Wednesday, October 5, 2016

Is Chlorophyll a Reliable Indicator of Designated Use in Lakes and Reservoirs?

State water quality standards are established in accordance with Section 303(c) of the Clean Water Act and must include a designated-use statement and one or more water quality criteria. Over the past several years, the USEPA has been assisting the states to adopt/modify nutrient criteria. The criteria serve as measurable surrogates for the narrative designated use; in other words, measurement of the criteria provides an indication of attainment of the designated use. In addition, violation of the criteria is a basis for regulatory enforcement, which typically requires establishment of a TMDL. Thus, good criteria should be easily measurable and good indicators of the attainment of designated use.

Traditionally, the task of setting criteria has involved judgments by government and university scientists concerning the selection of specific water quality characteristics and the levels of those characteristics that are associated with the designated use. For example, consider the North Carolina chlorophyll a criterion of 40 ug/l, which was established in 1979. This criterion applies to Class C waters, which are freshwaters with use designations of secondary recreation, fishing, and aquatic life support. To establish this criterion, the NC Division of Environmental Management examined the scientific literature on eutrophication and then recommended a chlorophyll criterion level of 50 ug/l to a panel of scientists for consideration. After reviewing a study of nutrient enrichment in 69 North Carolina lakes, the panel responded that 40 ug/l reflected a transition to algal, macrophyte, and DO problems and thus represented a better choice. Following public hearings, 40 ug/l was adopted as the chlorophyll water quality criterion. The 40 ug/l criterion was developed from an ad hoc process of science-based expert judgment. In my view, we should be cautious in selecting a criterion level simply because it represents a change/transition point in waterbody response (e.g., transition to algal, macrophyte, and DO problems). For example, a DO level of zero is clearly a major transition point for an aquatic ecosystem, but it is unlikely to make sense as a water quality criterion protective of designated use. The criterion level should also reflect public values on designated use; good water quality criterion selection is not strictly a scientific endeavor.

In Reckhow et al. (2005), structural equation modeling and expert elicitation were used to quantitatively link candidate water quality criteria with designated use. This technique was applied to Lake Washington, with designated uses that protect, for example, salmon and trout, primary contact recreation, domestic water supply, wildlife habitat, commerce and navigation, boating, and aesthetic values. Dr. Eugene Welch, a Professor Emeritus at the University of Washington, was chosen as the expert for this study. Presented with Lake Washington’s designated-use statement, Dr. Welch identified boating as the most appropriate nutrient-related designated use to address based on his technical expertise. He selected water clarity, the absence of algal scums, odor, and interference from aquatic vegetation as desired properties of a “boatable” lake. In addition, the expert provided a conceptual model that included chlorophyll a, total phosphorus, Secchi depth, total zooplankton, and Daphnia biomass as the key environmental variables for assessing attainment of the designated use. He hypothesized that chlorophyll a would be the water quality variable most closely linked to the desirable properties of a boatable lake.

Consider the approaches of a few other states. In Florida lakes, a numeric chlorophyll criterion has been set to protect designated use associated with “an imbalance in the natural populations of the aquatic flora or fauna.” This is similar to the North Carolina perspective. In addition to the chlorophyll criterion, Florida also established criteria for nitrogen and phosphorus.

In Texas, PBS&J (2003) conducted a thorough and thoughtful analysis of nutrient water quality standards in lakes in the Trinity River Basin. They observed that “the (Trinity River Basin) study reservoirs are heavily used for recreation, water supply, and support healthy aquatic life communities. By that measure, all the reservoirs supported their designated uses.” However, based on the chlorophyll water quality criterion, seven of the nine reservoirs were not in compliance. Despite this inconsistency, PBS&J “determined that chlorophyll a was the parameter most directly related to uses, and that it should be the parameter selected for numerical criteria development.”

While recommending chlorophyll as the best nutrient criterion, PBS&J pointed out the difficulty in selecting the specific concentration of chlorophyll to serve as the compliance/noncompliance cutoff. The figures below from the PBS&J report identify an “optimal range” for the chlorophyll cutoff for the designated uses in the Trinity River Basin lakes. What is most striking about these figures is the size of the optimal range and the insensitivity of designated use to chlorophyll levels within this optimal range.



