According to the US EPA, the most common pollutants that affect lake and reservoir water quality are nutrients, yet a quick review of Missouri’s list of impaired waters (a.k.a. the 303(d) list) shows that only five lakes are listed for nutrient impairment. Are Missouri’s lakes really in that good of condition? The answer is probably “no”. The reason more lakes are not listed as being impaired by high nutrient levels is that Missouri, along with most other states, lacks nutrient criteria (There is criteria for nitrate in drinking water sources, but nothing for phosphorus or total nitrogen).

The good news is that state agency personnel are working on developing nutrient criteria, though this may be more challenging than it initially sounds. Let’s look at some of the issues that need to be taken into consideration during this process.

A Question of Approach
EPA (who is mandating the criteria as part of the Clean Water Act) suggested that states look at lake water quality according to ecoregion (a division of land area based on soils, topography, etc.—see map). The approach would involve gathering available data from each ecoregion, and listing the lakes according to nutrient levels. Criteria could then be set, based on these data, by selecting a statistically descriptive value (such as the median, lower 25%, or upper 25%). This approach would take the overall condition of the lakes within a region into consideration and create one set of criteria.

At a 2003 water quality conference in Chicago, states overwhelmingly reported that they would not follow this approach, but would instead look at developing criteria on a lake by lake basis. This would allow for a lake’s physical characteristics and watershed features to play a role in determining criteria. But it would also create a lot of work as each lake would have to go through the criteria development process. Another downfall of this approach is that there could be a fair amount of legal action taken as criteria for individual lakes were compared to each other.

In the end, the best approach may be to group lakes together based on use, size and watershed land cover, with different criteria for each lake-group. The criteria should protect those lakes that are currently unimpaired as well as identify those lakes that are impaired.

Factors to Consider
The first factor to consider when setting nutrient criteria for a lake is the lake's intended use. The Missouri DNR lists 12 designated and beneficial uses of stream and lake water. These include protection of aquatic life, irrigation, livestock and wildlife watering, body contact recreation, and drinking water source. Most people would agree that a lake used for irrigation does not need to be held to the same standard as a lake used for swimming or as a drinking water source.

Another issue that needs to be taken into consideration is the physical characteristics of the lake. Comparing a 15 acre lake to a 50,000 acre lake is definitely an apples-oranges approach. Among the physical aspects that need to be considered are lake depth, lake volume, watershed area, and flushing rate. Lake depth is important because shallow lakes have a tendency to mix throughout the year. Sediment and nutrients from a shallow lake’s bottom are constantly being mixed into its surface waters, thus leading to a higher level of nutrients than expected based on inputs from the watershed. Volume needs to be considered because a lake with a large volume can dilute inputs more than a lake with a small volume. Watershed area is important because larger watersheds have more potential nonpoint source inputs than smaller watersheds. And finally, flushing rate (which is a product of the lake’s volume and its watershed size) determines how fast water moves through the lake. The longer water stays in the lake, the more time nutrients have to settle to the lake bottom.

It has been said that a lake is a reflection of its watershed, and this is very true for Missouri’s reservoirs. Thin nutrient-poor soils in the Ozarks are quite different than deep, nutrient-rich soils of northern and western Missouri. Along with regional differences in soil type come regional differences in land-use. In Missouri, in-lake nutrient concentrations show a strong relation to both agricultural land use as well as urban land cover. These land uses are major sources for the nonpoint pollution that impacts our lakes.

What is the Goal?
At its simplest, nutrient criteria would tell us which lakes are so impaired by nutrients that the beneficial uses are endangered. With all of these factors to take into consideration it is easy to see how coming up with nutrient criteria is not going to be an easy task. The time line for the Missouri DNR includes lake criteria by early 2006, and stream criteria by 2008. LMVP data will be used in the development of the criteria, and will be important in identifying lakes that are impaired once nutrient criteria are in place. Interested citizens will also be able to attend stakeholder meetings during the development process to voice concerns and ask questions.

Dan Obrecht

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