A great lake - Lots of birds! Hypertrophic	A great lake - Gin-clear water! Oligotrophic

Lake eutrophication is "the nutrient enrichment of lakes."

Eutrophication of Florida lakes is caused by natural and by human factors. Every lake fits into a particular "trophic state", according to its degree of eutrophication, and all lakes change their trophic status over time. All lakes, even the most pristine, are undergoing nutrient enrichment and filling.

Eutrophication is caused by nutrients. Nutrients are nothing more than elements like phosphorus and nitrogen that make plants grow. Nutrients aren't pollution in the sense that gasoline, industrial waste and other things are pollutants. But too much nutrient input - or enrichment - is sometimes called "pollution" because it causes a chain of events that may have undesirable effects on lakes. On the other hand, increased nutrients sometimes has beneficial effects, such as increased fish and birds.

Read on to learn more about the "bad" and "good" effects of eutrophication.

TROPHIC STATES
To help understand eutrophication, scientists have invented a system in which lakes are classified according to their "trophic status" or "trophic state".

Because it is difficult to count individual algae cells in lake water, these categories are most easily determined by measuring the chlorophyll content of the water. Chlorophyll is a plant pigment found in algae. Measuring chlorophyll indicates how much algae is present in the water, and so provides a clue as to the amount of nutrients present. Therefore, measuring chlorophyll lets us classify lakes according to their trophic status.

A lake's trophic state is a measure of its "biological productivity", which, simply, is a measure of how many plants and animals are in a lake.

Four trophic states are recognized by lake scientists (limnologists):

Lake Como

Oligotrophic lakes are very low in nutrients, so few algae grow and the water is very clear. Oligotrophic lakes are biologically less productive lakes (they have the lowest level of biological productivity), and support very few plants and fish. Oligotrophic lakes occur mainly in the sandy ridges of the state from northern Florida down to Highlands County of southern Florida. For more about oligotrophic waters, go to this page on our web site. Oligotrophic: about 12% of Florida lakes visible depth greater than 12 feet less than 3 micrograms/liter total chlorophyll less than 15 micrograms/liter phosphorus

Lake Overstreet

Mesotrophic lakes are moderately productive, with slightly green water. For more about mesotrophic waters, go to this page on our web site. Mesotrophic: about 31% of Florida lakes visible depth between 8 and 12 feet 3 to 7 micrograms/liter total chlorophyll 15 to 25 micrograms/liter phosphorus

Lake Weir

Eutrophic lakes are productive lakes with murkier water, and/or lots of plants. For more about eutrophic waters, go to this page on our web site. Eutrophic: about 41% of Florida lakes visible depth between 3 and 8 feet 7 to 40 micrograms/liter total chlorophyll 25 to 100 micrograms/liter phosphorus

Lake Wauberg

Hypereutrophic lakes are very high in nutrients and their water is very clouded with algae. Hypereutrophic lakes are the most biologically productive lakes, and support large amounts of plants, fish and other animals. Hypereutrophic: about 16% of Florida lakes visible depth less than 3 feet greater than 40 micrograms/liter total chlorophyll greater than 100 micrograms/liter phosphorus

For an informative trophic state pamphlet, download this PDF file.

NATURAL CAUSES of LAKE TROPHIC STATES
In Florida, it is important to recognize that the four trophic states (and their intermediate states) occur naturally. There are reasons for this:

Geology. The primary determinant of a lake's trophic state is the soil beneath the lake. Regional geology can set the trophic state of a lake. Florida's diverse geology is the reason we have such a natural diversity of lake types compared to other regions of North America.

Phosphorus. A primary nutrient responsible for plant growth is phosphorus. In Florida we have areas that are rich in phosphorus; in many places there is enough phosphorus in the soils to create very eutrophic water bodies. There is so much phosphorus that we mine it for use in the manufacture of fertilizer - Florida supplies about 75% of the nation's and 25% of the world's phosphate. For more about phosphate mining in Florida, go to this DEP web site.

Lake depth. The depth of a lake may be counted as more than one factor. Deeper lakes have more volume to dilute the nutrients and algae, so they may be clearer than shallower lakes that have less volume. Less volume can mean higher concentration of nutrients and thus greener, more eutrophic water. Also lake depth helps determine the sedimentation rate of phosphorus and nutrient resuspension from the bottom: deeper lakes are less likely to become stirred by wind energy, so nutrients may more easily fall to the bottom, where they are not as likely to become resuspended in the water and be available for algal growth.

Hydraulic flushing rate. This is a measure of how the water moves through the water body. If the water flows through a system relatively quickly, and if the water volume is changed in the lake every 7 to 15 days, there is usually insufficient time for the nutrients to be used for algae growth - the nutrients simply flow downstream to some other water body. On the other hand, in lakes that have slow flushing rates, the nutrients can be used for algae and large plant production.

