Questions and Answers
Answers to the following questions are technical in nature. EPA's Implementation Workgroup addresses these questions from a policy standpoint and will post their answers on the Nutrient Water Quality Criteria website at http://www.epa.gov/waterscience/criteria/nutrient/faqs.htm.
SLudlow clarified: Question deals w/ how many days to average over. For example, during hot summer months, would you use a shorter averaging period or the same one as used in winter? The averaging period would be for possible adoption into SWQS regulation.
No generic lengths of index periods have been proposed or seem to be emerging from development of nutrient criteria by various states, however the period of compliance tends to be during summer. Many states are linking assessment of nutrient condition to assessments of biological and other physicochemical conditions, which tends to be during the summer months. Many impairments of both lakes and streams seem to be during the summer, whether due to warmer water temperatures and lower oxygen saturation levels in streams or plankton blooms in lakes and coastal embayments. The rationale for applying nutrient criteria to one time of year or another, or year-round versus just seasonally, has been addressed in the answer to T-REQS question 53 (not answered yet). Decision for the index period in which nutrient criteria apply should consider knowledge of when nutrient effects occur, relationships between nutrients and responses of concern, and consideration of both instream and downstream effects.
The length of the index period and frequency of sampling during the index period should depend on many factors, for example: data quality issues for 303(d) listing of sites versus initial assessments and practical aspects of monitoring, temperature-dependent rate of response of algae or oxygen demand to weather-related disturbances, and importance of historic conditions for causing impairments. 303(d) listing of sites should be based on high levels of certainty that impairment by nutrients has occurred. This can be accomplished with multiple lines of evidence that indicate nutrient impairment. For example, if phosphorus, nitrogen, chl a, and secchi/turbidity measures exceed criteria (and reasonable cause due to high levels of land use associated with nutrient loading), then data requirements for 303(d) listing may be deemed sufficient. If however, only one observation of an exceedence has been observed, additional data may be required to justify listing. These kinds of data quality issues are being addressed in other EPA guidance efforts. Repeated sampling during an index period is one way to meet these data requirements. If routine sampling for water quality monitoring involves only one indicator of nutrient concentrations, or perhaps only one of several nutrient criteria parameters of the P, N, chlorophyll and secchi/turbidity measures is exceeded, then intensive follow-up efforts may be required to justify listing.
Nutrient concentrations, and the biological responses to nutrients, do vary temporally for many reasons. The reasons for this variation, and the variation in nutrient parameters, should guide the frequency of sampling during an index period and the duration of the index period. Weather-related factors, such as scouring events in streams and nutrient loading in lakes, may determine whether nutrient-related problems will be observed during one year or another. Thus, manifestation of nutrient-related problems varies with time after the disturbance and from one year to another. In theory, index periods are probably shorter in streams and wetlands than lakes and coastal waters because of reaction times. In practices, most index periods have been defined to be 2-3 months long and reflect the period over which problems develop and the possibility that nutrient and response signals occur at different times. Differences in the timing of nutrient and biological (chlorophyll) response signals occur may result from biological uptake of nutrients and resulting development of large biomasses of algae. This is common in streams when nutrients are removed from the water column and stored in periphyton and metaphyton, but differences in this period of development may be as short as a week to 10 days.
In summary, the index periods for repeated observations of nutrient parameters should be during periods when nutrient problems develop and are manifested. Although no generic guidelines specify when and for how long this should be, most states are using index periods during the summer and with durations of 2-3 months. A sound scientific basis should be used to define the timing and duration of the index period and it should be related to effects of nutrients on valued ecological attributes, such as water clarity and biological conditions.
The answer is correct and I will just expand on this a little.
If you are interested in loading to a downstream water body, the sampling must be over the entire time period of interest, and high discharge events in rivers must be sampled and flow-weighted to get accurate estimates of loading. If you are interested in concentrations already in a water body leading to problems, then averaging is necessary for several reasons.
First, algae can take up excess phosphorus and keep dividing (called luxury consumption). Therefore, low phosphorus (particularly low inorganic P) at a single time point does not necessarily mean there will no algal bloom in both lotic and lentic environments.
