Geneva Lake Data
Aquatic Plant Survey - 2020


Over the years the Geneva Lake Environmental Agency has conducted several aquatic plant surveys on Geneva Lake. The first one was conducted in 1976 and the most recent one was completed during the summer of 2020. These surveys are the foundation for the aquatic plant management plan which is anticipated to be updated in 2021-22.   Based upon the type and numbers of plants found in the lake, aquatic plant management strategies will be implimented. 

The early plant surveys were conducted using the transect methods, with plants identified every given feet along predetermined transects. During the summer of 2015 a comprehensive survey was conducted using the point intersect method. for the first time.   Points were located every 50 meters, samples were collected, plants identified and quantified. A total of 2,685 sites were located by longitude and latitude. Similar methodology was used in 2019 and 2020. 

The data from the 2020 survey showed a healthy, rich, bio-diverse aquatic plant community.  No new beds of starry stonewort were identified in the 2020 survey. The three most common plants found in 2020 were the same as found in 2019, wild celery (Vallisneria americana) , coontail ( Ceratophyllum demersum ) and spiral ditch-grass (Ruppia cirrhosa) .   


A total of 1,268 sites were sampled with 623 sites containing vegetation. Although aquatic plants may have been identified as deep as 37 ft, the largest number of plants were found at 16 ft.  A total of 21 different plants were identified. Native species dominated the plant community lake wide. Three non-native plants were identified, Eurasion watermillfoil (Myriophyllum spicatum), Culy-leaf pondweed (Potamogeton crispus) and Starry stonewort ( Tolypella intricata).  Native species dominated the plant community lake wide.  

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An Aquatic Plant Management Plan

A product of the past aquatic plant survey was the preparation of an aquatic plant management plan (see map of recommended management below). This aquatic plant management plan will be reviewed and modified as needed when an assessment of the 2015 survey is completed.


In general, the lake was divided into three separate aquatic plant management areas. The “No Restriction Zone” where it is recommended that there be no restriction on the aquatic plant management strategies if they are compatible with State requirements. The “Native Zone” are areas that have a diverse and healthy plant community and thus no management is needed or recommended unless conditions change. The “Watch Zone” is the largest area in size and includes areas were the plant community may need some minor management but for the most part is relatively stable. An overlay “Riparian Zone” can be located in any one of the other three zones but is restricted to only a 30 ft. wide strip running perpendicular to the shore and extends out into deeper water. This zone is to allow for riparian management of aquatic plants in their swim, mooring or pier areas.

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Zebra Mussels

To download the final report, click here

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Zebra Muscle Substrate

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During the summer of 2020, three zebra mussels substrates were deployed in Geneva Lake to monitor their colonization by zebra mussels.  Sites were located on the east end of the lake, Rivera pier, west end of the lake, Gordy's pier,  and on the south eastern shore, Hillside Rd pier.  Deployment this year was different than in past deployments as the substrates were counted only once at the end of summer.  The substrates were in the water for 115 days.  In the past, substrates were removed, counted and cleaned each month during the summer resulting in the maximum deployment without cleaning being 63 days.   

2020 mussel colonization rates ranged from 25 mussels per day per substrate to 37 mussels per substrate.  These rates are higher than those listed for 2016.  That may be the results of the substrates not being cleaned during the 2020 deployment.  During the 2016 the substrates were cleaned at the end in July and August.


The west end of the lake showed the highest colonization rate and ultimately the highest density during 2020 and during the later two periods of 2016 deployment. 

Although there were a few mussels observed on the substrates that looked like Quagga mussels, there has not been a confirmed identification of them in Geneva Lake as of 2020.     

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Trophic State Index

A trophic state index assigns a numerical value to specific measurement found in the lake that are representative of the lake’s overall condition. There are several trophic status indexes that are used but for the purpose of understanding Geneva Lake’s trophic condition, a modified version referred to as the Wisconsin Trophic State Index is used. Generally, the higher the trophic value the worse the water quality.

Trophic state refers to the amount of biological activity in a lake. The more trophic a lake is the more fertile it is. A very biologically active fertile lake with a lot of plants would be considered a eutrophic lake. A lake with very little plant life and not much fertility is called an oligotrophic lake. Lakes the fall between very fertile and a not very fertile are referred to a mesotrophic lakes.

Three measurement used to evaluate a lake’s fertility are; total phosphorus. chlorophyll ”a” and secchi disk or water clarity. Phosphorus is a vital nutrient that is often the limiting nutrient for plant growth in lakes. The more phosphorus in a lake, the more plant growth. Chlorophyll ”a” is a color pigment found in green plants. A measure of the chlorophyll ”a” in the water column is an indirect measurement of plant density in the open water or plankton community. Secchi disc reading measure the depth of light penetration into the water. All three are related, the more phosphorus the more chlorophyll” a” and the more chlorophyll”a”, the more algae in the water and the less light penetrates into the water.

