ecosystem function: Continuously measured DO and pH are good indicators that capture heterotrophic and autotrophic responses, are generally sensitive to nutrient stress, and provide a clear linkage to aquatic life. The workshop participants acknowledged the routine monitoring of fish and macroinvertebrates, and the public’s recognition of the linkage between adverse effects on fish and invertebrates and impairment of aquatic life. While they concluded that commonly used fish and macroinvertebrate indices may be less sensitive nutrient pollution indicators than other indicators (e.g., algae), refined and/or species-level metrics for macroinvertebrates specifically calibrated to be responsive to nutrient effects continue to show promise as indicators. However, there can be a significant temporal lag between high nutrient concentrations and adverse effects to some higher trophic levels, making it difficult to proactively prevent nutrient impairment.
How can Criteria be Structured in a Combined Approach?
When there is uncertainty around the relationship between nutrient concentrations and the health of the aquatic community, some experts suggested it might be useful to combine numeric nutrient criteria into a decision framework with other indicators, but there was not universal agreement on the defensibility of such an approach. One approach might be to establish an upper nutrient concentration, above which designated uses are impaired, and a lower nutrient concentration, below which designated uses are attained. The concentrations between these upper and lower values make up a “grey zone,” within which a numeric nutrient criterion, expressed as a decision framework, could be applied.
- Considering indicators that are commonly available to states, the following combination of indicators are sensitive to nutrient pollution: nutrient concentrations (TN and TP); chl-a; and, to a lesser extent, DO.
- Considering all possible indicators, the following combination of indicators are sensitive to nutrient pollution and may provide early warning of impairment: nutrient concentrations (TN and TP); a measure of algal biomass (chl-a, ash-free dry mass [AFDM], or visual percent cover); a measure of the primary producer assemblage (mostly based on diatoms); and, to a lesser extent, a measure of ecosystem function (e.g., diel DO or pH).
- Adverse responses from any of these possible indicators should be sufficient to indicate nutrient pollution-related impairment.
- Sufficient data and robust stressor-response models are imperative for numeric nutrient criteria development and assessment. Participants were concerned about the lack of sufficient data for nutrient criterion development and assessment.
- Proper classification of data by attributes, such as expected trophic state or physical factors, is fundamental to reducing natural variability in nutrient responses in different types of streams.
What Information Gaps Exist Regarding Nutrient Criteria Development in Streams?
The workshop participants identified the following research needs: (1) development of a single standardized primary producer indicator that integrates the productivity of various producers into a single indicator; (2) improved understanding of the linkages between nutrient measures, primary producer measures, and higher trophic levels that often are used to quantify aquatic life impairment; (3) identification of a minimum data set necessary to characterize stressor-response relationships; and (4) development of regional stressor-response relationships linking nutrient concentrations to algal assemblage indicators, algal abundance, and nutrient-sensitive macroinvertebrate indicators.
Workshop Proceedings
Full proceedings of the workshop discussions can be accessed by selecting the link.