Trophic State Levels
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Eutrophication Trophic State Categories Water Quality TSI Index Calculate the TSI TSI at LOZ Food Web Another Approach

The Trophic State

While many of us are unaware of the term, "the trophic state", we are familiar with the threatening term, "eutrophication". We may not know exactly what that means, but we know that when it happens to a lake, it's bad news. In years past we read news accounts about EPA and environmentalists' fears that, because of high phosphorus concentrations, the Great Lakes were threatened to extinction by "eutrophication". The end result of eutrophication is a lake's death.

What Is Eutrophication

Eutrophication is defined as

The process by which a body of water acquires a high concentration of nutrients, especially phosphoruss and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish. Eutrophication is a natural, slow-aging process for a water body, but human activity greatly speeds up the process.

Eutrophication is the process by which a lake approaches or increases it's position in or toward the trophic state of "eutrophic". And there is that trophic word again. The word trophic is used in many contexts and simply means "relating to nutrition", but when used in describing the state of a water body it is used as part of the phrase, "trophic state". Trophic state, then, refers to the biological production, both plant and animal life, that occurs in a lake. The eutrophic state is one of three primary classifications for a lake's trophic state and this is the level at which the total biomass (all living material) is at it's highest. On the other extreme, the point at which the lake is totally sterile or there is relatively little living biomass present, is called the oligotrophic state. If there is an up and there is a down, obviously there must be a middle and that is called the mesotrophic state.

The Three Main Trophic State Categories

Practioners of every specialty have their own name and those scientists who specialize in the study of lakes, being no different, call themselves limnologists. In the study of lakes, Limnologists strive to categorize different lakes in accordance to their relative position in the trophic state and have therefore divided lakes into the following categories:

  1. oligotrophic:lakes with low nutrient (primarily phosphorusinches and nitrogen) concentrations and very little plant growth such as is often typified by larger, deeper lakes with clear water, rocky or sandy shorelines, and adequate dissolved oxygen throughout.
  2. Mesotrophic: an intermediate category with characteristics between the other two groups.
  3. Eutrophic: (well-nourished) lakes having high nutrients and high algae growth as is typified by more shallow water bodies with mucky bottoms, extensive rooted plant growth and regions of oxygen depletion. As lakes increase in their relative position in the eutrophic state, they approach eutrophication and the dreaded "bloom" or great increase of phytoplankton (microscopic plant-like organisms such as algae) becomes more evident. Negative environmental effects include hypoxia (the depletion of oxygen in the water) which induces reductions in specific fish and other animal populations whereas undesirable species (such as rough fish like carp and other low oxygen tolerant fish) may experience an increase in population.

More on Eutrophication

Our efforts as amateurs in striving to understand the often complex language of biologists and other scientists such as the limnologists is frequently hindered by our ignorance of much of their vocabulary. But never fear, when you come across a word or term you do not understand, look it up quickly on The EPA Biouniversity Glossary. For example, that glossary defines eutrophication as: "The normally slow aging process by which a lake evolves into a bog or marsh and ultimately assumes a completely terrestrial state and disappears. During eutrophication the lake becomes so rich in nutritive compounds, especially nitrogen and phosphorus, that algae and other microscopic plant life become superabundant, thereby "choking" the lake, and causing it eventually to dry up. Eutrophication may be accelerated by human activities." Is this something that could happen here at the Lake of the Ozarks? Not likely, not to that extent of total eutrophication because of the beneficial continual flushing action in the lake as new water enters in the Osage via Truman Dam and other tributaries. But, as Tony Thorpe of the Lakes of Missouri Volunteers' Program has pointed out:
Be aware that lakes with high flushing rates typically have poorer water quality than lakes with low flushing rates. The water that "flushes" into lakes is generally made up of runoff; If a lake has a high flushing rate, that means the lake drains more land than other lakes and receives more runoff. Lake of the Ozarks is different because of Truman Dam. Truman buffers LOZ from much of the influence of all the agricultural runoff to the west. LOZ is also helped by its size. The water is much more eutrophic at the upper end of the lake than near the dam. As the water moves down lake, biological processes and settling move nutrients toward the bottom.

Water Quality and The Trophic State

The trophic state should not be considered a measure of water quality. The extreme end of the oligotrophic category would be a totally sterile water. That might be ok for a swimming pool, but would be of no value as a recreational lake. On the other hand, the extreme end of the eutrophic category would be a swamp (or worse) and that also would not be much fun as a recreational lake. A balance is necessary to provide a lake that is a good producer of fish but at the same time not so 'eutrophic' as to result in algae blooms.

