Okoboji Protective Association

THE ‘TURN-OVER’ OF LAKE OKOBOJI!

Did you know that like our Iowa climate, Lake Okoboji also has "seasons"? During its annual journey around the sun, our earth experiences differences in temperature and day length that cause the seasonal changes we recognize as winter, spring, summer and fall. These changes are also experienced by lakes, and influence the life cycles and ecology of plants and animals that live in them. However, the change in lake seasons of West Okoboji do not have names like ‘winter’, ‘spring’, ‘summer’ or ‘fall’. Instead, the term "turn-over" is used to describe these changes.

What is "turn-over"? Turn-over refers to the mixing of lake water. Most Iowa lakes are so shallow the wind easily mixes- or turns over- the water. Shallow lakes may turn over several times a season or even in a day. But not West Okoboji. Okoboji is so deep (Okoboji is the deepest natural lake in Iowa) it does not mix well, no matter how hard the wind blows! In fact, Okoboji only mixes completely in the spring and in the fall. Okoboji is called a "dimictic" or twice-mixing lake, while the shallow lakes that mix more frequently are called "polymictic", or many-mixing lakes.

How does turn-over happen? Lets begin with summer. During the summer the sun heats the surface water faster than it heats the bottom layer. Warm water is lighter, or less dense, than cold water. As a result, West Okoboji forms stable thermal layers, with the warmest water on the top - about 30 feet deep- and the coldest water at the bottom. In between is a narrow layer of water. No matter how hard the wind blows on a summer day, these layers of water do not mix until fall.

In the fall, the air temperature begins to drop. The surface water begins to cool, and become colder than the bottom layer. As the surface cools, heat escapes and on many a fall day you can see the heat rising as vapor from the lake surface. Fall turn-over is approaching! The water on the surface becomes cooler than the bottom layers. Because cold water is heavier than warm water, it sinks, and displaces the warmer, bottom layer of water which rises to the surface. Fall turn-over has happened!

As the warmer water rises to the surface it brings up nutrients up from the bottom sediments. These nutrients feed fall algae blooms. This is a normal, annual event. The fall algae are mostly the golden-brown kind called diatoms. If you pay close attention, you may a change in the water color to a golden-green-brown. Unlike the blue green algae blooms of the hot summer months, these diatom blooms do not form noxious scums and are in fact quite nutritious and an important source of food for the zooplankton which the little fish eat. Next, the bigger fish eat the smaller fish. Can you understand how the change in seasons is very important to the aquatic food chain of Okoboji?

Over the winter ice forms on the surface. Underneath the ice, the water is the same temperature from top to bottom: just under freezing. The shield of ice and snow does not allow the sun to differentially heat the top and bottom layers as it does in summer.

In the spring the lake mixes, or turns over, again when ice melt causes the surface water to become cooler and denser than the bottom layer of water. Once again, the heavier surface layer sinks and displaces the warmer, underlying layer that rises to the surface bringing nutrients with it. Again, the nutrients brought up from the bottom feed the algae blooms that feed the aquatic food chain.

When summer arrives, the longer days and rising air temperatures warm the surface water, and the cycle begins again. Summer tourists and residents return to enjoy the summer weather at Lake West Okoboji.

- Jane Shuttleworth, Environmental Education Coordinator, Iowa Lakeside Laboratory
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"Seaweed" is the common term for what aquatic biologists call "macrophytes" or "submerged aquatic plants". In this article we will use the term "macrophyte". In Latin, "macro" means large and "phyte" means plant. Biologists invented this term to distinguish these large plants from the other dominant green organisms in aquatic systems, the "micro" algae. Macrophytes are different than algae in ways other than size, however. They have leaves, root-like structures, and stems whereas the algae lack these structures.

The distribution of macrophytes is limited to the shoreline zone of lakes where the water is shallow and clear enough for light to reach the bottom. That is why we see them growing next to our docks and not in the deep water in the middle of West Okoboji.

From a swimming and boating point of view, macrophytes may pose a nuisance. From a fisheries and water quality point of view, macrophytes are a benefit.