Second, floods may scour algal biomass and algal blooms in lakes can crash for various reasons, so one point sampling of algal biomass as a response variable also can lead to erroneous values.
If you look at the empirical relationships between total N or P and lake phytoplankton biomass, they are almost always summer means for the epilimnion. This averaging tends to smooth out variance in the relationships. Likewise, the published relationships between water column nutrients and periphyton biomass in streams are also done using seasonal means. Point measurements are not a closely related to algal biomass because of the problems noted above. Averaging also allows regulators to account for the fact that inputs may be pulsed into some systems. For example, there may be flush of nitrate in the first rain following spring crop fertilization. Averaging may allow this type of activity to continue if it does not substantially alter the long-term averages. As the original answer pointed out, seasonal averaging may also allow regulators to key in on problem times of year (e.g. low flow conditions in streams where low dissolved oxygen concentrations are of concern).
I agree with the above responses, and would add that you may need to consider different averaging periods for the nutrients and for the algal responses. In many cases, algae may not respond to a short-term flush of nutrients (especially if accompanied by high flows/low residence times), and you would be justified in a month-long or seasonal averaging period for the nutrients. Conversely, algal blooms may be short-term adverse events - say, several days or a week of excess filamentous algae in a river, or blue-greens in a lake. When averaged over a season, short-term blooms may not register as an impairment, but animal biota, users and constituents may view it as impairment and react accordingly. Thus, in addition to a seasonal average measure of algal response, you may also want to consider the number or frequency of adverse bloom events in a season.
SLudlow clarified: Whether there should be different criteria by seasons. If different criteria by seasons, would the nutrient limitations in permits also vary by season?
There is little doubt that season should be considered for nutrient criteria in lakes and streams. In streams an extended period at baseflow, particularly in warmer times of year, increases the probability of anoxic or hypoxic events. In lakes, summer stratification and warmer temperatures increase the probability of cyanobacterial blooms. Unfortunately, this is also the time of year where there is the lowest degree of dilution of point and non point source waters.
It is possible that nutrients are not important at some times of year (e.g., during highly turbid spring runoff events). However, the potential for luxury P uptake makes this assumption a bit risky.
Empirical relationships between chlorophyll and nutrients for lakes are primarily constructed using seasonal means. This is because variance is too great with point measurements. In streams particularly, there can be transient pulses of nutrients that do not correlate well to algal biomass and floods can temporarily lower algal biomass. Lakes also have transient events such as hypolimnetic entrainment associated with storms or nutrient input with floods that can temporarily increase nutrients in the epilimnion.
Probably the best method is to ensure that seasonal means of nutrient concentration do not exceed the criteria. However, it may be advisable to use shorter time periods in situations where there are critical times (e.g., certain summer months with low flow and high temperatures where dissolved oxygen problems are very problematic). Response variables on the other hand, may also be considered on a point bases. For example, peak algal biomass in addition to mean algal biomass may be important to assess the effects of nutrients on producing blooms. The acute effects of nutrient enrichment may be more closely reflected by maxima of algae rather than median or mean biomass.
The rationale for applying nutrient criteria to one time of year or another, or year-round versus just seasonally, have also been addressed in the answers to Implementation #6 and T-REQS question 53 (not answered yet). Those answers address knowledge of when nutrient effects occur, importance of anticedent conditions, relationships between nutrients and responses of concern, and consideration of both instream and downstream effects. Those answers complements Dr. Dodds' answer.
Several states have discussed using a single variable index, such as a
TSI, to translate criteria into a more meaningful number or to allow relationships between response variables to drive nutrient criteria, such as, setting chlorophyll criteria and then using the relationship between Chl and Phosphorus to set the phosphorus criterion level.
Multiple variable criteria are a different situation, as described in the Lakes Criteria Manual, p 7-16. Under Developing Nutrient Criteria Implementation Procedures, it states: "The four initial criteria variables include two causal variables (TN and TP) and two response variables (chlorophyll a and Secchi depth or a similar indicator of turbidity). Failure to meet either of the causal criteria should be sufficient to indicate a criteria "excursion," and usually the biological response, as measured by chlorophyll a and Secchi depth, will follow this nutrient trend. However, if the causal criteria are met but some combination of response criteria is not met, then there should be some means of determining if the lake in question meets the nutrient criteria.