Why is this so important? This insensitivity of designated use to chlorophyll concentration is of particular concern in situations where attainment of a single-number chlorophyll criterion is expected to be extremely expensive, with little evidence of designated use improvement. In those situations, it is essential that water quality improvements (or, designated use improvements/attainment) justify the costs. This may be the situation in Falls Reservoir (North Carolina) where, despite little evidence of designated use impairment, compliance with the 40ug/l chlorophyll criterion is estimated to cost approximately $1 billion.

For much of my career, I have been a strong supporter of chlorophyll a as the best nutrient criterion for surface waters in the US. Since chlorophyll integrates the effect of nitrogen and phosphorus, it effectively serves as a site specific indicator of eutrophication. However, in my advocacy of chlorophyll as the best nutrient criterion, I neglected the fact that chlorophyll is not highly correlated with many common designated uses over the range of chlorophyll levels found in many US lakes, as demonstrated in the PBS&J figures.

What is the alternative? Dissolved oxygen is often a water quality criterion for nutrient enrichment. However, DO can be quite variable in space and time, so monitoring DO to assess designated use compliance poses challenges. It is an exaggeration but worth noting that a user survey to assess designated use may be more cumbersome than DO monitoring for compliance, but at least the user survey is a direct assessment of what matters most.

Water clarity, typically measured with a Secchi disk (SD), is another plausible alternative, as the SD depth is a good indicator of aesthetic appeal. Perhaps, as in Chesapeake Bay, all three (chlorophyll, DO, and SD) should be nutrient criteria, but I am not sure that this is sufficient. I think that these three criteria need to be accompanied by a direct assessment of designated use attainment in situations, like in Falls Reservoir, where compliance costs are extremely high. This will help ensure that our limited resources for environmental protection are wisely spent.


References

Reckhow, K.H. G.B. Arhonditsis, M.A. Kenney, L. Hauser, J. Tribo, C. Wu, K.J. Elcock, L.J. Steinberg, C.A. Stow, S.J. McBride. 2005. A Predictive Approach to Nutrient Criteria. Environmental Science and Technology. 39:2913-2919.

PBS&J. 2003. Analysis of Use and Nutrient Data on Selected Reservoirs of the Trinity River Basin. Austin, Texas.


1 comment:

  1. Ken –
    This is a good commentary on the challenge of establishing appropriate water quality criteria within the Clean Water Act regulatory framework that can actually work to cost effectively help achieve attainment of designated use for water bodies.

    You describe a conundrum with Falls Reservoir in NC, where the existing chlorophyll criteria does not seem appropriate for this lake that exhibits little evidence of impairment. You ask whether SD, chlorophyll and DO might all be used as nutrient criteria. But confounding this, chlorophyll seemingly misses the mark – since if I understand correctly the measured levels exceed the existing 40 mg/l criterion value yet there is little impairment of use. DO is likely also problematic. I would expect that DO, as you suggest, likely has a substantial swing within each day as daytime photosynthesis produces high DO and nighttime respiration and decomposition consumes DO. The greater the algal biomass, the greater this daily DO swing will likely be. Accordingly, perhaps 24-hour DO swing (i.e., max value minus min value within a 24-hour period) would be a more useful criterion than direct DO concentration measurement (either single value or average of multiple measurements).

    But as an alternative to individual criteria, perhaps a multi-metric criterion could be developed – an index that incorporates a weighted scoring based upon two or three parameters. This could include water clarity (SD), DO swing, and chlorophyll – and/or perhaps others. For each parameter, a weighted sub-score (based upon the relative importance of the respective parameter and its measured value compared to a threshold or an optimum range) would be determined. The sum of the weighted sub-scores would be compared to values developed through assessment of calculated historic data collected from an appropriate universe of lakes. The target value would be established as a breakpoint from the index score database distinguishing those water bodies in attainment and those that are impaired. How to best assess attainment status, and which designated uses to focus on? I know, this is a further challenge. If it were easy, we’d have long since been there. But as you state – this is important and we need to try to wisely spend our limited resources for environmental protection.

    Guy Jamesson
    Columbus, Ohio

    ReplyDelete