Because macrophytes attach to the bottom they help stabilize the sediments and help keep the water clear. Keeping the water clear allows plants to absorb sunlight and produce oxygen through photosynthesis. In addition, the long stems and leaves of macrophytes act like condominiums for literally 1000s of species of algae and tiny animals that live and hide on their surfaces. These organisms are fed upon by aquatic insects, micro-crustaceans, snails and other invertebrates which, in turn, are fed upon by minnows and baby fish, which in turn are fed upon by fish-eating-fish. Macrophyte beds also serve as import fish spawning, nursery and foraging areas. When macrophytes are removed, so is an important the basis of the lake food chain, and there is a decline in water quality and ecological health.

Nature Abhors a Vacuum

By the 1920s macrophytes began a drastic decline. In East Okoboji Lake, for example, 40 species were once described. By the 1970s, only two or three species were common.

Several interacting factors were responsible for this decline. Considered a nuisance, they were often removed by chemical and mechanical methods. Boating destroyed plants, especially in more shallow waters. We also believe nutrient enrichment from raw human sewage, livestock waste, soil erosion and agricultural runoff played a significant role, for at the same time macrophytes began to decline the noxious blue-green alea blooms began to make their annual appearance in our lakes. Blue greens grow rapidly in nutrient rich waters and can out compete macrophytes for nutrients. They also out compete macrophytes for light because they float near the water surface. Blue-greens further benefit from macrophyte removal because without the bottom stabilizing effect of macrophytes, sediments become suspended into the water column. Suspended sediments release nutrients and favor more algal growth at the expense of macrophytes and other aquatic life.
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There is one macrophyte we are on the look-out for, a non-native species called Eurasian watermilfoil. It grows rapidly in nutrient rich waters and forms thick mats that are truly are a nuisance to boating and a danger to swimmers.

Eurasian watermilfoil can become established in a new lake simply by transport of fragments on a boat propeller. The OPA has been very supportive of state legislation passed to inspect Iowa lakes on an annual basis for this species. This legislation also included an outstanding public education campaign about this plant.

Eurasian watermilfoil looks similar to our native northern watermilfoil. The best way to learn to identify it is to become familiar with all of the aquatic plants growing in our lakes. The OPA encourages you to learn to recognize plants by their shapes and sizes. Eurasian watermilfoil has 12 to 21 leaflets, our native northern watermilfoil has only 5 to 10. If you think you have identified Eurasian watermilfoil, keep it in a bucket of water and call the DNR at the Spirit lake Fish Hatchery for identification verification.

Research indicates lakes with sedimentation problems and nutrient rich waters seem to be especially vulnerable to colonization by Eurasian watermilfoil, so this is another reason why it is so important to protect water quality through creation and enforcement of silt fence ordinances, establishment of vegetation buffers between lakes and lawns, and by supporting conservation agriculture practices. Being a member of the OPA also helps protect the health of our lake!

-Jane Shuttleworth, Environmental Education Coordinator, Iowa Lakeside Laboratory.
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Water Quality

To protect natural lakes and their watersheds in Dickinson County, the Okoboji Protective Association has supported the water quality monitoring programs of Iowa Lakeside Lab and the Friends of Lakeside Lab. Data is collected annually by volunteers through the Cooperative Lakes Area Monitoring Project (CLAMP). See the following links for more information.

Fact Sheets from 1999 - 2006 for West Okoboji and each of the other Dickinson County lakes monitored by CLAMP.

Raw Data for 2005 & 2006 from CLAMP.

Summaries of water quality data for the Iowa Great Lakes for 2003 and prior years, including data collected by CLAMP.

If you are interested in monitoring the health of our lakes, contact Jane Shuttleworth, CLAMP Volunteer Coordinator, at 337-3669 ext. 3 or by e-mail at lakesidejane@yahoo.com.

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How to Control Algae Blooms

Phil Peterson

Our greatest asset in the Iowa Great Lakes area is our lake water quality. It is the driver of our economic engine. An algae bloom reduces our enjoyment of the lakes and is one of those lake water quality distractions we would like to see disappear.

This past August and September we had heavy rains. Just after these heavy rains our lakes had very bad algae blooms. Although it is difficult to pin point the exact cause – and there could be multiple causes – many knowledgeable people feel the heavy rain washed a dry summer’s accumulation of lawn fertilizer into the lakes. It is well understood the major culprit for algae blooms is phosphorus.