The figure below shows the plotted TSI values for Geneva Lake’s total phosphorus(TP), Chlorophyll (chl”a”) and secchi over the last 18 years. Geneva Lake’s secchi disk TSI is mostly in the oligotrophic range. The Chl”a” TSI value moves between mesotrophic and oligotrophic. Only the total phosphorus TSI ever reaches eutrophic and only on few occasions. It is mostly in the mesotrophic range. Based upon this information, Geneva lake would be classified as a relatively clean lake that is in transition between oligotrophic and mesotrophic states.

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Spring Chlorophyll"a"

Chlorophyll a

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Chlorophyll "a" is a color pigment found in plants.  It is measured as an indirect means of measuring the amount of free floating plants (phytoplankton) in the lake. 

Total Phosphorus

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Secchi Disk Trends

A secchi disk is a round disk, 20 cm. in diameter,  that is colored alternately in black and white quadrants.  It is used to measure water clarity.  The disk is lowered in the water and the depth at which it can no longer be seen is recorded.  It is then slowly brought back up and the depth at which it reappears is recorded.  The two depths are then averaged for a Secchi depth recording. It is used to measure water clarity.


                             A Secchi Disk

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Temperature and Oxygen Stratification

As is typical with most deeper lakes in the northern hemisphere, Geneva Lake stratifies during the summer and winter and mixes during the spring and and fall. When the ice goes out in the spring the lake mixes from top to bottom until the surface waters are warmed by the longer, warmer days.  The lake stays stratified with the warmer water at the surface and the colder waters in the deeper areas till fall.  As fall approaches and the days become shorter and cooler the surface waters cool and the lake become uniform in temperature.  This allows for the waters to mix from top to bottom.  As the lake freezes over, the lake again stratifies, but this time the warmer water (39 F.) are at the bottom and the cooler waters (32 F.) are at the top. The lake remains in this winter stratification until the days begin to get longer and warmer and the ice melts, starting the annual cycle again. 

Although variable, Geneva Lake's summer stratification starts in  late May and lasts till November.  Fall mixing last till January when the lake freezes over.  Ice-of is in late March.  Spring mixing takes place till May when summer stratification begins to set up. 

Lake stratification impacts the distribution of gases, nutrients and fauna in the lake.  Of significant importance is the distribution of oxygen in the lake.  Summer stratification can be is so strong that the deeper, colder waters may run out of oxygen, limiting the distribution of aquatic fauna to only the upper warmer but oxygen rich waters.  


















The August 16, 2020 temperature and dissolved oxygen profiles  show the lake is in strong summer stratification.  A strong thermocline has been established between the depths of 32.5 ft. (10.0 m) and 49.2 ft. (15 m).  The thermocline separates the upper circulating warm layer of the lake’s epilimnion  from the deep cold waters of the hypolimnion.  At the time of sampling, the epilimnion existed from the lake’s surface to a depth of 32.5 ft.  (10.0 m), and the hypolimnion existed from 45.2 ft. (15 m) to the measured bottom at 139.8 ft. (43 m). The thermocline was recorded as being 16.4 ft. (5.0 m) thick.  The thermocline is defined as that part of the water column where the water temperature decreases at a rate of 1 C per meter or greater. 


The thermocline will continue to erode and sink as the circulating warmer epilimnion waters wears away at the top of the thermocline. This action will continue until the fall when cooling surface waters reach the same temperature as the thermocline.  This will cause the thermocline to disappear.  The lake will then enter into fall turnover where temperature and dissolved oxygen will be uniform from the top waters of the lake to the bottom waters. The lake experiences complete top to bottom mixing. On Geneva Lake, this generally happens in early to mid-November.


At the lake’s surface, the dissolved oxygen concentration was measured as 8.12 mg/L. At the top of the thermocline, oxygen concentration was 8.33 mg/L. Oxygen concentration slowly decreased  in the thermocline.  Below the thermocline theoxygen level increase to 6.61 mg/l at 62.5 Ft. (19 m.).  It steadily decreased to the bottom where it was recorded as 0.34 mg/l. ,


At a depth of 91.9 ft (28 m), oxygen levels drop below 5.0 mg/L. At 121.4 ft (37 m), oxygen levels drop below 2.0 mg/L. Fish experience higher levels of stress when dissolved oxygen levels drop below 5 mg/L. Most fish species cannot survive for any period of time where dissolved oxygen is less than 2 mg/L.


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