Fast flush rate. Typically in a Florida spring, the water is very, very clear. Yet because of our geology, the spring water is typically very rich in nutrients. So why isn't spring water green and murky, instead of blue and clear? Because of their fast flushing rate -- by the time the algae starts to grow, the water has moved way down stream. Here's a little experiment you can do: the next time you visit a Florida spring, take home a jar of water and put it on a sunny window sill for a few days - watch the color turn from clear to green as the algae grow in the phosphorus-rich water.

Slow flush rate. Many of our lakes don't have streams going into them, or leaving them. These are called seepage lakes. Seepage lakes get their water from groundwater (underground) movement and from runoff. They have very slow hydraulic flushing rates, so algae and plants have plenty of time to use the nutrients.

HUMAN CAUSES of LAKE TROPHIC STATES
As soon as people move next to a lake or into its watershed, we have the potential to accelerate the process of eutrophication. We may contribute nutrients through wastewater disposal, agricultural practices, and urban runoff from yards, golf courses, shopping centers, roads...

While it's easy to focus on the folks who have their homes on the edge of the lake, research has shown that human activities many miles away may have an even greater effect on the lake than development on the shoreline.

The bottom line is, we are all part of the process of accelerated eutrophication of our lakes. (See the human impacts page of this web site.)

EFFECTS OF EUTROPHICATION
Eutrophication of our lakes has certain effects, some visible, some not so visible:

Effects on algae growth and water clarity. Rightly or wrongly, many people judge the quality of a lake by its water clarity. Clear water lakes might be considered to be high quality, whereas murky lakes might be considered to be low quality. However, a wildlife lake's murky water might be considered high quality; a swimming lake's clear water might be considered low quality. A lake's perceived quality is really a function of the purposes of the lake and the WANTS of the person judging it.

What controls water clarity in Florida lakes? Besides suspended sediments, the major factors are 1) nutrients and algae, and 2) water color. The more nutrients, the more algae, and the less clear the water. Also the more color, the less clear the water. Water color comes from natural plant chemicals such as tannins and lignins that run off from our swamps and forests, especially after heavy rains. Learn more about water color on this page of our web site.

To measure water clarity, we use a small black and white disk called a Secchi disk. This disk is simply dropped into the water and where it disappears from view is called the Secchi disk depth. In oligotrophic lakes with low algal levels and no color or suspended sediments, the Secchi disk depth may be 30 feet or more. In eutrophic lakes, where the algae is denser, the Secchi disk may disappear very rapidly, perhaps at 5 feet or so. If the algae continues to increase, the water clarity continues to drop. In hypereurophc lakes, the Secchi disk may disappear in only a few inches of water.

It is important to know that beyond a certain amount of algae in the water, water clarity is constant. In other words, once the total chlorophyll measurement is at about chlorophyll 50 micrograms per liter, then the water simply can't look any murkier, no matter how much algae increases. Once water contains a certain high level of nutrients and algae, the water clarity will not change with the addition of more nutrients. Conversely, if a lake's water has high levels of nutrients and algae, and we were to somehow reduce the algae by 2/3, the water would not look any clearer - it would remain just as green and murky as before. The nutrients and algae would have to be reduced even more for there to be a noticeable change in the greenness of the water. (See the limiting nutrients page of this web site.)

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A PARABLE

Suppose you're the first person to build a house on a nice clear lake. Suppose you can see down 24 feet. You're very happy with this lake. You want to share your good fortune, so you invite your friends to build their houses on the lake.

What happens to the lake? You and your friends add nutrients; you accelerate eutrophication. Suppose the added nutrients increased the algae, and the water clarity went down to 12 feet. The friends who moved in might find 12 feet very acceptable. After all, your lake is still very clear compared to the lake down the road. However, you remember when you could see down to 24 feet. You might complain that environmental damage has occurred. You would be correct. You and your neighbors have had an effect on the lake.

Now suppose that more people move onto the lake and the surrounding watershed and add more nutrients. Depending on the lake, the water clarity may be reduced some more; the Secchi disk depth might go down to two feet.

When that happens, everyone is going to be upset.

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Effects on large plants. Lake eutrophication also affects the growth of large plants. Large plants (macrophytes) generally increase with increased eutrophication, just as algae do.

Oligotrophic lakes can support few, if any, macrophytes, while eutrophic lakes can be absolutely full of them. When plants grow "too much", they are considered aquatic weeds and might need to be controlled. "Too much" depends on who's looking.

A major question for plant managers is,

How many plants should we leave on a lake?