One option is to establish an index that accomplishes the same result by inserting the data into an equation that relates the multiple variables in a nondimensional comprehensive score much the same way an index of biotic integrity (Karr, 1981) does. An example of an enrichment index approach is presented in Table 7.2 of the Lakes and Reservoirs Technical Guidance manual: http://www.epa.gov/waterscience/criteria/nutrient/guidance/lakes/chapter7.pdf [^]Such enrichment index scores are not intended at this time to be surrogate nutrient criteria. They may, however, serve as a "translator" to implement multiparameter criteria.
As a result, multiple variable criteria are discouraged. This makes sense if criteria are viewed as the nutrient concentrations that control the response variables such as algal biomass. The prevailing presumption is that a single limiting factor can limit plant growth at one time; criteria should be set at levels that would assure that the response variables would be at acceptable levels. This would be difficult to do from a management standpoint if multiple variables were included in a single criterion.
It would seem that a multiple-variable index would mean that all the variables incorporated in the index would have to be managed instead of only the limiting nutrient.
Rivers and streams may present a more difficult case, because the relationship between nutrient concentration and algal response variables are not necessarily clear cut. It is not clear how a multiple variable criterion can remedy this problem.
Nutrient criteria will work best if they are established as the simplest approach to achieve a reasonable endpoint. We are gaining some appreciation of the limiting nutrient in freshwaters and understand that there may be communities that show both N and P limitation. We also know that chlorophyll per unit of phosphorus is a bit higher when nitrogen is abundant. These findings are of basic interest but seem too detailed to center nutrient criteria around.
In Missouri we will use total phosphorus as the focus of nutrient criteria.
The fist reviewer's final revision looks good to me. Single-variable chemical criteria are preferred.
NPDES permits, TMDLs, and criteria are three complementary regulatory tools of a set that are used for managing pollutants. This set also includes designated uses of a waterbody, antidegradation clauses, and tiered criteria for both valued attributes (VA) and stressor criteria. Designated uses of waterbodies are based on levels of valued attributes that are acceptable for that use, such as the aesthetics of clear water, aquatic life, or recreational contact. Criteria for stressors, such as pollutants and habitat alterations, are established to protect valued attributes by using reference and/or stressor-VA relationships (e.g. Figure 1, see attached file). Stressor criteria, such as nutrient criteria, also serve as TMDL goals or targets. A total maximum daily load (TMDL) for a waterbody is calculated based on stream discharge (or lake volume) to maintain pollutants below stressor criteria. National pollution discharge elimination system permits (NPDES permits) are assigned to regulated dischargers to maintain loads and concentrations of pollutants below levels that have negative impacts on aquatic ecosystems (i.e., that impair designated use with pollution above their criteria).
Tiered criteria can be used to protect high quality waters and provide incremental restoration goals for lower quality waters (Davies and Jackson 2006). Tiered criteria for VAs and stressors can be linked as well as tiered. Linked VA and stressor criteria would be related in a way that approved stressor concentrations would protect the VA in acceptable condition (Figure 1). The following example illustrates one approach for linking VA and stressor criteria (from Stevenson 2006).
A stressor-response relationship can identify a stressor level that would support the desired level of a valued attribute. Non-linear responses help justify where to establish the stressor criteria (Stevenson et al. 2002 and 2004), because levels on one side of the thresholds (sometimes lower as in Figure 1) have relatively little effect on VAs versus the other side. VAs or their indicators with linear responses to stressors should be used as parameters for criteria because they respond sensitively along the entire range of stressor conditions. Linear responses can be used to justify stressor criteria if unacceptable levels of VAs can be clearly identified and justified.
Tiered and linked criteria for VAs and stressors require identifying more than one level of a VA that is acceptable. For example, perhaps all waters of a state must have all functions fully maintained even though some sensitive-rare species had been lost (sensu Davies and Jackson 2006). However, some waters are near natural and would benefit from protection at a higher level of designated use. Different stressor criteria would be required to support these uses (Figure 2).