The Trophic State Better Defined

As you might expect, the scientists are not satisfied with general classifications and have quantified the trophic state of bodies of water into a numerical system which can more accurately describe the trophic state. Robert E. Carlson developed such a quantification which he published in 1975 as A Trophic State Index For Lakes

The abstract of that paper reads as follows:

A numerical trophic state index for lakes has boon developed that incorporates most lakes in a scale of 0 to 100. Each major division ( 10, 20, 30, etc.) represents a doubling in algal biomass. The index number can be calculated from any of several parameters, including Secchi disk transparency, chlorophyll, and total phosphorus.

Wow! Thanks to Carlson, we now have a way to pin-point a lake's position in the trophic state by making measurements of any one of three parameters. The simplest, and the one many of us who have been involved as volunteer lake samplers are familiar with, is the estimation of water clarity by use of the Secchi disc. The other two require some chemistry work but that chemistry can easily be done on water samples we collect. The Lakes of Missouri Volunteers Program, for example, routinely collects such samples which are then analyzed at the Univerity of Missoui for the other two, phosphorus and chlorophyll.

Determination of the Carlson Trophic State Index

The equations for determination of the Carlson Trophic State Index, as provided by EPA in their Biouniversity are as follow:

  1. Based on the Secchi Disk: TSI = 60 - 14.41 ln Secchi disk (meters)
  2. Based on chlorophyll: TSI = 9.81 ln Chlorophyll a (µg/l) + 30.6
  3. Based on phosphorus: TSI = 14.42 ln Total phosphorus (µg/l) + 4.15

If working with natural logs is not something you care to do, just input your values in the boxes provided here and we will make the calculation for you. Enter the Secchi disk depth in inches (the calculator automatically converts this to meters) or the phosphorus or chlorophyll concentrations in µg/l, hit enter or tab and see the resulting TSI.

Interpreting the TSI Indexes

The Carlson Trophic State Index is based solely on the assumption that algae is the only biomass present. Deviations from the theoretical assumption that the TSI will be the same when based on chlorophyll or Secchi diskdepth or phosphorus can therefore be expected. What these deviations might mean can give us even more information about the trophic state of our lake. For example, in A Trophic State Index,this reference provides the following possible interpretations relative to differences in the TSI Index: (note, the table below was updated from Carlson 1983)

Relationship Between TSI Variables Conditions TSI(Chl) = TSI(TP) = TSI(SD) Algae dominate light attenuation; TN/TP ~ 33:1 TSI(Chl) > TSI(SD) Large particulates, such as Aphanizomenon (blue-green algae) flakes, dominate TSI(TP) = TSI(SD) > TSI(CHL) Non-algal particulates or color dominate light attenuation TSI(SD) = TSI(CHL) > TSI(TP) Phosphorus limits algal biomass (TN/TP >33:1) TSI(TP) >TSI(CHL) = TSI(SD) Algae dominate light attenuation but some factor such as nitrogen limitation, zooplankton grazing or toxics limit algal biomass.

Tropic State As Function of Carlson Trophic State Index

The following table was modified based on information from the secchidipin.org reference:

TSI Trophic State Chlorophyll in µg/l Secchi disk in meters Phosphorus in µg/l
<40 Oligotrophy:
Clear water, oxygen throughout the year in the hypolimnion
<2.6 >4 <12
40-50 Mesotrophy:
Water moderately clear; increasing probability of
hypolimnetic anoxia during summer
2.6 - 7.3 4 - 2 12 - 24
>50 Eutrophy:
Anoxic hypolimnia, macrophyte problems possible
>7.3 <2 >24

Trophic State Index at Bagnell Dam

The LMVP has provided the data needed to calculate the Carlson TSI values up and down the Lake of the Ozarks (and most other Missouri state lakes) for many years. As an example of what these values might look like, see below the TSI values calculated for samples collected near Bagnell Dam during the 2011 season: calculated TSI values at Bagnell Dam

The graph shows that all three tracked fairly close together in the early Spring and that the TSI based on chlorophyll and the Secchi diskcorrelated fairly well throughout the season. However, from early to late summer the TSI based on phosphorus trended considerably below the other two. Based on the interpretations discussed above, this would indicate that, during the summer, algae growth is limited by the amount of available phosphorus.