How does the amount of macrophytes affect water clarity? To answer this question, we need to understand the relationship between the amount of macrophytes and the amount of algae in the lake. Scientists have found that in nutrient-rich eutrophic and hypereutrophic lakes, there often is a great abundance of macrophytes. They've also found that in these plant-filled lakes, the water is relatively clear. The periphyton (clinging algae) that are on the macrophytes are using up the nutrients that the free-floating algae might have used.

What happens if we remove a large amount of the macrophytes? Often, the water turns greener - the algae proliferate, or they become free-floating instead of clinging to plant surfaces.

The water may be clear in a plant-filled lake because the wind cannot generate enough energy for waves to reach the bottom and stir up the sediments. So the particles in the sediments are not being resuspended to cloud the water and the nutrients in the sediments are not being resuspended to add to algal growth.

We can't predict how cloudy the water will be after we remove lots of large plants. However, scientists have learned that leaving or planting a small fringe of macrophytes around the edge of a lake, say 15% of the lake's total area, will not make the water noticeably clearer. A small fringe of plants may be aesthetically pleasing, would attract birds and other wildlife, would help stabilize shorelines from erosion, and act as a protective area for fishes, but these relatively few plants would have little effect on the overall water clarity of the lake.

To cause or maintain water clarity in eutrophic lakes in Florida, the submersed plant volume must be 30-50% or more. And this amount is often considered to be an aquatic weed problem...

Effects on fisheries. Another key interest for judging the quality of a lake is its fishery. How does eutrophication affect the fish and fishing?

Read "Using Lake 'Trophic State' to Predict Mercury in Fish," (PDF, 256 KB) a Florida LAKEWATCH article.

A common practice for fish farmers is to actually add plant fertilizer to ponds; they do this because they know it will lead to increased fish production. When nutrients are added to the lake, algae grow, small animals eat the algae, small fish eat the small animals, and larger fish eat the small fish. This is true in natural lakes as well as in artificial fish ponds. So, increased nutrients means increased fish. They may not be the biggest fish or the "best" fish, but there will be more fish.

A SECOND PARABLE

Sometimes the media portrays nutrient enrichment (eutrophication) as leading to a "dead lake". However, scientific research shows that as nutrients increase, the number of fish increases. The best sportfishing lakes in Florida and elsewhere are eutrophic lakes. Eutrophic lakes produce large amounts of bass, speckled perch, bluegills and shellcrackers.

Eutrophication can cause a change in fish species composition. As lakes become more eutrophic, some types of fish grow more rapidly than others. In Florida's eutrophic lakes, catfish and gizzard shad may become dominant.

For example, hypereutrophic Lake Apopka is sometimes said to be a "dead lake". But actually it has one of the highest fish populations in Florida. Most types of fish, with the exception of largemouth bass, are present in large numbers there. While some people may complain that they don't catch as many bass as they did before the lake became hypereutrophic, there is a good speckled perch fishery and a commercial fishery that is valued at more than $1 million per year (1990).

Setting fish managment objectives is simply another problem of lake management. In oligotrophic lakes, you can have good fishing, but the absolute number of fish is always much lower than in eutrophic lakes. It'll take longer to catch a fish in an oligotrophic lake than in an eutrophic lake, and it will take longer to produce a replacement fish in that oligotrophic lake.

NUTRIENT MANAGEMENT
As discussed earlier, the nutrient phosphorus is of primary interest when considering lake eutrophication. Because more phosphorus causes more algae to grow, thus making water less clear, some lake managers have come to believe that reducing phosphorus can cause some Florida lakes to clear up. Indeed, in certain instances in northern U.S. lakes, phosphorus control efforts have been highly successful. In other cases, nutrient control efforts have had no noticeable effects.

Can we manage eutrophication in Florida lakes? Sometimes yes, but oftentimes no.

In Florida there are many lakes where phosphatic soils alone contribute all the phosphorus necessary for dense algal growth. In these lakes, no matter what we do, even if all human nutrient inputs were stopped, we would not be able to reduce the phosphorus below algae's critical growth needs and then clear up the water. Likewise, we wouldn't reduce aquatic weed problems. Lakes situated in phosphatic soils are naturally eutrophic and management efforts other than nutrient control are required.

However, nutrient control can have an effect on phosphorus content. For example, records show that the Tsala Apopka chain of lakes have not increased in phosphorus levels during the past 40 years, in spite of heavy population growth and lake use. It is believed that this fact might have to do with proper use and maintenance of the area's septic tanks or the 85% cover of vascular plants.

Some of our naturally oligotrophic lakes might be good candidates for starting nutrient control programs. On some lakes nutrient control efforts might work, but on others we literally would be wasting time and money.