NPDES permits and TMDLs are intended to be set at levels that prevent concentrations of pollutants from exceeding a criterion, whether it is a single criterion for all waters or individual tiered criteria. The ranges of conditions between tiered criteria are irrelevant, directly, to assessment of whether waters attain uses or not. The management goal of TMDLs and NPDES permits is to maintain conditions below a pollutant’s criterion (assuming that higher levels of a condition are bad, which may not be the case for a few pollution parameters and most biological parameters). If a pollutant concentration exceeds the criterion for the designated use of a specific waterbody, then use is not supported - whether the measured concentration still supports a use that is lower than the designated use for that waterbody (Figure 3, scenario 4) or no use is supported below the exceeded criterion. If concentration of a pollutant is managed so well that it is lower than the criterion that protects a use that is higher quality, then the specific designated use of that water body is supported and exceeded substantially (Figure 3, scenario 3).
Ranges between criteria are indirectly important in assessment because they define the sustainability and restorability of ecosystems for different designated uses (sensu Stevenson 1998). Here sustainability and restorability were defined as the difference between observed condition in a waterbody and the criteria. If observed condition was better than criteria, then the difference between the two is a measure of sustainability. If observed condition is worse than criteria, then the difference between the two is an inverse measure of restorability. Thus, tiered criteria and ranges between criteria provide the benchmarks and yardsticks for preventing anti-degradation of high quality waters and restoration goals for waters that can be improved with better management. Stakeholders can use measures of sustainability and restorability to prioritize restoration and protection projects (Stevenson 1998). In this way, tiered criteria provide reasonable management targets for all waters, protection benchmarks for high quality waters, and incremental restoration targets for lower quality waters (Davies and Jackson 2006).
Davies, S. P., and S. K. Jackson. 2006. The biological condition gradient: a descriptive model for interpreting change in aquatic ecosystems. Ecological Applications 16:1251-1266.
Stevenson, R. J. 1998. Diatom indicators of stream and wetland stressors in a risk management framework. Environmental Monitoring and Assessment 51:107-118.
Stevenson, R. J. 2006. Refining diatom indicators for valued ecological attributes and development of water quality criteria. Pages 365-383 in N. Ognjanova-Rumenova and K. Manoylov, editors. Advances in Phycological Studies. Pensoft Publishers, Moscow, Russia.
Stevenson, R. J., Y. Pan, and P. Vaithiyanathan. 2002. Ecological assessment and indicator development in wetlands: the case of algae in the Everglades, USA. Verhandlungen Internationale Vereinigung für Theoretische und Andgewandte Limnologie 28:1248-1252.
Stevenson, R. J., R. C. Bailey, M. C. Harass, C. P. Hawkins, J. Alba-Tercedor, C. Couch, S. Dyer, F. A. Fulk, J. M. Harrington, C. T. Hunsaker, and R. K. Johnson. 2004. Interpreting results of ecological assessments. Pages 85-111 in M. T. Barbour, S. B. Norton, H. R. Preston, and K. W. Thornton, editors. Ecological Assessment of Aquatic Resources: Linking Science to Decision-Making. Society of Environmental Toxicology and Contamination Publication, Pensacola, Florida.
If I understand the sustainability and restorability argument, it’s basically the amount of stressor that can be added before the VA degrades to below the next criterion line (sustainability), or the amount of stressor that must to be removed before we expect the VA to be restored to its criterion (restorability). The concepts of sustainabilty and restorability also imply some estimate of the amount of effort to sustain and restore (technical issues and resources), not just the quantity of stressor.
The basic concept of tiered uses is that criteria would be established for each tier and a waterbody assigned to the appropriate tier based on its beneficial and/or designated uses. These criteria are not ranges, they are specific values defining the lower bound of each tier. For a waterbody within a specific tier, this criterion (lower bound) would be the target used for NPDES permitting or for TMDL targets. If a waterbody is raised to a higher tier, then the new criterion would apply.