The Food Web: Another Aspect on the Trophic State

What is meant by "The Food Web" of a lake? Can you get to it by clicking on a URL? No, afraid not. the reference, Food Web for Freshwater Lakes describes it as follows:

The food web in a freshwater lake shows how different levels of aquatic life interact and depend on one another for survival. A lake's food web, which consists of multiple food chains, includes plants, microorganisms and fish.

The term "Food Chain" is also often used to define the relationship between the lowest organism on the total pole (the plant-like microorganisms) and the highest (bass, for example). But "Food Web" better describes such relationships because even the bass begin life as tiny larvae that initially grow by feeding on the microorganisms and then work their way up the food chain as they grow (assuming they themselves are not eaten first.) Understanding this food web provides us with a better understanding of a lake's ecosystem. Energy is the fundamental requirement for life and, as with all other life, that within a lake's ecosystem is derived from sunlight. The sunlight is utilized through the process of photosynthesis by the algae and other plant-like organisms to produce the food needed to sustain the ecosystem. These "primary producers" are the algae and cyanobacteria discussed elsewhere on this web-site.

As we travel up the food chain in the food web, each new level is called a trophic or feeding level Fitting Algae into the Food Chain Be careful not to confuse this use of the word trophic with that discussed earlier in reference to the trophic state. Remember, the word trophic simply means "related to nutrition".

Traditional biology has taught us that the food chain commences with the mircroscopic food producers and proceeds to the next level, the zooplankton (the animal microorganisms) which eat the primary producers. That is, the zooplankton have been characterized as algae feeders. However, a recent study reported in ScienceDaily and published in "Proceedings of the National Academy of Sciences" states that nearly a third of zooplankton diets are supported by material that originates on land in lake watersheds. Lead author and Cary Institute of Ecosystem Studies limnologist Dr. Jonathan J. Cole comments,

Our work changes the paradigm for how we describe the environment that supports fish. Zooplankton are one of the pillars of the aquatic food web. And while they do feed on algae, they also rely on materials derived from maple leaves, pine needles, and whatever else comes in from the surrounding watershed.

Cole further notes,

Historically, lake ecosystems have been studied in isolation. Yet we know lakes are connected to their watersheds and organic matter from land enters lakes in the form of run-off or ground water." Adding, "Our study adds to the growing body of evidence that aquatic food webs are subsidized by these inputs.
We still do not want any of you to dump those Fall leaves into the lake, but it is of some relief now to know that those leaves will,in part at least, be consumed by zooplankton and become part of the lake's food chain.

Another Aspect of the Food Web

Another interesting aspect of the food web was presented by Tony Thorpe in the Lakes of Missour Volunteers Program (LMVP) Fall 2005 issue of the Water Line with the title, "Top Down vs Bottom Up." In this article, the point is made that the food web of a lake can be modified either by controlling the growth of algae (the bottom trophic level) such as by reducing the nutrients like phosphorus or by contolling the proliferation of piscivores (fish eaters, i.e. like bass that eat other fish.)

We recommend you read the Water Line article, but the essence of it is that there are basically five trophic levels in a lake ranging from nutrients like phosphorus on the bottom to fish like bass at the top. The big bad fish eat the little fish (like shad) that eat the little aquatic animals (the zooplankton) that eat the algae which grow by photosynthesis with the help of nutrients like phosphorus. Obviously, the algae cannot grow beyond the limit set on them by the nutrient concentration in the water.

However, in theory it is also possible to limit the algae growth by encouraging the growth of the fish-eating fish like bass and walleye. As that population increases, they will tend to reduce the population of shad and other such fish which live by eating the zooplankton. This, in turn, will allow for an increase in the population of zooplankton and, since the zooplankton eat the algae, the algae in the lake will be reduced. cartoon of fish eating
modified from LMVP reference

But remember, since it is a web, the actual relationships are not as simple as that in the explanation above. Nevertheless, we know that the Lake of the Ozarks is a great bass lake. Does this mean, therefore, that our lake can withstand a higher phosphorus level than would normally be acceptable in an lake with a lower population of game fish like bass?

Tie It All Together

The Lake of the Ozarks Watershed Alliance (LOWA) was formed to protect and preserve our lake and watershed. Education is one way we strive to do that and we hope that this LOWA web presentation on nutrients, algae, and the trophic state will provide you with a better understanding of the complex relationships between what we as humans do to our watershed and the result that will have on the quality of the Lake of the Ozarks.


  1. EPA's Biouniversity
  2. Food Web for Freshwater Lakes
  3. Fitting Algae into the Food Chain
  4. Science Daily report on food of zooplanktons
  5. Fall 2005 issue of the Water Line
  6. A Trophic State Index
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